Crossing Over: The Basics of Evolution - Workbook for Teachers

Compiled by the Social Cohesion and Identity Research Programme of the Human Sciences Research Council in association w...

Author: Edith Dempster

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Compiled by the Social Cohesion and Identity Research Programme of the Human Sciences Research Council in association with the Africa Genome Education Institute Published by HSRC Press Private Bag X9182, Cape Town, 8000, South Africa www.hsrcpress.ac.za © 2006 Human Sciences Research Council First published 2006 All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. ISBN 0-7969-2119-9 Copy editing by Karen Morrison Typeset by New Leaf Design Illustrations by R Nanni and Robert Hichens Cover design by Richard Mason Print management by comPress Distributed in Africa by Blue Weaver PO Box 30370, Tokai, Cape Town, 7966, South Africa Tel: +27 (0) 21 701 4477 Fax: +27 (0) 21 701 7302 email: [email protected] www.oneworldbooks.com Distributed in Europe and the United Kingdom by Eurospan Distribution Services (EDS) 3 Henrietta Street, Covent Garden, London, WC2E 8LU, United Kingdom Tel: +44 (0) 20 7240 0856 Fax: +44 (0) 20 7379 0609 email: [email protected] www.eurospanonline.com Distributed in North America by Independent Publishers Group (IPG) Order Department, 814 North Franklin Street, Chicago, IL 60610, USA Call toll-free: (800) 888 4741 All other enquiries: +1 (312) 337 0747 Fax: +1 (312) 337 5985 email: [email protected] www.ipgbook.com

The Crossing Over Pilot Teacher Trainer Research Programme (a project of the Human Sciences Research Council) was a week-long course held in Cape Town and attended by 33 teachers with representivity from all nine provinces and from rural, urban, private and state schools. We chose the name Crossing Over as it suggests not only the transmission of knowledge from one to another, but the shift from one state of knowledge to another too. It is also the act that chromosomes perform in the event of meiosis. Crossing over is fundamental to change. Crossing Over was designed to cover the basic content necessary for teaching the key concepts of comparative functioning, relationships and the development of change, otherwise known as evolution, in molecular biology. There was a special emphasis on lesson planning skills and a series of exciting visits to appropriate environments. The teachers had access to the best that is available in the country both in terms of facilitators and sites (a glance at the Acknowledgements reveals this). This book is a compilation of the material that was developed for the Crossing Over Pilot Course for the GET (General Education and Training) curriculum and the FET (Further Education and Training) curriculum. A further book, Reading Scientific Images: The Iconography of Evolution, has also been produced by the project for educators interested in the cusp between art and science and more specifically in reading scientific images. We hope that through this publication, Crossing Over is able to add value and bring satisfaction to educators beyond the team of the course facilitators, the group of participants and their learners. We thank the Royal Netherlands Embassy and the Human Sciences Research Council for the financial support and the research resources that they respectively contributed to the project. Sandra Prosalendis Project Coordinator: School Curriculum and Scientific Literacy Project Social Cohesion and Identity Research Programme Human Sciences Research Council

HSRC Workbook

Preface

Acknowledgements Putting together the Crossing Over initiative was a collaborative effort and required the expertise and the imagination of the team named here. Thank you to Mr Utando Baduza (HSRC), Ms Pam Barron (HSRC), Ms Colleen Dawson (Consultant HSRC), Dr Edith Dempster (UKZN), Dr Helen De Pinho (UCT), Mr Luvuyo Dondolo (HSRC), Mr Adrian Hadland (HSRC), Ms Cathy Hastie (SABC), Dr Wim Hoppers (RNE), Professor Wilmot James (HSRC), Mr Richard Mason (UCT), Professor Tony Morphet (UCT), Mr Kanthan Naidoo (Gauteng Education Department), Professor John Parkington (UCT), Ms Sandra Prosalendis (HSRC), Dr Jaishree Raman (MRC), Mr Trevor Samson, Ms Gillian Warren Brown, Ms Lynne Wilson (HSRC) and Ms Jean Witten (HSRC). The contributions of the UCT MicroBiology Department, Kirstenbosch Gardens, Central Methodist Mission in Cape Town, the MTN Science Centre, the South African Museum, the Fossil Park, Sivuyile Tourism and Information Centre and Sivuyile Teacher Training College in Gugulethu, and Women Unite are also acknowledged. We thank the teachers listed below for their enthusiastic and considered participation: Mr Thabo Msutu (Mzontsudu S.S. School, King William’s Town), Ms Kuzeka Gecelo (Lingelethu J.S. School, Cala), Mr Anthony Pandaram (Westville Boys High, Wandsbeck), Mr Barry Booysen (Ebenhaeser School, Wepener), Mr Ezekiel Moyaha (Tidimane Middle School, Mogwase), Ms Nandipha Mapukata (Blorhweni J.S.S, Ntabankulu), Mr Setshwaro Mokgethi (SaHeso Intermediate School, Roodepoort Farm), Mr Philemond Nkuna (Noto High, NW Province), Mr John Visagie (Intermediate School Keimoes, Northern Cape), Mr Dumisani Dlodlo (KwaDomba High, Nongoma), Mrs R. Ramgoolam (Greytown Sec. School, Greytown), Mr Stephan le Roux (Stanford Lake College, Haenertsburg), Mr NP Sebone (Marumofase H. School, Indemark), Mr Thomas Jafta (Newslands East Sec. School, Marblerary), Mrs G.N Links (St. Boniface H. School, Kimberley), Mr Johnny Witbooi (Bridgton Sec. School, Oudtshoorn), Mr Nicholas Smith (St Boniface High, Kimberley), Ms Chairmaine Stalmeester (Bridgton H. School, Oudtshoorn), Mrs Nozuko Phakela (Fezeka Sec. School, Cape Town), Mr Thomas Mathew (Somavugha High, Mahwelereng), Mr Ashley Engelbrecht (Simunye High, Cape Town), Mr NE Nyawose (Durban Natural Science Museum, Durban), Mr Retsisang Moreku (Mhwayi Primary, Kabokweni), Mr Siyanda Mcwango (Gordon Memorial High, Dundee), Mr Benjamin Chipulu (Janjo High, Gopano), Mr Lesetja Seopa (Mapule Sec. School, Bakone), Mr Frans Bodigelo (Mmadikete Intermediate, Brits), Mr Amos Rangata (Emadwaleni High, Soweto), Mr KW Kgopane (Sango Combined School, Laersdrif) Mr N Kamteni (Sophumelele Sec. School, Cape Town) Mr Thabo Tsunyana (Sinako Sec. School, Cape Town) Ms Alticia Klaasen (Villiersdorp Sec. School, Villiersdorp). The author and publishers would like to thank the following people and organisations for photographs and micrographs used in the publication: The University of KwaZulu-Natal Centre for Electron Microscopy; Mike van der Wolk; Harcourt Education.

Introduction

vi

The Natural Sciences curriculum

viii

Using the workbook

xiii

Part 1: Understanding Genes & Inheritance Contents Overview 1. The cell 2. Measuring very small structures 3. The cell cycle 4. The chromosomes and cell development 5. Mitosis 6. Inheritance 7. Selection 8. General principles of reproduction 9. Sexual reproduction Solutions to activities

1 3 4 9 14 16 19 23 32 36 38 44

51 52 55 57 60 62 70 75 94 95

Part 2: Introducing evolution Contents The Life Sciences curriculum Overview 1. Charles Darwin and the voyage of The Beagle 2. Darwin’s theory of evolution 3. What evidence supports Darwin’s theory? 4. Present day evidence of evolution 5. Genetics and evolution Resources Solutions to activities

HSRC Workbook

Contents

Introduction Did you know? The Human Genome Project (1990 – 2003) was an international effort which aimed to identify the 20 000 – 25 000 genes in human DNA, find out more about the chemical composition of DNA, store the information in databases, and investigate the ethical and legal issues related to their findings. Although the project is finished, the information that scientists have found will be analysed for many years to come. You can find out more about this project online at: www.ornl.gov/sci/techresources/ HumanGenome/home.shtml

The Human Genome Project is one of the greatest scientific efforts of the current age. As a result of the HSRC’s interest in this project, we ran a teacher-training project to help teachers become aware of the importance of the Human Genome Project and to increase their understanding of genetics and the ways in which characteristics are transferred or inherited from one generation to the next. This short course was developed to provide teachers with the knowledge and understanding required to do so. Part 1 of this workbook is designed for Senior Phase teachers. It forms the basis for the more advanced concepts needed in the FET phase which are covered in Part 2: Introducing Evolution. An understanding of evolution is very important for teachers, particularly as this is a new topic in the school curriculum. We believe that understanding the theory and examining some of the supporting evidence, will lead you to agree with the famous evolutionary biologist, Theodosius Dobzhansky, who said ‘Nothing in Biology makes sense except in the light of evolution.’ When we first trialled this material, we ran a week-long course for 35 teachers from all over South Africa. We were very lucky to run the course in Cape Town, which is within a famous centre of evolution, the Cape Floral Kingdom. The itinerary below shows you how the course itself helped teachers to raise questions and develop their own understanding of the evolutionary process. Day 1 Kirstenbosch

experience the incredible variety of plants in the Cape Floral

Botanical Garden

Kingdom

University of

extract DNA from onions

Cape Town

use models to discover how hereditary information is stored in

– MicroBiology

DNA, and then translated into protein molecules

Laboratory

investigate simple hereditary characteristics such as blood groups, widow’s peak, attached ear lobes, and hair on the middle finger joints.

Some of the teachers were amazed to find out that each of us inherits an equal number of genes from our mother and our father, and that our children inherit equal numbers of genes from their mother and father. They asked questions like this: ‘I have 7 brothers and sisters, and all of them inherited equal numbers of genes from our mother and father. Why do we look different?’ It was quite a challenge to answer all the questions! Day 2 West Coast

view 5-million year old fossils being excavated.

Fossil Park

The fossils were of large, sturdy giraffe-like animals all lying where they had been buried five million years earlier. For most of the teachers, it was the first time they had seen fossils, and it made them aware of changes in the Earth’s surface, and changes in the life forms that have existed on the Earth. It also made them aware of the long history of life on Earth, because five million years ago is just the other day in geological time.

vi

Introduction

HSRC Workbook

Day 3 South

activities on the process of natural selection

African

trying to find patterns of similarity and difference in flowers

Museum

visit the museum to look at similarities in the bone structure of vertebrate forelimbs.

By then, some of the teachers had realised that evolution was not something mysterious and dangerous, but a very good way of answering the ‘why’ questions in Biology. Some of these questions are:

Why is there so much duplication in organisms that occur in different parts of the world? For example, each continent in the southern hemisphere has a different species of large flightless bird – the ostrich in Africa, the emu in Australia, the moa in New Zealand (now extinct), the rhea in South America, and the cassowary in New Guinea.

Why do we find fossils of organisms that no longer live on Earth? How do characteristics pass from parents to their children?

Why do children resemble their parents?

It has been estimated that there are about 20 million species living on Earth at present.Why do we find such enormous diversity of life on Earth today?

Why do we find patterns of similarity and difference within the diversity of life?

What has happened to those organisms?

Why are there so many different species filling the same niche on different land masses?

A few teachers made all the links, and realised that evolution is about variation in the genes, and that through the process of natural selection, certain individuals have a better success rate in breeding and therefore passing on their genes to the next generation. It all seemed simple and so logical because the experiences were built up in this way and teachers could see that evolution progresses through natural selection in each successive generation.

Crossing Over: The Basics of Evolution

vii

The Natural Sciences curriculum The Revised National Curriculum Statement (RNCS) is the current policy document on education. The RNCS places outcomes at the centre of learning and teaching, but it also specifies content topics that must be covered, and those must constitute 70% of the teaching time in each grade. The remaining 30% of the time is available to extend the core knowledge, or to introduce content from local contexts. The core content in Natural Sciences draws knowledge from four main areas: l Life and Living l Energy and Change l Planet Earth and Beyond l Matter and Materials. The Natural Sciences Learning Area is constructed around three learning outcomes: Learning Outcome 1: Scientific Investigation The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts. Learning Outcome 2: Constructing Science Knowledge The learner will know and be able to interpret and apply scientific, technological and environmental knowledge. Learning Outcome 3: Science, Society and the Environment The learner will be able to demonstrate an understanding of the interrelationships between science and technology, society and the environment. Natural Sciences educators are required to construct learning programmes which will help learners to make progress in all three learning outcomes throughout the GET band. The learning outcomes are further described by a set of Assessment Standards that specify the levels of achievement within each learning outcome in each Grade. The Assessment Standards for Grades 7 – 9 are shown in Table 1.

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HSRC Workbook

Table 1: Learning Outcomes and Assessment Standards for Grades 7 – 9

Learning Outcome

Assessment Standard

Grade 7

Grade 8

Grade 9

LO1: Scientific Investigation

1. Planning investigations

Learner plans simple tests and comparisons, and considers how to make them fair.

Learner identifies factors to be considered in investigations and plans ways to collect data on them, across a range of values.

Learner plans a procedure to test predictions or hypotheses, with control of an interfering variable.

2. Conducting investigations and collecting data

Learner organises and uses equipment or sources to gather and record information.

Learner collects and records information as accurately as equipment permits and investigation purposes require.

Learner contributes to systematic data collection, with regard to accuracy, reliability and the need to control a variable.

3. Evaluating data and communicating findings

Learner generalises in terms of a relevant aspect and describes how the data supports the generalisation.

Learner considers the extent to which the conclusions reached are reasonable answers to the focus question of the investigation.

Learner seeks patterns and trends in the data collected and generalises in terms of simple principles.

1. Recalling meaningful information when needed

Learner, at the minimum, recalls definitions and complex facts.

Learner, at the minimum, recalls procedures, processes and complex facts.

Learner, at the minimum, recalls principles, processes and models.

2. Categorising information to reduce complexity and look for patterns

Learner compares features of different categories of objects, organisms and events.

Learner applies classification systems to familiar and unfamiliar objects, events, organisms and materials.

Learner applies multiple classifications to familiar and unfamiliar objects, events, organisms and materials.

3. Interpreting information

Learner interprets information by identifying key ideas in text, finding patterns in recorded data, and making inferences from information in various forms such as pictures, diagrams and text.

Learner interprets information by translating tabulated data into graphs, by reading data off graphs, and by making predictions from patterns.

Learner interprets information by translating line graphs into text descriptions and vice versa, by extrapolating from patterns in tables and graphs to predict how one variable will change, and by identifying relationships between variables from tables and graphs of data, and by hypothesising possible relationships between variables.

4. Applying knowledge to problems that are not taught explicitly

Learner applies conceptual knowledge by linking a taught concept to a variation of a familiar situation.

Learner applies conceptual knowledge to somewhat unfamiliar situations by referring to appropriate concepts and processes.

Learner applies principles and links relevant concepts to generate solutions to somewhat unfamiliar problems.

1. Understanding science as a human endeavour in cultural contexts

Learner compares differing interpretations of events.

Learner identifies ways in which people build confidence in their knowledge systems.

Learner recognises differences in explanations offered by the natural sciences and other systems of explanation.

2. Understanding sustainable use of the Earth’s resources

Learner analyses information about sustainable and unsustainable use of resources.

Learner identifies information required to make a judgement about resource use.

Learner responds appropriately to knowledge about the use of resources and environmental impacts.

LO2: Constructing Science Knowledge

LO3: Science, Society and the Environment


ix

In an outcomes-based framework, the content is the vehicle that educators use to facilitate learners’ achievement in each assessment standard. The Natural Sciences curriculum statement provides a list of core content topics that must be covered, but it provides very little detail about the depth and breadth of each topic. This means that educators must have good subject-matter knowledge in all four content areas of science, as well as access to a wide range of resources so that they are able to construct learning experiences that are interesting and valid in Natural Sciences. Each content area is further divided into two or three sub-strands, giving a total of ten sub-strands in the learning area. Teachers are required to draw on appropriate content from all of the sub-strands to build the assessment standards and ultimately the three learning outcomes for the Natural Sciences learning area. This workbook aims to help you build your own understanding of topics that fall within the content area Life and Living. Although the Natural Sciences learning area statement does not specifically mention the term evolution, many of the content topics relate directly to evolution and the processes involved therein. Table 2 shows the content areas that relate directly to evolution, together with an explanation of how they relate to evolution.

The National Sciences curriculum

Content area and sub-strand

Knowledge statement

Link with evolution

Life and Living: Biodiversity, change and continuity

Unifying statement: The huge diversity of forms of life can be understood in terms of a history of change in environments and in characteristics of plants and animals throughout the world over millions of years.

Change in environments is the engine that drives change in life forms, or evolution. The history of life on Earth is the evolutionary history of life.

South Africa has a rich fossil record of animals and plants which lived millions of years ago. Many of those animals and plants were different from the ones we see nowadays. Some plants and animals nowadays have strong similarities to fossils of ancient plants and animals. We infer from the fossil record and other geological observations that the diversity of living things, natural environments and climates were different in those long-ago times.

This content statement introduces learners to the idea that life has a very long history (millions of years) and that fossils tell a story of changing life on Earth. Similarities between fossils and living species are best explained by evolution. Long periods of time, changes in the Earth’s surface and climate, and the similarities between fossils and living species were three of the observations that led Charles Darwin to propose the theory of evolution.

Offspring of organisms differ in small ways from their parents and generally from each other. This is called variation in a species.

Variation provides the raw material for natural selection to act upon. The variation is controlled by genes, and is transmitted from one generation to another through the genes.

Natural selection kills those individuals of a species which lack the characteristics that would have enabled them to survive and reproduce successfully in their environment. Individuals which have characteristics suited to the environment reproduce successfully and some of their offspring carry the successful characteristics. Natural selection is accelerated when the environment changes; this can lead to extinction of a species.

Natural selection is the mechanism whereby evolution results in changes within a species. Accumulated change in isolated populations eventually results in new species forming. The fact that characteristics are inherited through the genes means that individuals that reproduce successfully pass on their genes to the next generation. Species that do not adapt to an environmental change become extinct.

Variations in human biological characteristics such as skin colour, height, and so on have been used to categorise groups of people. These biological differences do not indicate differences in innate abilities of the groups concerned. Therefore, such categorisation of groups by biological differences is neither scientifically valid nor exact; it is a social construct.

Variations in physical characteristics are largely controlled by the genes. Changes in skin colour in humans result from a minor alteration in the genes, which is not associated with differences in intelligence or any other characteristic.

Extinctions also occur through natural events. Mass extinctions have occurred in the past, suggesting that huge changes to environments have occurred. However, these changes occurred very slowly, compared to the fast rate at which humans can destroy plant and animal species.

Extinction means that life on Earth is not the same as it was thousands, millions or hundreds of millions of years ago. The fossil record provides evidence of species that have existed on Earth in the past, and are no longer present on the Earth. The fossil record also provides evidence of periods of relatively rapid turnover in species, which are associated with massive environmental change, such as that caused by a meteorite impact.

Unifying statement: The Earth is composed of materials which are continually being changed by forces on and under the surface.

The Earth is constantly undergoing change: it is not the same as it was thousands or millions of years ago. Through realising that the surface of the Earth changes, Charles Darwin began to understand that life forms could evolve in response to the changing surface of the Earth.

Fossils are the remains of life forms that have been preserved in stone. Fossils are evidence that life, climates and environments in the past were very different from those of today.

Fossils provide tangible evidence of life forms that have existed in the past, and through which we can trace the evolutionary history of the species living on Earth at present. Fossils therefore provide strong evidence that life has evolved.

Many of the organisms in South Africa’s fossil record cannot be easily classified into groups of organisms alive today, and some are found in places where presentday conditions would not be suitable for them. This is evidence that life and conditions on the surface of Earth have changed through time.

Fossils provide direct evidence of differences and similarities in life-forms presently living on Earth. They also support the idea that evolution has resulted in changes in living organisms in response to changes in the Earth’s environment over very long periods of time.

Planet Earth and Beyond: The changing Earth

Table 2 shows that a large number of topics have direct relationships with evolution. The Natural Sciences learning area provides a number of the foundational concepts on which the theory of evolution was built. An understanding of genetics provides us with a deeper understanding of the actual mechanisms involved in the process of evolution.


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HSRC Workbook

Table 2: Content topics in the Natural Sciences learning area statement and their relationship to evolution

Thus, evolution underpins the Natural Sciences learning area, although it is never named in the document. In South Africa we are very lucky as we have many unique attributes that make it easy to teach the learners about evolution in practical and meaningful ways. l Within our country’s borders, we have at least seven hotspots of biodiversity, where we can see the results of rapid evolution in particular areas. l We have a rich fossil record that extends from some of the oldest fossil bacteria in the world (3 600 million years old) to human fossils of the last 100 000 years. l We have a network of museums where learners can see and touch fossils, as well as world heritage site, the Cradle of Humankind, where learners can see evidence of the evolution of humans. l Through the work of geneticists in South Africa, we can draw on DNA analysis to understand human history extending back to 100 000 years ago and beyond.

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HSRC Workbook

Using the workbook This workbook is written in the form of an interactive text so you can think and make notes in the book as you work through it. The activities are designed to develop your own understanding, but you may find that some activities are suitable for use with learners. Feel free to adapt the activities to suit your learners, but remember that some activities may be too difficult for school use. You may also translate portions of the text into the language of learning and teaching at your school. The learning outcomes, assessment standards and content topics covered in each section are listed at the beginning of the section to help you to structure your learning programme for Natural Sciences. Most of the material on genes and chromosomes is not included in the prescribed content for the Natural Sciences learning area, but it is important for your own understanding of the relationship between genetics and evolution. If you also teach Life Sciences, you may find sections of this workbook useful in your teaching in Grades 10 – 12. This workbook is available as a downloadable .pdf document from the HSRC website (www.hsrcpress.ac.za) and you may print pages to make your own audio-visual aids or worksheets for your learners. When we trialled the workbook, we found that the practical activities helped participants to enjoy and understand the concepts. In particular, allowing learners to touch and see real fossils helps them to understand that life existed millions of years ago, and that the life-forms were different from the life we see around us now. When you teach this material, try to arrange for learners to visit a museum, or borrow some fossils from the nearest museum so that learners get first-hand experience of fossils. You can buy an inexpensive cast of the skull of Mrs Ples, a famous pre-human fossil, from the Transvaal Museum. This book is only an introduction to some of the basic ideas of genes and inheritance. At the end of Part 2, you will find a list of resources that you can use to help you find out more about evolution. As more information becomes available each year, it is a good idea to search the Internet or your local library for new books and videos on the topic of evolution. New genetic discoveries are often reported in newspapers and magazines such as African Geographical and local newspapers.


xiii

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HSRC Workbook

Understanding Genes & Inheritance

Part

1

Contents Overview

3

1. The cell

4

The structure of a cell

5

2. Measuring very small structures

9

Using scale bars

10

14

Cells divide Events in the cell cycle

14 15

4. The chromosomes and cell development

16

5. Mitosis Steps in the process Cell division

3. The cell cycle

19

19 21

23

23 24 25 26

28 28

32

32 34 34 34

6. Inheritance Parents and offspring What is a species? Chromosomes carry genes The genetic basis of inheritance The relationship between chromosomes and the whole organism Measuring variation 7. Selection Natural selection Natural selection and evolution Artifi cial selection Artifi cial selection and genetics

8. General principles of reproduction Reproduction ensures the survival of a species

36 36

9. Sexual reproduction Males and females Meiosis Fertilisation

38

38 40 42

Solutions to activities

44

1

HSRC Workbook

Overview Part 1 of this workbook explores the way every cell carries a plan for its development and functioning. The plan is stored in a coded form in the chromosomes. You will learn how this plan is copied and passed from cell to cell and from parent to offspring. Humans use the knowledge about these plans to breed animals and plants that are useful to us.

Skills Interpreting micrographs, converting measurements, carrying out a survey, drawing and interpreting frequency distribution graphs.

Outcomes When you have finished Part 1, you should be able to: l Identify structures in cells l Describe the cell cycle in drawings and words l Explain the role of chromosomes and genes in transferring hereditary information from cell to cell, and from parents to offspring l Draw and interpret a frequency distribution bar graph l Explain the uses of genetic control of development in tissue culture, cloning, plant and animal breeding l Describe the role of natural selection in evolution.


1

The cell

Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Learning Outcome 1: Scientific investigation Assessment Standards Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.

Cells are the smallest units of life that can grow, reproduce and carry out metabolic functions. Most cells are too small to see with the naked eye. Your body consists of millions of cells, but your eyes are not powerful enough to distinguish even one cell without a microscope. Cells were first discovered in 1665 by a scientist called Robert Hooke. He sliced a piece of cork into a thin sheet. He then studied the thin sheet of cork using a homemade microscope. (A microscope makes things look much larger than their usual size). Hooke discovered that cork was made up of tiny boxes. He decided to call these boxes cells.

Figure 1. What Robert Hooke saw through his microscope.

Other scientists observed many living things under microscopes and came to the conclusion that all living things are made up of cells. Some organisms consist of only one cell. We say they are unicellular organisms. Organisms that are made up of many cells are called multicellular organisms.

4

Part 1: Understanding Genes and Inheritance

HSRC Workbook

The structure of a cell Cells come in many shapes and sizes. When we talk about a ‘typical’ cell, we are really talking about the characteristics that are shared by most cells. You can think of a cell as a tiny bag which holds water, thousands of very tiny molecules, and some other microscopic structures.

Q

Why do all living things consist of cells?

A

Cells carry out chemical reactions as part of their metabolism. By keeping the molecules in a restricted area, the reactions can take place much more efficiently. Scientists think cells evolved because it is more efficient for metabolic reactions to take place in a restricted environment than if the molecules were floating freely in a large amount of water.

Did you know? South African scientists have discovered fossils of cells in rocks that are about 3.5 billion years old. Life has existed on the Earth for more than 3.5 billion years!

cell wall

Mitochondrion cell membrane Nucleus Endoplasmic reticulum Chloroplast Vacuole Cytoplasm

Figure 2. The structure of a typical plant cell.

The cell membrane Earlier you read that you can think of a cell as a tiny bag. We call the bag that holds the cell contents the cell membrane. The cell membrane is a layer that separates the cell from the cells around it, or from the water or air that surrounds the cell. It is like a very thin skin surrounding the cell contents.


Diffusion is the process whereby a substance moves from a region of high concentration to a region of lower concentration.

The cell membrane plays a very important role in the metabolism of a cell. It keeps all the molecules that take part in chemical reactions inside the cell. It allows small molecules like oxygen, carbon dioxide, water and some salts to diffuse freely into and out of the cell. The cell membrane helps certain molecules like sugars to move into or out of the cell. We call the process whereby a cell membrane helps molecules to move into or out of the cell active transport. Diffuse (say dife-fuse) means that a substance moves from where it is plentiful to where it is scarce. So, if carbon dioxide is plentiful in a cell, and scarce outside the cell, carbon dioxide will diffuse out of the cell. It diffuses through the cell membrane. If oxygen is plentiful inside a cell and scarce outside the cell, oxygen will diffuse out of the cell. It diffuses through the cell membrane. Animal cells use oxygen during the metabolic process called cellular respiration. Oxygen is scarce inside the cell, but plentiful outside the cell. Oxygen diffuses into each cell through the cell membrane. Cellular respiration produces carbon dioxide inside the cell. Carbon dioxide is plentiful inside the cell and scarce outside the cell. Carbon dioxide diffuses out of each cell through the cell membrane.

Activity 1 Imagine a cell in the leaf of a green plant. Underline the word or words that correctly complete these sentences about a cell during the daytime. During daylight, green plant cells make sugar by the metabolic process of (cellular respiration / photosynthesis). The process uses the gas (oxygen / carbon dioxide), which enters the cell by (active transport / diffusion) through the cell membrane. During the daytime, green plant cells produce the gas (oxygen / carbon dioxide). It leaves the cells by (diffusion / active transport) through the cell membrane. Photosynthesis produces molecules of (sugar / protein) in the leaves. Root cells break down (protein / sugar) in the metabolic process of (cellular respiration / photosynthesis). Sugar must move from the cells of the leaf to the cells of the root. Sugar cannot (diffuse / be actively transported) through the cell membrane. The cell membrane assists sugar to (enter / leave) the cells of the leaf and to (enter/ leave) the cells of the root.

Cytoplasm

Cytoplasm is the name given to everything inside a cell except the nucleus.

The substances and structures inside the cell membrane are called cytoplasm (say sy-toe-plaz-im). Cytoplasm consists of cytosol (sy-toe-sol) and organelles (say organ-els). The cytosol contains water, dissolved substances like sugar, salts, oxygen and carbon dioxide, and molecules that take part in the metabolic processes of the cell. The organelles are microscopic structures floating in the cytosol.


HSRC Workbook

Activity 2 Look at Figure 2. Find the following organelles in the cytoplasm: a. Chloroplasts (say klaw-ro-plasts) b. Mitochondria (say my-toe-kon-dree-a) c. Endoplasmic reticulum (say en-doe-plas-mic re-tic-you-lum) d. Vacuole (say vac-you-ole)

Each organelle is separated from the cytoplasm by its own membrane. Some organelles have folded membranes inside an outer membrane. The whole cytoplasm contains membranes that are folded and branched inside the cell. Each organelle has a particular function in the cell. l Photosynthesis takes place in the chloroplasts. l Cellular respiration takes place in the mitochondria. l Proteins are manufactured in the endoplasmic reticulum. l Water, small molecules, and waste products are stored in the vacuole.

Organelles are microscopic structures in a cell.

Q

Mitochondria occur in both animal and plant cells, but only plant cells contain chloroplasts. Why is this?

A

Only plant cells carry out photosynthesis, which takes place in the chloroplasts. Therefore, we expect that only plant cells will have chloroplasts. The reactions of cellular respiration take place in the mitochondria. Both animal and plant cells carry out cellular respiration, therefore we expect that both plant and animal cells will contain mitochondria.

Activity 3 Would you expect to find chloroplasts in the cells of a plant root? Explain your answer.


The nucleus In Figure 2 you saw that the nucleus is the biggest structure in the cell. When cells were first seen, the microscopes were powerful enough to enable scientists to see that a cell contained a nucleus and cytoplasm, but they could not see all the organelles that we now recognise. We use the term cytoplasm to mean everything inside the cell except the nucleus. Cytoplasm

Chromosomes

Figure 3. Chromosomes in a cell.

Chromosomes are structures in the nucleus that carry hereditary information.

8

The nucleus is separated from the cytoplasm by a membrane which has a large number of tiny holes, or pores, in it. The nucleus contains very important structures called chromosomes (say kroam-o-somes). A chromosome is a structure in the nucleus that carries the hereditary information for the cell. Hereditary information is information that passes from one cell or organism to its offspring. It is the set of instructions that control the way a cell develops and functions. Normally, we cannot see the chromosomes in a nucleus, but when a cell is about to divide, we see the chromosomes as threads in the cell. You can see a photograph of chromosomes in a cell in Figure 3. A chromosome is made up of a very long molecule called deoxyribonucleic acid (DNA) combined with proteins. The chromosomes carry all the instructions for the growth, reproduction and metabolism of the cell and for the whole organism.


HSRC Workbook

2 M easuring very small structures Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Learning Outcome 1: Scientific investigation Assessment Standards Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.

The smallest division on most rulers is one millimetre, but cells are much smaller than one millimetre. You could fit ten human egg cells into one millimetre. Scientists use a special set of units to measure very small structures. These units and their relationship to one metre are shown in Table 1.

Units

Millimetre (mm)

Micrometre (μ)

Number of units in one metre

1 000

1 000 000

Units expressed in scientific

1 x 10

Nanometre (nm) 1 000 000 000

1 x 10

-3

-6

1 x 10 -9

notation as fraction of (m)

Scientific notation is a special way of representing very large or very small quantities in science. You can convert numbers from normal form to scientific notation easily: 1 000 = 10 x 10 x 10 = 1 x 103

Q A

There are three zeroes in 1 000. To what power is ten raised in the scientific notation for 1 000?

Table 1: Units for measuring small structures

Did you know? In mathematics, division is the inverse, or opposite of multiplication, so when we are expressing numbers as a fraction of another, the index, or power is negative. A micrometre is onethousandth of a millimetre, so we say it is 1 x 10–3 mm, or 1 x 10–6 m.

Three.

It is difficult for us to picture such tiny amounts, but try to imagine that one millimetre division on your ruler divided into one thousand segments. Each segment would be one micrometre.


9

Using scientific notation, we can say that one micrometre is 1 x 10 -3 mm. Micrometres are also called microns. If one micrometre was divided into one thousand equal segments, each segment would be one nanometre. Using scientific notation, we say that one nanometre is 1 x 10 -6 mm.

Using scale bars Biological drawings often have a scale bar on the diagram which gives you an idea of the actual size of the specimen.

A

0.4 mm

Figure 4. Transverse section of a leaf.

Scale bars are used to work out the magnification of the drawing and the actual size of the specimen. The magnification on the bar tells us how much bigger or smaller than the real specimen the drawing or micrograph is. For example, the scale bar on Figure 4 shows how long 400 µm would be at the magnification of the drawing. How much bigger or smaller is the drawing than the real specimen? 1. Measure the length of the scale bar. It is 20 mm. 2. 20 mm represents 400 µm of the actual specimen. Convert the 20 mm to µm by multiplying by 1 000. 3. 20 x 1 000 = 20 000 µm. 4. Now divide the actual measurement of the scale bar by the measurement that it represents. 5. 20 000 ÷ 400 = 50. The diagram shows the actual object magnified 50 times. We write the magnification like this: 50x or x50 You can also use the scale bar to measure the actual size of various parts of the diagram. For example, let’s say you want to know how tall the cell marked A is. 1. Measure the height of cell A with your ruler. It is 10 mm high.

10


HSRC Workbook

2. According to the scale of the diagram, its magnification is 50x. Cell A is drawn 50 times bigger than its real size. To calculate its real size, divide 10 mm by 50. 3. The answer is 0,2 mm or 200 µm. Cell A is part of a layer of similar cells, but the cells are not all the same size. Measuring only one cell is not a true reflection of all the cells in that layer. In order to make a general statement about the height of cells in layer A, you should measure a number of cells and calculate the average. For example, measure five cells. 10 mm, 9 mm, 9,5 mm, 11 mm, 10 mm. To calculate the average height, total all the measurements and divide by the number of measurements. 10 + 9 + 9,5 + 11 + 10 = 49,5 = 9,9 mm 5 Convert the average height in the diagram to the actual height using the magnification (x50), in other words, divide by 50. 9,9 = 0,198 mm, or 198 µm. 50

Activity 4 1. Figure 5 shows some cells from the inside of a human’s mouth. a. Use the scale bar to work out the actual diameter of five cells. b. Work out the average diameter of the cells.

Figure 5. Human cheek cells with scale bar.


11

2. Figure 6 shows some cells from the surface of a leaf. a. Use the scale bar to work out the actual length and width of five cells. b. Work out the average length and width of a cell from the surface of a leaf.

Figure 6. Epidermal cells with scale bar.

3. Figure 7 shows some bacterial cells. Use the scale bar to work out the average length and width of the round bacterial cells.

Figure 7. S canning Electron Micrograph of some bacteria from the gut of a nyala.

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HSRC Workbook

4. You have now measured three different kinds of cells: plant cells, animal cells and bacterial cells. Compare the sizes of the three kinds of cells you have measured. What do you notice?

5. Draw lines to match each cell structure with its function: Cell membrane

carries out photosynthesis.

Nucleus

carries the hereditary information of the cell.

Vacuole

carries out cellular respiration.

Mitochondria

manufactures proteins.

Chloroplast

stores water, small molecules and waste products.

Endoplasmic reticulum

allows substances to enter and leave the cell


13

3 The cell cycle Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Categorising information to reduce complexity and look for patterns Interpreting information Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.

It was clear in Unit 1 that the smallest unit of life is a cell. Life cannot be created from non-living materials, even under the most carefully controlled conditions. No-one has managed to create a living cell by mixing together various chemicals and supplying energy. Since cells cannot be created, where do new cells come from? Your body, consisting of millions and millions of cells, grew from a single fertilised egg. A tree grows from a single fertilised egg cell in the flower. Where do all the extra cells come from? Cell theory says two things about cells: l All living organisms are made up of cells. l All cells arise from other cells. How does one fertilised egg cell grow into millions and millions of cells in a human body?

Cells divide The first fertilised egg cell of any organism grows into many cells by dividing. We say cells pass through a cell cycle. A cell grows to its full size, and then divides into two daughter cells. Each daughter cell grows to its full size, and then divides into two daughter cells. The cell cycle is the time from the formation of a daughter cell to its own division into two daughter cells. The cell cycle is illustrated in Figure 8. ild egd\Zcn XZaah

14

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Figure 8. A sequence of cell divisions. The letter A marks the start of a cell cycle, and B marks the end of a cell cycle.

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HSRC Workbook

Notice that when a cell divides into two daughter cells, the original cell effectively disappears. All cells in a developing embryo go through repeated cell cycles, but once cells have become specialised for a particular function, they no longer divide. Cell division only takes place in certain specialised parts of the adult body. For example, cells under the surface of your skin divide continuously to replace cells that are worn off on the outside of your skin. The duration of a cell cycle varies from a few hours to several weeks. The cells at the tip of many roots have a cell cycle of about 12 hours, while cells in human skin have a cell cycle of about 12 hours. Bacterial cells have a cell cycle of 20 minutes under very good conditions.

Events in the cell cycle Growth I A newly-formed daughter cell is about half the size of an adult cell, so the first phase of the cell cycle is taken up with growing. The cell makes new organelles, more cytosol is produced, and the cell wall increases in size. The growth phase is the longest part of the cell cycle, and the most variable. Some cells never progress beyond the growth phase, others become specialised for particular functions and stay in the growth phase until they die.

Replication (say rep-li-cay-shun) Each chromosome in the nucleus makes an identical copy of itself in the replication phase. At the end of the replication phase, the cell contains a double set of chromosomes.

Growth II The cell grows a little bigger. The second growth phase is shorter than the first growth phase.

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The first part of cell division is called mitosis (say my-toe-sis). We will describe mitosis in more detail in section 5. During mitosis, the two sets of chromosomes produced in the replication phase separate and move to opposite sides of the cell. In the second part of cell division, a new cell membrane forms across the middle of the parent cell, dividing it into two daughter cells. The organelles of the parent cell are shared between the two daughter cells.

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Figure 9. The cell cycle. The dark shaded area is called interphase.

15

4 T he chromosomes and cell development This section contains some advanced knowledge and exercises that may not be suitable for all Senior Phase classes. Use your discretion, or adapt the activities to suit your class. Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Categorising information to reduce complexity and look for patterns Interpreting information Knowledge area: Life and Living Substrand: Biodiversity, change and continuity. Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.

Activity 5 1. Where do you find chromosomes in a cell?

2. What are chromosomes made of?

3. What is the important function that chromosomes carry out in a cell?

4. Think about all the metabolic functions that take place in a cell. What controls the metabolic functions?

5. Think about the fact that every human being develops from a single fertilised egg cell. What controls the way an embryo develops?

6. What makes sure that arms grow in the right places, legs grow in the right places, and all the organs form in the right places?

7. What makes sure that the embryo grows into another human being, and not into another kind of organism?

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HSRC Workbook

The answer to all these questions is that the chromosomes carry all the information that a cell needs to grow and to carry out its functions. The chromosomes direct each cell in the path of its development. As each cell develops in response to the instructions from the chromosomes, it forms part of a structure in the embryo. You should remember from section 1 that each chromosome consists of a long molecule of DNA, combined with protein. Chromosomes carry the hereditary information, that is, the information that passes from one cell to the next and from an adult organism to its offspring. Hereditary information is vital for a cell to function, and for an organism to develop and function correctly.

Figure 10. A human baby grows from a single fertilised egg cell.

Some cells become bone cells, some cells become muscle cells, some become nerve cells, some become skin cells, and some cells become blood cells. Almost every single cell in an organism has a full set of chromosomes for the whole organism. We say ‘almost’, because a few specialised cells, like red blood cells in mammals, and phloem cells in plants, do not have chromosomes. Chromosomes pass from parent cell to daughter cell. Normally, we cannot see the chromosomes in a cell, but when a cell is about to divide, the chromosomes become visible under a microscope.


17

Look at Figure 11, which shows the cells in the root tip of an onion. You can see that some cells have a large, dark-stained nucleus, while others have small threads in the cell. The threads that you can see in some cells are the chromosomes. D

Figure 11. We can only see chromosomes when cells are dividing. A

B

C

E

Scientists use special techniques to spread out the chromosomes of a cell. They have found that all the cells of a particular kind of organism have the same number of chromosomes. For example, each cell in a human body has 46 chromosomes; each cell in the body of a fruit-fly has 4 chromosomes, and each cell in a maize plant has twenty chromosomes. Before a cell divides, each chromosome makes an exact copy of itself during the replication phase of the cell cycle. A cell in a human body after the replication phase has twice as many chromosomes as a normal cell: it has 92 chromosomes. A fruit-fly cell has eight chromosomes after replication, and a maize cell has 40 chromosomes after replication. The chromosomes in a cell that is about to divide are arranged in pairs, which we call chromatids (say kroa-ma-tids). At this stage in a cell cycle, each chromosome consists of two identical chromatids, joined together by a structure called the centromere (say sen-tro-meer)

Activity 6 Match each term with its meaning:

18

Chromosome

The place where two chromatids are attached.

Spindle

The process of producing new individuals without fertilisation.

Chromatid

The threads that stretch across a cell that is about to divide.

Centromere

A structure in the nucleus that carries hereditary information.

Cloning

One half of a chromosome after replication.


HSRC Workbook

5 Mitosis

This section contains some advanced knowledge and exercises that may not be suitable for all Senior Phase classes. Use your discretion, or adapt the activities to suit your class. Learning Outcome 2: Constructing science knowledge Assessment Standards Recalling meaningful information when needed Applying knowledge to problems that are not taught explicitly Categorising information to reduce complexity and look for patterns Interpreting information Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: The cell is the basic unit of most living things, and an organism may be formed from one or many cells. Cells themselves carry on life processes such as nutrition, respiration, excretion and reproduction, which sustain the life of the organism as a whole.

Steps in the process Prophase At the beginning of mitosis, the chromosomes coil tightly, like a spring, so that they become thicker and fatter. At this stage, each chromosome consists of two chromatids, joined at the centromere. The membrane that normally surrounds the nucleus now disappears, so the chromosomes float in the cytosol.

Metaphase Fine threads form in the cytosol, making a pattern called a spindle in the cell. The chromosomes attach themselves to the spindle at the centromere. The spindle threads become tight and begin to pull the chromosomes, so that they all line up in the middle of the spindle.

Anaphase The two chromatids of each chromosome suddenly pull apart and move towards opposite sides of the cell.

Telophase The threads of the spindle disappear. The chromosomes begin to uncoil and cluster together to form two new nuclei. A new nuclear membrane forms around each nucleus. Mitosis is now complete, but before the cell cycle has ended, the cytoplasm must divide into two.


19

Chromosomes condensing. Each chromosome has already divided into two chromatids

Cell membrane Spindle

Chromosomes arranged in an orderly manner about the plane of the spindle

Chromosomes moving to the equator of the spindle in a disorderly manner

Spindle

Aster

Centriole

Figure 12. Mitosis in animal cells – micrographs.

20

Figure 12. Mitosis in animal cells – drawings.


Cell membrane

Spindle fibres. Each daughter centromere has a spindle fibre attached to it. Contraction of the fibre pulls the daughter centromeres to opposite poles

Cleavage furrow grows inwards

Chromosomes at poles. Nuclear membranes enclose the chromosomes at each pole

Figure 12. Mitosis in animal cells – micrographs.

Figure 12. Mitosis in animal cells – drawings.

Cell division A new cell membrane starts to form between the two nuclei. Once the new membrane has divided the cytoplasm into two new cells, cell division is complete. The two new cells begin another cell cycle.

Q

Each new cell has the same number of chromosomes as the original cell. Why is this important?

A

Each cell in an organism must have a complete set of chromosomes to ensure that all the functions of the cell can be carried out. If the cell did not replicate its chromosomes before it divides, some daughter cells may be missing essential chromosomes and die.


21

HSRC Workbook

One of each pair of chromatids moves to each pole. Once they are separated, the chromatids are called chromosomes

Activity 7 1. Figure 11 is a micrograph of cells in the tip of a root. You can see different phases of mitosis in some cells. Certain cells have been labelled A, B, C, D, and E. a. What phase of mitosis is each labelled cell in? A. B. C. D. E. b. Draw each cell and label the chromosomes, spindle, nucleus and cell wall wherever you can identify those structures.

2. Suggest a simple, practical way that you could demonstrate mitosis to a Grade 9 class. You could use pieces of wool or string to represent the chromosomes, and a sheet of paper to represent the cell. Demonstrate your model of mitosis to your colleagues. 3. Draw a diagram representing the first five cell cycles after an egg has been fertilised. Assume that every cell divides in each cell cycle. How many cells are present in the embryo after five cell cycles?

4. A fertilised egg cell in a maize plant has twenty chromosomes. a. How many chromosomes are present in each cell of the adult maize plant? b. How many chromatids are present in a cell that is about to divide? c. How many chromosomes are present in each cell after cell division? 5. Before mitosis begins, each chromosome makes an exact copy of itself. Why is replication such an important event in the cell cycle?

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HSRC Workbook

6 Inheritance

Learning outcome 2: Constructing science knowledge Assessment Standards Applying knowledge to problems that are not taught explicitly Interpreting information Recalling meaningful information when needed Learning outcome 1: Scientific investigation Assessment Standard Conducting investigations and collecting data Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topics: Offspring of organisms differ in small ways from their parents and generally from each other. This is called variation in a species. Sexual reproduction is the process by which two individual plants or animals produce another generation of individuals. The next generation’s individuals look like the parents but always have slight differences (‘variation’) from their parents and from each other.

To inherit something means to receive it from a previous generation. You can inherit money from a late parent if he or she left it to you in their will. You can also inherit physical characteristics from your parents. For example, children can inherit certain diseases from their parents. Children also inherit physical characteristics from their parents. Usually, the children of tall parents will also be tall, and the children of short parents will be short. The children inherit the physical characteristic of their height from their parents. This section explores the biological mechanisms of inheritance.

Parents and offspring All living organisms inherit characteristics from their parents. It sounds strange to talk about the ‘parents’ of a plant or a fungus. In a biological sense, the male parent of any organism is the individual that supplied the sperm. The female parent is the individual that supplied the egg. So, in a maize plant … l The male parent is the plant that supplied the pollen which contains the sperm cells. l The female parent is the plant that supplied the cob, containing egg cells. Most organisms produced by sexual reproduction have two parents: a male parent and a female parent. We refer to the products of fertilisation as the ‘offspring’ of two individuals. Your own children are your offspring, and you are the offspring of your parents.

Similar, identical and variable In this section we will use words like resemble, looks like, similar, the same as, identical, varies, different, exactly and the same as. It is important that you know what these words mean.


23

Resemble, looks like and similar are words that mean that the objects share certain characteristics. For example, look at the herd of cows in Figure 14. The cows share certain characteristics: they all have four legs, a body, a head, a neck and a tail. They all have horns and their skin is covered with fur. We say the cows resemble each other, they look alike, or they are similar to each other.

Figure 14. Cows and goats.

Identical and exactly the same as are words that mean that two or more objects share all their characteristics. Look at the herd of cows in Figure 14, notice that most of the cows are similar to each other, but they are not identical. Find two cows that have exactly the same pattern of markings and the same shape of their horns. We say these two cows are identical. They are exactly the same as each other. Varies and different are words that mean that objects differ in certain characteristics. For example, in Figure 14 you can see that cows are different from goats. Cows are bigger than goats, for one thing. Cows and goats make different sounds. When you look at all the cows, you can see that most of the cows differ from each other in their coat patterns. We say that their coat pattern varies.

What is a species? One of the fundamental characteristics of life is that particular kinds of organisms always produce offspring that resemble themselves. A group of individuals that breed and produce offspring that resemble themselves is called a species (say speeseez). A species is a group of organisms that resemble each other, and that breeds and produce offspring that resemble themselves. Here are some examples of offspring that grow up to resemble their parents: l Baby monkeys grow to look like monkeys. l Maize seeds grow into maize plants.

24


l l

HSRC Workbook

l

Hen eggs hatch into chickens that grow into domestic fowls. Duck eggs hatch into ducklings that grow into adult ducks. Thorn tree seeds grow into thorn trees.

In all the examples of species above, the young grow from a single fertilised egg. One sperm cell from the father fertilises one egg cell from the mother, and the offspring grows from that one fertilised cell. The offspring inherit something from the parents that makes sure they grow into the same species as the parents. Whatever it is that passes to the offspring is carried in the sperm cell and the egg cell, so it must be very small but very powerful. It must carry all the instructions to make sure that the fertilised egg cell grows into a whole new organism that resembles the parents. The structure that offspring inherit from the parents is a set of chromosomes. Each species has a set of chromosomes that ensure that all individuals of that species will look alike. Each fertilised egg must have a complete set of chromosomes for the species.

Figure 15. One sperm fertilises an egg cell.

Chromosomes carry genes In section 5, you learnt that chromosomes carry all the instructions that a cell needs to grow and carry out its functions. Chromosomes consist of long molecules of DNA, wound around protein molecules.

Q

A

What kind of instructions does a chromosome carry?

The chromosomes carry codes for the sequences of amino acids in particular proteins.

Q

How do proteins control the way a cell grows, develops and functions? How can proteins make different colours in skin, and different shapes to leaves?

A

Cells make different kinds of proteins. Most of the proteins that control the way a cell grows, develops and functions are enzymes. Enzymes speed up chemical reactions in cells. The chemical reactions build new substances, break down old ones, and enable cells to do different things.

A gene is a section of a chromosome that carries the code for a particular protein, and therefore controls a particular characteristic or process. Some proteins are not enzymes, but they form structures in cells. For example, skin cells produce a protein called collagen that makes skin strong. Hair cells produce a protein called keratin that supports the hair. Each chromosome carries codes for several thousand protein molecules. The codes for a single protein molecule are arranged in a particular area of the chromosome. We call the section of a chromosome that carries codes for a particular protein a gene (say jean).


25

Big sections of chromosomes do not code for any proteins. Most of the genes in a particular cell are never decoded. Imagine that the genes are switched off. But almost every cell in your body has all the genes needed to make your whole body grow and function correctly.

Q

How do we know when a gene is ‘switched on’?

A

Genes that are active swell up and form ‘puffs’. Scientists can see parts of the chromosomes where genes are active.

The genetic basis of inheritance The action of genes is to produce proteins that gradually build a structure, a substance or break down structures in a cell. Each gene controls one protein, but it takes many protein molecules to make a structure. We must assume that most physical features of a living organism are the result of the work of many proteins, and therefore many genes. Each cell in the human body has about 30 000 genes – that is the total number of genes that are needed to make a whole human being from a single fertilised egg cell. Identical genes produce identical proteins, which make identical structures. So, if two individuals are identical in some genetically-controlled characteristic, we assume that their genes are identical. The opposite of this is that if two individuals differ in the appearance of a genetically-controlled characteristic, we assume that their genes differ. All members of a species share a number of important characteristics, so we must assume that thousands of genes for each species are identical. However, individuals within a species vary in a number of characteristics. For example, in Figure 14, you noticed that cows vary in the patterns of their coats.

Activity 8 1. Write down three ways that cows vary in their physical characteristics.

2. Write down three ways that humans vary in their physical characteristics.

3. Think of your own family. Do all the children of one mother and father look exactly the same?

26


HSRC Workbook

Even very closely related members of the same species vary in detail. You have probably noticed that children of one mother and father often resemble each other. Sometimes we can tell who a child’s parents are by looking at the child!

You have to be a Robertson! You have the Robertson eyes and nose.

I can see that you are Themba Mtshali’s brother – you look just like him.

You are surely from the Khan family. They are all short and thin like you.

You must be the daughter of Lindiwe Mbanjwa.

Figure 16. What physical characteristic have these children inherited from their father?

Activity 9 1. Nonhlanhla and Ntombifuthi are identical twins, while Zanele and Zodwa are sisters from the same mother and father. What can you say about the genes of Nonhlanhla and Ntombifuthi?

2. What can you say about the genes of Zanele and Zodwa?


27

The relationship between chromosomes and the whole organism 8=GDBDHDB:H

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Figure 20. Natural selection leads to changes in the coat colours of mice.


33

Natural selection and evolution If natural selection continues over a long period of time, a new species may form. The new species will not be able to breed with the old species from which it originated. Its genes have changed so much that any offspring will be sterile, or fertilisation will not take place. Natural selection is the mechanism whereby evolution takes place. The huge diversity of living organisms that we see in the world today has evolved from species that are now extinct. Evolution is the most important concept in Biology, because it helps us to understand the processes that result in the diversity of life around us, similarities and differences in plants and animals, and the way organisms are distributed on the continents around the world.

Artificial selection Imagine that you were alive 10 000 years ago. You would most likely have lived in a small band of people who hunted for their meat and collected plant bulbs, roots, and leaves to eat. Let’s assume that while hunting you found some goats that were easy to catch and kill for meat. The other goats were very wild and difficult to capture. You realise that life would be much easier if you could capture a few of the tame goats and keep them near your settlement. When you do that, you discover that some of the offspring of the tame goats are also tame. You kill the wilder offspring to eat them and keep the tame offspring to breed. After a number of generations of selecting the tame goats to breed in each generation, you have a flock of tame goats that stay near the humans. The early farmers did not know about genes, but they did know that certain characteristics were passed from one generation to the next. By selective breeding of plants and animals, humans produced new breeds of plants and animals to provide for their needs in terms of food, work, sport, shelter and hobbies. Artificial selection works in a similar way to natural selection, but humans make the choices about which animals or plants will breed in each generation. In natural selection, the natural environment determines which individuals are best adapted, and therefore most likely to breed. Through artificial selection, the world’s population has been able to expand to its present huge numbers. Artificial selection, resulting in increased productivity, has been one of the key reasons for the success of the human species.

Artificial selection and genetics Artificial selection began with mass selection. Breeders selected the animals or plants that showed the best characteristics for a particular purpose. The breeder collected seed only from the best plants, and allowed only the best animals to breed. Mass selection is slow and unpredictable, although it does eventually produce the desired results.

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HSRC Workbook

After the discovery of genes in 1900, breeders were able to work out more accurately what the results of a particular breeding were likely to be. They could use mathematical models to predict how many offspring of a particular cross were likely to have the desired characteristic. Modern breeding methods produce much quicker and more reliable improvements in agriculturally useful organisms.

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l

Each

cell passes through a cell cycle, in which it grows, replicates its chromosomes and divides into two daughter cells. Chromosomes carry the hereditary material for the development and functioning of the whole organism. Each cell inherits a full set of chromosomes from the parent cell. Mitosis is a precise sequence of events that results in a cell dividing into two daughter cells. Each daughter cell receives a complete set of chromosomes, and daughter cells are genetically identical to each other and identical to the parent cell. The hereditary units on chromosomes are called genes. Each gene controls the production of a single protein, which contributes to the development of a particular characteristic in the organism. Organisms that are physically and physiologically identical are genetically identical. Organisms that are physically and physiologically different are genetically different. Each species has a common set of genes that ensure that all members of the species look alike. Each individual within a species also has some genes that are variable, and that result in all individuals being slightly different from each other. Natural selection ensures that the individuals in each generation that are best adapted to the environment are most likely to breed, and therefore pass on their genes to the next generation. Natural selection results in evolution. Artificial selection refers to human selection of individual plants or animals for breeding. Artificial selection is the basis of animal and plant breeding techniques.


35

8 G eneral principles of reproduction

Learning outcome 2: Constructing science knowledge Assessment Standards Applying knowledge to problems that are not taught explicitly Interpreting information Recalling meaningful information when needed Learning outcome 1: Scientific investigation Assessment Standards Planning investigations Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity Content topic: Sexual reproduction is the process by which two individual plants or animals produce another generation of individuals. The next generation’s individuals look like the parents but always have slight differences (‘variation’) from their parents and from each other. Substrand: Life processes and healthy living Content topic: Human reproduction begins with the fusion of sex cells from mother and father, carrying the patterns for some characteristics of each.

Reproduction ensures the survival of a species Every individual organism of a species has a life cycle, which lasts from the time it is born until it dies. A life cycle varies from twenty minutes for some bacteria to thousands of years for some trees. All living things, even if they are single-celled bacteria, will die. Remember that all life comes from existing life. The only way that a new individual can be produced is if another individual of the same species reproduces. Reproduction ensures that each species of organism present on Earth continues to survive. If all the individuals of a species stop reproducing, the species becomes extinct. Reproduction ensures that hereditary information passes from one generation to the next. Remember that each cell of every organism contains nuclear material. The nuclear material carries the hereditary information - the coded instructions that control the way every organism functions and develops. During reproduction, the hereditary information passes from one generation (the parents) to a new generation (the offspring). The hereditary information ensures that the offspring will resemble the parents and each other. The offspring will belong to the same species as their parents.

36


HSRC Workbook

Organisms reproduce sexually and asexually Methods of reproduction can be divided into two categories: l In sexual reproduction, two specialised sex cells (gametes) join and grow into a new organism. An individual produced by sexual reproduction has two parents, a male parent and a female parent. l In asexual reproduction, one individual (the parent) produces offspring without the fusion of specialised sex cells. Cloning is a form of asexual reproduction. An individual produced by asexual reproduction has only one parent.

Activity 11 1. Explain why all individuals of a species that reproduces asexually are genetically identical to each other.

2. Explain why all individuals of a species that reproduces sexually are genetically different from each other.

3. Complete the table to summarise the differences between asexual and sexual reproduction. Asexual reproduction

Sexual reproduction Two parents

No special sex cells required All offspring are genetically identical to the parent and to each other Occurs in plants and animals


37

9 Sexual reproduction

Learning outcome 2: Constructing science knowledge Assessment Standards Applying knowledge to problems that are not taught explicitly Interpreting information Recalling meaningful information when needed Learning outcome 1: Scientific investigation Assessment Standards Planning investigations Conducting investigations and collecting data Evaluating data and communicating findings Knowledge area: Life and Living Substrand: Biodiversity, change and continuity. Content topic: Sexual reproduction is the process by which two individual plants or animals produce another generation of individuals. The next generation’s individuals look like the parents but always have slight differences (‘variation’) from their parents and from each other. Substrand: Life processes and healthy living Content topic: Human reproduction begins with the fusion of sex cells from mother and father, carrying the patterns for some characteristics of each.

Organisms that reproduce sexually produce special cells for reproduction, called the sex cells or gametes. Gametes are usually produced in special parts of the plant or animal.

Males and females In asexual reproduction, there is only one parent, so we do not distinguish between males and females. However, sexual reproduction involves two parents that produce two different kinds of gametes. In sexual reproduction, male individuals or organs produce sperm that can swim or that move from their parent. Female individuals or organs produce much larger gametes that do not move. The large gametes are called ova (singular ovum). Ova contain stored food. Some individual plants or animals produce both sperm and ova, while in others the sexes are separate.

Q A

38

We can easily tell which is the male and which is the female in mammals, but how do we tell which is the male and the female in other organisms?

The individual or organ that produces sperm is always called the male, and the individual or organ that produces the ova is always called the female.


HSRC Workbook

We use the word ‘egg’ in ordinary life when we refer to structures that have a shell and that contain a developing embryo. A hen egg is an example. In Biology, an egg is the unfertilised female gamete, as well as the structure that contains the developing embryo. The word ovum (plural ova) refers to the unfertilised egg. We will use the term ovum or ova wherever we refer to an unfertilised egg in this unit.

Gametes Male gametes have a head, which contains the nucleus, and, in many species, a long tail. The tail propels the sperm towards an ovum of the same species. Sperm are produced in the testes of animals, or in pollen grains of flowering plants. Look at Figure 21 to see the relative sizes of sperm from some organisms.

Human sperm 1500x

Chicken sperm 500x

Frog sperm 500x

Rat sperm 500x

Figure 21. Sperm from several organisms.

Actual Sizes

Female gametes (ova) have a nucleus, cytoplasm, and variable amounts of stored food. Ova are produced in organs called ovaries in animals and in the ovules of flowering plants. Look at Figure 22 to see the relative sizes of ova of different species.

Human ovum 20x

Frog ovum Human ovum Rat ovum

Coat of jelly Protective membrane Egg membrane Yolky cytoplasm Nucleus

Hen ovum

Figure 22. Ova from several organisms.

Activity 12 1. Work out the actual sizes of the sperm cells in Figure 21. 2. Compare the sizes of ova and sperm of the same species as shown in Figures 21 and 22.


39

Meiosis Meiosis (say mayo-sis) is a kind of cell division that results in half the normal number of chromosomes as in a normal cell.

Meiosis is a special kind of cell division that happens before the production of gametes. Remember that mitosis results in two cells with the same number of chromosomes as the parent cell. Meiosis results in four cells with exactly half the number of chromosomes of the parent cells. Every cell in a human body carries 46 chromosomes, but human sperm and ova have 23 chromosomes each. The 46 chromosomes in human body cells consist of 23 pairs, which are called homologous pairs. The two chromosomes of a homologous pair are the same length and shape, and control the same characteristics as each other. Every genetically-controlled characteristic in your body has two genes: one inherited from your mother, and one from your father. The two genes are on the two chromosomes of one homologous pair. Each cell in an organism produced by sexual reproduction carries a double set of chromosomes, one set inherited from the male parent, and the other set from the female parent. Cells that carry a double set of chromosomes are called diploid (2n). During meiosis, the number of chromosomes in the cells is reduced from the diploid number to half of that number. The cells now carry only one chromosome from each homologous pair. We say these cells are haploid (n).

Activity 13 1. Figure 23 shows the process of meiosis. How many chromosomes are present in the parent cell at the start of meiosis? ________ 2. How many chromatids are present at the start of meiosis? ________ 3. How many chromosomes are present in each cell at the end of meiosis? ________ 4. What happens during crossing-over?

5. How many cells are produced at the end of meiosis? ________ 6. What do you notice about the combination of chromosomes in the gametes compared to the chromosomes in the parent cell?

7. Does any gamete carry chromosomes from only one parent? ________

40


HSRC Workbook Diploid (say diployed) means having a double set of chromosomes.

Figure 23. The stages in meiosis.

Haploid (say haployed) means having only one chromosome of each homologous pair.


41

Fertilisation

Zygote (say zygoat) is a fertilised ovum.

42

Sexual reproduction involves the act of fertilisation, when the nuclei of the sex cells combine. One sperm enters an ovum of the same species, and the two nuclei fuse. The fertilised ovum is called a zygote. Meiosis and fertilisation are significant events in the production of genetic variation in the offspring of two parents. l During meiosis, the homologous chromosomes separate randomly into the gametes. Each gamete receives a mixture of chromosomes inherited from both parents. l Crossing-over means that many chromosomes contain a mixture of genes from both parents. l Fertilisation brings together chromosomes from two different individuals. Through the processes of meiosis and fertilisation, every individual produced by sexual reproduction has its own combination of genes inherited from both parents. Fertilisation is such an important event that many organisms have special mechanisms to ensure that sperm meet ova. Organisms that live in the sea, such as fish, seaweeds, sponges and coral, release their ova and sperm into the water at the same time. The ova release a substance that is attractive to sperm of the same species. Sperm swim to the ova and fertilise the ova in the water. Organisms that live on land have special mechanisms to ensure that the sperm reach the ova.


HSRC Workbook

Activity 14 1. Match each term with its correct description. __ g amete

a) A kind of cell division that halves the number of chromosomes __ sperm b) A cell that has two sets of chromosomes. __ ova c) A general name for sex cells. __ diploid d) One half of a chromosome in a cell that is about to divide. __ haploid e) A male sex cell. __ meiosis f) A female sex cell. __ chromosome g) Two chromosomes that are the same shape and size. __ chromatid h) A structure that carries hereditary material in the cell. __ homologous pair i) A cell that has only one chromosome from each homologous pair. 2. Complete the table that compares sperm and ova. Sperm

Ova

Small Do not move much Have a food store Have a tail for moving

3. Each muscle cell in a species of lobster has 250 chromosomes. a. H ow many homologous pairs of chromosomes are present in each cell of the lobster? __ b. H ow many chromatids are present in a cell that is about to divide by mitosis? __ c. How many chromosomes are present in each sperm cell? __ d. How many chromosomes are present in each ovum? __ 4. What would happen if sex cells were produced by mitosis instead of meiosis?


43

Summary l

l

l

Reproduction

is necessary for the survival of a species, but not for the survival of an individual. Asexual reproduction is the production of new organisms without the fusion of sex cells. All offspring produced by asexual reproduction are genetically identical. Sexual reproduction involves the production of haploid gametes, which fuse to form a diploid zygote. All offspring produced by sexual reproduction vary in their genetic makeup.

Solutions to activities Activity 1 Photosynthesis; carbon dioxide; diffusion. Oxygen; diffusion. Sugar; sugar; cellular respiration; diffuse; leave; enter.

Activity 3 You would not expect to find chloroplasts in the cells of a root because roots do not photosynthesize.

Activity 4 1. The cells are not circular, so choose diameters where you can see the outer limit of the cytoplasm most clearly. Below is an example using five cells, you may choose different cells, but the method of calculation will be the same. Following the method on page 11 measure the scale bar as 34,5 mm long and convert 34,5 mm to μm: 34,5 x 1000 = 34 500 μm To get the magnification of the micrograph, divide the actual measurement of the scale bar (in μm) by the measurement that it represents: 100 μm.

44

34 500 ÷ 100 = 345 The magnification of the micrograph is x 345 Measure 5 cells and obtain the measurements in mm: 30; 22; 14; 14; 15


HSRC Workbook

Convert each measurement to μm: 30 000; 22 000; 14 000; 14 000; 15 000

Divide each measurement in μm by the magnification factor (345) to get the actual size of each cell: 86,96 μm; 63,77 μm; 40,58 μm; 40,58 μm; 43,48 μm

Calculate the average size of the five cells as follows: (86,96 + 63,77 + 40,58 + 40,58 + 43,48) ÷ 5 = 55,07 μm

2. Using the same method as for question 1, you will find that the micrograph is shown at a magnification of x 1425. Length mm

Length μm

Actual length μm

Width mm

Width μm

Actual width μm

66

66 000

46,32

20

20 000

14,04

36

36 000

25,26

29

29 000

20,35

61

61 000

42,81

55

55 000

38,60

29

29 000

20,34

18

18 000

12,63

32

32 000

22,46

33

33 000

23,16

Mean

31,44

21,76

3. Using the same method as for questions 1 and 2 you will find a magnification of x 7000. Length mm

Length μm

Actual length μm

Width mm

Width μm

Actual width μm

5

5 000

0,71

5,5

5 500

0,79

6

6 000

0,86

4,5

4 500

0,64

5

5 000

0,71

5,5

5 500

0,79

5,5

5 500

0,79

5

5 000

0,71

4,5

4 500

0,64

5

5 000

0,71

Mean

0,74

0,73

4. The animal cells, with an average diameter of 55,07 μm, are larger than the plant cells, which have an average length of 31,44 μm and an average width of 21,76 μm. The bacterial cells are much smaller than plant or animal cells, since they have an average diameter of 0,74 μm.


45

5. Cell membrane

Allows substances to enter and leave the cell

Nucleus

Carries the hereditary information of the cell.

Vacuole

Stores water, small molecules and waste products.

Mitochondria

Carries out cellular respiration

Chloroplast

Carries out photosynthesis

Endoplasmic reticulum

Manufactures proteins

Activity 5 1. Chromosomes are found in the nucleus of a cell. They are made of DNA and proteins. Chromosomes carry the information necessary for the development and functioning of the whole individual, including the cell. 2. Questions 2 – 5 encourage the reader to think about the key problems of cell and embryonic development that are solved by the chromosomes.

Activity 6 Chromosome

A structure in the nucleus that carries hereditary information.

Spindle

The threads that stretch across a cell that is about to divide.

Chromatid

One half of a chromosome after replication.

Centromere

The place where two chromatids are attached.

Cloning

The process of producing new individuals without fertilisation.

Activity 7 1. a. A – Prophase – the chromosomes are beginning to become visible in the nucleus. B – Metaphase – the chromosomes are lined up on the equator of the cell. C – Anaphase – the chromatids have separated and been pulled to opposite sides of the cell. D – Telophase – a new cell wall has formed between the two daughter cells, and the nuclear membrane has re-formed, but we can still see chromosomes in the nuclei.

b. cell wall cell wall

nucleus

spindle

spindle

chromosomes

chromosomes chromosomes

46


nuclei

HSRC Workbook

3.

After five cycles of cell divisions, there will be 32 cells in the embryo. Each block in the diagram above represents a cell. 4. a. 20, the same number as in the fertilised egg. b. 40 c. 20 5. Replication is an important event in the cell cycle because it ensures that each daughter cell receives a complete set of the hereditary information needed to make a new individual.

Activity 8 This activity is designed to make the reader think about variation in familiar organisms. It is an open-ended activity.

Activity 9 1. Nonhlanhla and Ntombifuthi are identical twins, therefore they have identical genes. 2. Zanele and Zodwa have inherited genes from the same mother and father, but their genes are different. We know this because the two children are not identical in all physical characteristics, like Nonhlanhla and Ntombifuthi.


47

Activity 10 1. a. T he figures along the x-axis of the graph show the height categories of the boys. b. The figures on the y-axis show the numbers of boys. c. 157 and 162. d. Eight boys have genes for being more than 165 cm tall at twelve years of age. e. Food has a strong impact on height. Children who do not receive correct nutrition are shorter than those who eat well. f. Height (cm)

155 – 157

Number of 3 boys

157 – 159

159 – 161

161 – 163

163 – 165

165 – 167

167 – 169

169 – 171

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9

11

9

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48


HSRC Workbook

4. This survey should come close to the 3:1 ratio in the general population. 5. Sometimes we see the zebras in side view, sometimes front view, and sometimes rear view. We therefore need to choose the stripe pattern that is easiest to identify: the pattern of stripes around the rump is quite distinctive in each of the four zebras shown here. 6. Cell; chromosomes; genes; characteristics (any other physical characteristic could be mentioned here, e.g. hair colour, height) the father; the mother; fertilisation; an embryo child / offspring; parents; identical twins; physical

Activity 11 1. All individuals of a species that reproduces asexually are genetically identical because there is never genetic mixing as when a sperm cell from one individual fertilises an egg cell from another individual. Since all the cells are produced by mitosis, there is never mixing and random segregation of chromosomes into gametes, as occurs during sexual reproduction. 2. All individuals of a species that reproduces sexually are genetically distinct because of mixing of genetic material that occurs during crossing over in meiosis, random segregation of chromosomes into gametes, and fusion of two genetically distinct gametes during fertilisation. 3. Asexual reproduction

Sexual reproduction

One parent

Two parents

No special sex cells required

Two special sex cells – sperm and egg – are required

All offspring are genetically identical to

All offspring are genetically distinct from

the parent and to each other

each other and from the parents

Occurs in plants and animals

Occurs in plants and animals

Activity 12 1. You will need a piece of string to work out the actual sizes of the sperm cells. The human sperm is 85 mm long. Its real size is 85 ÷ 1500 = 56,7 μm. The chicken sperm is 77 mm long. Its real size is 77 ÷ 500 = 154 μm The frog sperm is 52 mm long. Its real size is 52 ÷ 500 = 104 μm The rat sperm is 92 mm long. Its real size is 92 ÷ 500 = 184 μm 2. Notice that the human sperm is about three times smaller than the rat sperm! The rat sperm is the largest of the four species, but its ovum is the smallest. The chicken sperm is the second largest, but its ovum is the largest. The frog sperm and ovum are the third largest of the four species. The human sperm is the smallest, and its ovum the second smallest of the four species. In order of size from smallest to largest: Sperm – human; frog; chicken; rat Ova – rat; human; frog; chicken


49

Activity 13 1. 6 2. 12 3. 3 4. During crossing over, segments of chromosomes from homologous pairs exchange positions. The result is that each chromosome ends up containing a mixture of genes inherited from its male parent and its female parent. 5. 4 6. Each gamete contains one member of each chromosome, but each chromosome contains a mixture of genetic material from the mother and father of the individual producing the gametes. 7. No, each gamete carries a mixture of chromosomes from both parents.

Activity 14 1.

c) Gamete e) Sperm f) Ova b) Diploid i) Haploid a) Meiosis h) Chromosome d) Chromatid g) Homologous pair

2. Sperm

Ova

Small

Larger than sperm

Very active; swim by undulating the tail

Do not move much

No food stored

Have a food store

Have a tail for moving

No tail

3. a. 125 b. 500 c. 125 d. 125 4. Each time fertilisation occurred, the number of chromosomes in the zygote would double. Meiosis is necessary so that the number of chromosomes remains constant after each fertilisation.

50


Contents The Life Sciences curriculum

52

Overview 1 1. Charles Darwin and the voyage of The Beagle

55

59

Reactions to the publication of the theory

59

2. Darwin’s theory of evolution

60

3. What evidence supports Darwin’s theory?

62

The age of the Earth

62

Changing organisms

63

Intermediate forms

68

70

Descent with modification

72

Comparative anatomy supports evolution

73 75

Searching for genes

77

Discovering the nature of genetic material

77

Nucleic acids

78

DNA replication

80

Why is DNA so important?

81

RNA

81

How does DNA affect the development and functioning of an organism?

82

What is the genetic code?

83

Protein synthesis: Translating the genetic code into actions

84

How does genetic variation arise?

88

Using genetic information to work out evolutionary histories

91

The Human Genome Project

92

Resources

94

Solutions to activities

95

2

70

Geographic distribution of species

5. Genetics and evolution

Part

57

Developing and publishing the theory

4. Present day evidence of evolution

HSRC Workbook

Introducing evolution

51

The Life Sciences curriculum From 2006, the subject ‘Life Sciences’ will replace the subject known as ‘Biology’. The new curriculum will mean that previous teachers of Biology will have to adapt their teaching methodology to fit the Revised National Curriculum Statement (RNCS) and to work towards outcomes that learners must achieve by the end of the FET phase of schooling. There are three learning outcomes for Life Sciences: Learning Outcome 1: Scientific Inquiry and Problem-solving Skills The learner is able to confidently explore and investigate phenomena relevant to Life Sciences using inquiry, problem-solving, critical thinking and other skills. Learning Outcome 2: Construction and Application of Life Sciences Knowledge The learner is able to access, interpret, construct and use Life Sciences concepts to explain phenomena relevant to Life Sciences. Learning Outcome 3: Life Sciences, Technology, Environment and Society The learner is able to demonstrate an understanding of the nature of science, the influence of ethics and biases in the Life Sciences, and the interrelationship of science, technology, indigenous knowledge, the environment and society. As a Life Sciences educator, you should construct your learning programme so that learners make progress in all three learning outcomes throughout the FET band. The learning outcomes are further described by a set of assessment standards that specify the levels of achievement within each learning outcome in each Grade. The assessment standards are shown in Table 1.

52

Part 2: Introducing evolution

Table 1: Learning Outcomes and Assessment Standards for Grades 10 – 12

Assessment Standard

Grade 10

LO1: Scientific Inquiry and Problem-solving Skills

1. Identifying and Identify and question questioning phenomena phenomena. and planning an investigation

HSRC Workbook

Learning Outcome

Grade 11

Grade 12

Identify phenomena involving one variable to be tested.

Generate and question hypotheses based on identified phenomena for situations involving more than one variable.

Plan an investigation using instructions

Design simple tests to measure the effects of this variable.

Design tests and/or surveys to investigate these variables.

Consider implications of investigative procedures in a safe environment.

Identify advantages and limitations of experimental design.

Evaluate the experimental design.

2. Conducting an investigation by collecting and manipulating data

Systematically and accurately collect data using selected instruments and/ or techniques and following instructions.

Systematically and accurately collect data using selected instruments and/or techniques.

Compare instruments and techniques to improve the accuracy and reliability of data collection.

3. Analysing, synthesising, evaluating data and communicating findings

Display and summarise the data collected. Analyse, synthesise, evaluate data and communicate findings.

Select a type of display that communicates the data effectively.

Manipulate data in the investigation to reveal patterns. Identify irregular observations and measurements. Allow for irregular observations and measurements when displaying data.

Compare data and construct meaning to explain findings.

Critically analyse, reflect on and evaluate the findings. Explain patterns in the data in terms of knowledge.

LO2: Construction and Application of Life Sciences Knowledge

Draw conclusions and recognise inconsistencies in the data.

Provide conclusions that show awareness of uncertainty in the data.

Assess the value of the experimental process and communicate findings.

Suggest specific changes that would improve the techniques used.

1. Accessing knowledge

Use a prescribed method to access information.

Use various methods and Use various methods and sources to sources to access information. access relevant information from a variety of contexts.

2. Interpreting and making meaning of knowledge in Life Sciences

Identify concepts, principles, laws, theories and models of Life Sciences in the context of everyday life.

Identify, describe and explain concepts, principles, laws, theories and models by illustrating relationships. Evaluate concepts, principles, laws, theories and models.

LO3: Life Sciences, Technology, Environment and Society

Interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts.

3. Showing an understanding of the application of Life Sciences knowledge in everyday life

Organise, analyse and interpret concepts, principles, laws, theories and models of Life Sciences in the context of everyday life.

Analyse and evaluate the costs and benefits of applied Life Sciences knowledge.

1. Exploring and evaluating scientific ideas of past and present cultures

Identify and investigate scientific ideas and indigenous knowledge of past and present cultures.

Compare scientific ideas and Critically evaluate scientific ideas indigenous knowledge of past and indigenous knowledge of past and present cultures. and present cultures.

2. Comparing and evaluating the uses and development of resources and products, and their impact on the environment and society

Describe different ways in which the uses and development of resources and products, and their impact on the environment and society.

Comparing and evaluating the uses and development of resources and products, and their impact on the environment and society.

3. Comparing the influence of different beliefs, attitudes and values on scientific knowledge

Analyse and describe Compare scientific ideas and the influence of different indigenous knowledge of past beliefs, attitudes and values and present cultures. on scientific knowledge and its application to society.


Evaluate and present an application of Life Sciences knowledge.

Comparing and evaluating the uses and development of resources and products, and their impact on the environment and society.

Critically evaluate and take a justifiable position on beliefs, attitudes and values that influence developed scientific and technological knowledge and their application in society.

53

In an outcomes-based framework, the content is the vehicle that educators use to facilitate learners’ achievement in each assessment standard. The curriculum statement provides a list of content topics that must be covered, but it provides very little detail about the depth and breadth of each topic. This means that educators must have good subject matter knowledge in Biology, as well as access to a wide range of resources so that they are able to construct learning experiences that are interesting and valid in Life Sciences. The content topics in the RNCS for Life Sciences are arranged in four knowledge areas: l Tissues, cells and molecular studies l Structures and control of processes in basic life systems l Environmental Studies l Diversity, change and continuity. The core knowledge comprises 80% of the content to be covered. The remaining 20% must be used by the teacher to incorporate local knowledge into the curriculum, or to adapt specific knowledge to local circumstances. Part 2 of this workbook will help you to build your own understanding of topics that fall within two of the knowledge areas: Diversity, change and continuity, and Tissues, cells and molecular studies. The knowledge area ‘Diversity, change and continuity’ requires South African teachers and learners to include a basic understanding of evolution in the curriculum for Life Sciences. This is a new and very welcome addition to school Biology in South Africa.

54


HSRC Workbook

Overview Learning Outcome 3: Life Sciences, Technology, Society and the Environment (Grade 12) Assessment Standard: We know this when the learner is able to critically evaluate scientific ideas and indigenous knowledge of past and present cultures. Knowledge area: Diversity, Change and Continuity Content topics: Changes of knowledge through contested nature and diverse perceptions of evolution. Beliefs about creation and evolution.

For thousands of years, people have asked questions such as: l How did the world begin? l Where do plants and animals come from? l Where do I come from? l Where do humans come from? l What is the purpose of life? l Is there life in any other part of the universe? If you belong to a particular faith, your answers will probably reflect your faith. Every faith or culture in the world has a belief about the beginning of the world, the origin of humans and the purpose of life. Some faiths believe in a god or gods who created the world and everything in it. Other faiths or cultures believe the world developed from nothing, without a god or gods who created the world. Before entering a discussion about evolution, it is important to understand the difference between belief by faith, and proof by following the scientific process. l When we believe something by faith, we accept that it is true without requiring evidence to support the belief. l The scientific process requires that we seek explanations for natural phenomena by examining the evidence, by experimenting, and then explaining our findings in the light of what we have discovered and what we already know. In other words, science is constantly looking for explanations for what we see in our world. The question that people often ask is whether this means that scientists cannot also hold religious beliefs. And the answer is, of course they can! Through science we learn many wonderful things about the world. As a religious scientist, you may marvel at the wonders of your god or gods. A faith or set of cultural beliefs provide guidelines for daily living and interacting with other people. Many scientists derive great personal comfort from their religious beliefs. Occasionally, though, scientific findings conflict with people’s religion or traditional beliefs. In your work as a Science or Biology teacher you may have found certain concepts or topics that conflict with your own or the learners’ beliefs. In our work at the University of KwaZulu-Natal, we have experienced many such concepts. For example, many South Africans believe that albino people do not die, but simply disappear. Science tells us this is not possible, yet we have found this belief to be very widespread. Other examples include that AIDS is caused by witchcraft or that it is a punishment sent by God, that humans are reincarnated as animals, so we should not harm any other living creatures and that we should not eat foods from certain animals either because they are considered sacred or unclean.


55

Homo sapiens is the genus and species (taxonomic classification) of modern humans. Sapiens is the only existing species of the genus Homo.

56

As teachers we need to respond to confl icts between science and religious or traditional beliefs. The most effective way to do so is to openly acknowledge the beliefs, and to offer the scientifi c explanation as an alternative explanation. The subject of Part 2 of this workbook is evolution. The theory of evolution proposes that all the living organisms on Earth today have developed slowly and gradually from other organisms. The theory is supported by a huge amount of evidence from scientifi c studies. Most modern biologists believe that evolution has occurred and continues to infl uence life on Earth, but biologists differ on some of the details of evolution. Scientists don’t yet know everything there is to know about the process of evolution, but so far, they have found no evidence that contradicts the theory. The theory of evolution is an example of a scientifi c theory that has confl icted with major religions and with cultural beliefs about a single creation event. The accumulated evidence from many different sources now supports the idea that there was not a single creation event, and that evolution is a much more likely explanation. The major religions of the world have now accepted that the huge amount of scientifi c evidence in favour of evolution is convincing, and that the Earth and life on it have been and still are undergoing gradual changes. Modern geologists have calculated that the Earth is at least 4 500 million years old. Biologists have gathered evidence that life on Earth has developed from simple organisms such as bacteria, which fi rst lived on Earth about 3 800 million years ago, to the present variety of organisms that share our Earth at present. Modern humans (Homo sapiens) are fairly recent in terms of Earth’s history: there is no evidence of that Homo sapiens existed until about 100 000 years ago. Most species of organisms that once lived on Earth are now extinct, which means that there are no longer any living individuals of that species on Earth. In this part of the workbook you will be able to gain only a brief insight into a fraction of the evidence on which the theory of evolution is based. If you want to fi nd out more, you can read much more around the subject since this is the most important theory of modern Biology. There are also some excellent videos that cover the history of life and these make interesting viewing. South Africa has played, and continues to play, an extremely important role in the development of evolutionary theory. We have many outstanding fossils in different parts of the country, including some of the oldest known fossils, and some excellent fossils that tell us about human origins. Young South Africans deserve to know about this research and to be proud of our country’s contribution to this fi eld of scientifi c research. As the new curriculum comes into effect over the next few years, it will be possible for Life Science teachers to introduce learners to the evidence and underlying concepts of evolution. Evolution is the most important unifying theory in modern Biology. It enables us to make sense of most of our observations about life, and the patterns we see in living organisms and life processes.


HSRC Workbook

1 Charles Darwin and the voyage of The Beagle Learning Outcome 3: Life Sciences, Technology, Society and the Environment (Grade 12) Assessment Standard: We know this when the learner is able to critically evaluate scientific ideas and indigenous knowledge of past and present cultures. Knowledge area: Diversity, Change and Continuity Content topics: History and nature of science. Changes of knowledge through contested nature and diverse perceptions of evolution.

The theory of evolution is usually associated with the name of Charles Darwin. Charles Darwin was born in England in 1809. His father and grandfather were both doctors, so young Charles was sent to study medicine at Edinburgh University. However, Charles was not interested in medicine, so he changed his course and began to study theology so that he could become a minister. He was always very interested in nature and collected shells, birds’ eggs, beetles and plants. Darwin never completed his training for the ministry, because in 1831 he was invited to join an expedition which was going on a round-the-world voyage. The purpose of the voyage was to investigate life on other continents, and to map parts of the world that had not yet been mapped. Charles Darwin was only 22 years old, and he was very excited to be invited to join the team as its naturalist. The ship was called The Beagle, and during its fi ve-year voyage, Darwin saw many strange organisms that infl uenced his ideas about the origin of the Earth, and the origins of life on Earth. You can see the route taken by The Beagle during its fi ve-year voyage in Figure 2.

Figure 1. Charles Darwin.

Figure 2. The route taken by Charles Darwin and the crew of the The Beagle.


57

Darwin and the crew left England in 1832 and travelled first to Brazil in South America. They then made their way down the east coast of South America, visiting islands along the coast and stopping at various ports on the mainland. They rounded the southernmost tip of South America and sailed up the west coast. As you can see from the map, they made many stops as they mapped the coastline of South America. As they travelled, Darwin collected rocks, plants and animals from all the places they visited. The last stop before they left South America was a group of islands called the Galapagos Islands. Darwin’s visit to these islands was very important for his later ideas about evolution. After leaving South America, The Beagle crossed the Pacific Ocean, stopping at various islands. They stopped at the northernmost point of New Zealand, made several stops in Australia, and then sailed across the Indian Ocean to visit Mauritius, Madagascar and South Africa. From the Cape of Good Hope, The Beagle crossed the South Atlantic Ocean back to Brazil before returning to England. Darwin had collected hundreds of specimens from each country and island they visited. Each specimen was carefully labelled with details of exactly where and when he collected it. He noticed many curious phenomena, but it was another twenty years before he published his book on evolution by natural selection. Charles Darwin was not the first scientist to think that living organisms present today have evolved from previous living things. His grandfather, Erasmus Darwin, had published articles in which he presented his ideas that all presentday species have evolved by a gradual process of change from pre-existing species. However, before Charles Darwin, no-one could explain the mechanism of evolution. During his travels, Charles Darwin marvelled at the way living things are adapted to their surroundings. ‘Adaptation’ means ‘well-suited’ or ‘adjusted’. For example, we say that desert plants are adapted to living in dry areas. They have adaptations such as spines instead of leaves or thick fleshy leaves or welldeveloped root systems. Darwin was strongly influenced by a geologist called Charles Lyell. Lyell published a book called ‘Principles of Geology’ in 1830, in which he proved that the Earth is much older than the age calculated by theologians. Lyell proposed that the Earth was hundreds of millions of years old, not a few thousand, as most people thought. Charles Lyell is now considered to be the father of modern geology. Darwin took a copy of Charles Lyell’s book with him on the voyage of The Beagle, and he was able to confirm that geological features of the Earth form by slow acting forces that are still at work. Most of the Earth’s rocks form from fine particles of sand and silt that sink to the bottom of lakes, rivers and seas. The sediments change into rock as pressure from the water and more layers of sediment increases. Sedimentary rocks are still forming. Sedimentary rocks usually contain layers or strata with the oldest layers at the bottom and the most recent strata at the top. It takes thousands of years to form a layer, so a sedimentary rock hundreds of metres thick must be millions of years old. Darwin also collected fossils during his voyage around the world. Fossils are the remains of organisms preserved in sedimentary rock. Darwin was amazed to find fossils of marine organisms in mountains more than 4 000 metres above the present sea level. Darwin realised that the mountains must have once been below sea level. Observations such as these confirmed Lyell’s proposal that the surface of the Earth is slowly but constantly changing, and is shaped by natural forces such as Earthquakes, volcanoes, wind, and water. 58


HSRC Workbook

Developing and publishing the theory After returning from the voyage of The Beagle, Darwin spent many years studying all the specimens he had collected on his journey. He was convinced that evolution does occur, and he began to form and refine his idea about how it happens. He called the mechanism of evolution ‘natural selection’. Because his theory conflicted with the Old Testament creation story and he wanted to present as much supporting evidence as he could, he delayed making his ideas known for as long as possible. However, in 1858 he received a scientific paper written by a young British naturalist, Alfred Wallace. Wallace had been working in southeast Asia, and he had arrived at the same conclusion as Darwin: living organisms evolve by a process of natural selection. The arrival of Wallace’s paper forced Darwin to complete his book called ‘On the Origin of Species by Means of Natural Selection’ in 1859.

Reactions to the publication of the theory Darwin and Wallace announced their theory of evolution by natural selection together, since both had independently arrived at the same hypothesis, supported by evidence found in two different parts of the world. As expected, the book caused an outcry from the press, the Church and the general public, but it was sold out within hours of arriving in the bookshops. Although many people were able to accept that other organisms evolve, they found it hard to believe that humans have also evolved. Darwin wrote a book called ‘The Descent of Man’ in 1871. The book extends the theory of evolution to include human evolution. Although no fossil humans or pre-humans had then been found, Darwin suggested that the most likely place for such fossils was Africa. As it turns out, Darwin was right! The first fossil to shed light on the evolution of humans was found in South Africa in 1929. Since then, many fossils have been found in South Africa and East Africa, and many gaps in the story of human evolution have been filled. Darwin was not a healthy person for most of his life. He died in 1882, and was buried in Westminster Abbey. Since Darwin’s death, the theory of evolution by natural selection has been refined and changed as new evidence becomes available. In the next section we will study what Darwin proposed about the nature of evolutionary change. Remember that at the time that he wrote ‘The Origin of Species’, chromosomes and genes had not yet been discovered.

Activity 1 Arrange these sentences describing Charles Darwin’s life in the correct sequence: 1. Darwin published a book called ‘The Descent of Man’. 2. He studied medicine at Edinburgh University. 3. Charles Darwin was born in 1809. 4. He visited the Galapagos Islands, where he saw many new species of plants and animals. 5. Alfred Wallace published a paper describing his theory of evolution by natural selection. 6. Charles Darwin published a book called ‘On the Origin of Species by Means of Natural Selection.’ 7. He joined an expedition to map parts of the world. 8. Charles Darwin died and was buried in Westminster Abbey.


59

2 Darwin’s theory of evolution Learning Outcome 2: Construction and application of Life Sciences knowledge. Grade 12 Assessment Standards: We know this when the learner is able to use various methods and sources to access relevant information from a variety of contexts. We know this when the learner is able to interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts. Knowledge area: Diversity, Change and Continuity Content topics: Evolution theories, natural selection.

The word extinct refers to species that no longer live on the Earth.

In his book ‘On the Origin of Species by Means of Natural Selection’, Darwin explained that he believed that all organisms living on Earth today have slowly and gradually evolved from other organisms that are now extinct. We know that many extinct organisms once existed on Earth because scientists have found their fossils.

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Figure 3. Darwin’s theory of natural selection.

60

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Activity 2 Try to name a breed or variety that has the following characteristics: 1. A maize plant that produces white seeds. __ 2. A type of cattle bred especially for meat. __ 3. A kind of chicken bred for egg-laying. __ 4. A variety of bean plant that grows into a small bush. __ 5. A variety of orange that has no seeds. __


61

HSRC Workbook

The way in which evolution occurred was called ‘natural selection’. Darwin’s theory of natural selection was based on several key ideas, which are illustrated and explained in Figure 3. l Many more organisms are produced than survive to adulthood. For example, a tree produces thousand of seeds, but only a few will germinate. A fish lays thousands of eggs, but only a few will hatch into young fish. l Individuals of a population compete for resources. For example, the seedlings of a tree in a forest compete with other plants for space, sunlight, water and nutrients. Young fish will compete with each other for space, food and shelter. Some individuals will survive, but many will die. l Individuals produced by sexual reproduction vary within a species. For example, there are millions of people in South Africa, but we all look different (except for identical twins). Cattle in one herd differ in the colours of their coats. Some differences are not visible: some maize plants in a field may be more resistant to disease than others. l Those individuals within a population that are best adapted to their environment will survive to reproduce. Individuals that are less well adapted will die before they reach adulthood, or will not reproduce at all. This process is sometimes called ‘the survival of the fittest’. l Through differential reproduction rates, the characteristics that are best adapted to the environment will gradually spread through the population. Thus, although Darwin knew nothing about genes and heredity, he was aware that characteristics were passed on from one generation to another. l The process of natural selection results in a slow and gradual change in a population as it becomes progressively better adapted to its environment. If A population is a group of organisms of the same species living in one area. the environment remains unchanged for a long time, the population should also remain unchanged. However, a change in the environment may cause some species to become extinct, and other species to change. l Darwin believed that if natural selection continued for long enough, it would result in a new species. He believed it took thousands or millions of years for one species to change into a different species. Darwin used the example of artificial selection to illustrate the process of natural selection. If a plant breeder wants to produce plants with particular characteristics, for example maize plants that produce extra large cobs, she collects seeds from plants that have the biggest cobs. She grows the seeds from these plants, and again chooses the plants that have the biggest cobs. By doing this repeatedly, she can produce plants that have much bigger cobs than the plants she started with. The process of artificial selection has resulted in an improvement in all the food crops we grow commercially. It is also responsible for changes in domestic animals, such as cows that are specially bred to produce lots of milk, chickens that grow very fast and can be slaughtered at the age of six to eight weeks, and sheep that produce thick layers of wool.

What evidence supports Darwin’s theory? Learning Outcome 2: Construction and application of Life Sciences knowledge (Grade 12) Assessment Standards: We know this when the learner is able to use various methods and sources to access relevant information from a variety of contexts. We know this when the learner is able to interpret, organise, analyse, compare and evaluate concepts, principles, laws, theories and models and their application in a variety of contexts. Knowledge area: Diversity, Change and Continuity Content topics: Biological evidence of the evolution of populations. Fundamental aspects of fossil studies.

Darwin could show that selective breeding brought about changes in a population, but he couldn’t prove that artifi cial selection could produce new species. Although many different breeds of cattle now exist, all the breeds can still interbreed and produce fertile young. However, if you allow a horse and a donkey to interbreed, they produce a sterile offspring called a mule. This proves that horses and donkeys are separate species, but different breeds of cattle are all one species. Darwin believed that natural selection has to work for a very long time before a new species forms. The human life span of about 70 years is too short to witness the evolution of new species.

The age of the Earth So, one of the fi rst things that Darwin needed to show to convince people that his theory was true, was that the Earth could be millions of years old. This had to be the case if there was to be enough time to produce the huge variety of life forms that we see today. In Darwin’s time, people believed the Earth was 4 000 years to 20 000 years old. According to Darwin’s theory, this would not have been enough time for evolution to have occurred. However, after Charles Lyell published his book, in which he proposed that the Earth was really hundreds of millions of years old, Darwin had the support he needed for his theory of evolution. The Earth constantly changes its form due to very slow movements of its surface. Occasionally we become aware of these movements through earthquakes and volcanoes. Modern techniques of dating rocks have shown that the Earth is about 4 500 million years old. Samples of rock collected on the moon have confi rmed the age of the Earth and the moon as 4 500 million years. The moon formed at the same time as the Earth, so its rocks should be the same age as the Earth’s rocks.

62


HSRC Workbook

Changing organisms The second thing Darwin needed to support his theory of evolution was to show that organisms change over time. This requirement was met by studying fossils. Most organisms start to decay once they die. After some time, there is no trace of their existence. However, sometimes an organism dies in a place where it does not decay immediately, but is preserved and becomes fossilised. Sometimes the dead organism is preserved by freezing, or by becoming embedded in a hard substance that oozes from trees. However, most fossils occur in sedimentary rock. Figure 4 shows how fossils form in sedimentary rock.

Fault in rock Younger fossil in young rock

Dead organism

Organisms die. Their bodies sink to the sea bed and become buried in the sediment.

Skeleton fossilised

Erosion of rock exposes fossils.

Dead organism Older fossil in deeper rock

As the sediment hardens into rock, the remains turn to stone. New layers form over the old.

The rocks fold and are raised up out of the sea. They are now exposed to wind and rain

Erosion and faults in the rock expose the different layers and the fossils they contain.

Figure 4. The process by which fossils are formed.

A dead organism sinks to the bottom of a sea or lake. The body is covered with layers of sediments such as sand, mud or silt. Further layers are added to the first layer, building up enormous pressure on the dead body. Slowly, the sediment turns to rock, slowing down the decomposition of the dead body. As the body slowly decomposes, the decayed parts are replaced by minerals. The dead organism turns into stone in the newly-formed sedimentary rock. Once this happens we say a fossil has formed. Soft tissues like internal organs do not fossilise well, but bones, teeth, external skeletons and woody tissues of plants do fossilise well. We even find fossilised dung, fossilised nests, and fossilised footprints of animals! After many thousands or millions of years under the water, the sedimentary rocks may be exposed due to the Earth’s crust moving. Weathering by wind, rain and temperature changes may result in the fossils being exposed and discovered by humans. Many fossils have been discovered during road-building or in quarries. As the sedimentary rocks are blasted away, fossils that have lain undisturbed for millions of years may be exposed.

Figure 5. A fossilised ammonite from northern KwaZulu-Natal.


63

Using modern techniques for dating the rocks, scientists can work out the ages of fossils found in the rocks. Dating techniques have become very accurate, and scientists always cross-check using two or more techniques before they publish the age of a particular fossil. Fossils had been known for hundreds of years before Darwin began his work. People believed that fossils were the remains of organisms that still existed ‘somewhere’ in the world. Some of the fossils of enormous animals like dinosaurs and woolly mammoths were so big that they could not easily hide away from humans. However, as travellers explored more and more parts of the Earth, it became clear that these organisms no longer existed – they were in fact extinct, and the only evidence that they ever existed was in the form of their fossilised remains.

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Figure 6. Number of taxonomic groups found at different times in Earth’s history.

According to Darwin’s theory, the simplest organisms should be found in the oldest rocks. One should be able to see a gradual increase in complexity of the organisms, and also an increase in diversity of life as we examine fossils from successively younger rock layers. After carefully examining millions of fossils, scientists have found that, in general, Darwin was correct. Figure 6 shows the numbers of different families at various times in the history of the Earth. (When we talk about ‘families’ in this way, we mean groups of species, not human families.)

64


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