9A Quick Quiz - Pearson Education Classify different types of variation. 1 Identify examples of inherited variation. 2 Explain how inherited variation is caused. ...
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Quick Quiz
3 A gene is:
A the first part of an organism’s scientific name.
B a short section of a chromosome.
C a large, blue, ghost-like entity that lives in a lamp and grants wishes.
D a molecule found in the nucleus of cells.
4 The structure of the long molecule found in chromosomes was worked out by:
A Holmes and Watson.
B Bonnie and Clyde.
C Watson and Crick.
D Adenine and Thymine.
9Ad 1 Which of these is least likely to cause the
endangerment of a species?
A changes in physical environmental factors
B competition from other organisms
C decrease in predators
D human activities
2 Which of these is least likely to be used for conservation?
A banning the sale of items made from a certain animal
B creating a reservoir
C setting up a nature reserve
D building a zoo
3 Which feature of this animal suggests that it is prey for larger animals?
A spines on its body
B long nose
C small ears
D whiskers
4 Which of these would you expect to find in a gene bank?
A gametes
B single genes
C money for conservation projects
D sand
9Ae 1 This bird lives along the seashore. It feeds on
shellfish buried in the sand. There are plenty of shellfish for the birds and no diseases.
Some of the adult birds have slightly longer beaks and others have slightly shorter beaks. The cause of this is most likely to be:
A environmental factors.
B how much the birds use their beaks.
C how much the birds stretch their beaks.
D genes.
2 Think about the birds in the previous question. What will happen to the number of birds with the slightly longer beaks, compared to those with slightly shorter beaks, when there is plenty of food?
A The number of longer-beaked birds will go down compared to the number of shorter-beaked birds.
B The number of longer-beaked birds will go up compared to the number of shorter-beaked birds.
C The numbers of longer-beaked birds and shorter-beaked birds will not change much.
D Longer-beaked birds will disappear altogether.
3 Evolution is:
A what happens after an organism becomes endangered.
B a gradual change in the characteristics of organisms over time.
C when organisms decide to change their features to cope with a change in conditions.
D something that caused changes to happen in animals, such as dinosaurs, but does not occur any longer.
4 The theory of evolution that most scientists believe today was put forward by:
The Quick Quiz is to see how much you already know about a subject. It also gives you some idea of the things you will soon be learning about. Record your answers in the answers column. Shade in or tick the ones you get right.
Topic Answers I can already…
1 State what variation is.
2 Identify examples of environmental variation.
3 Explain how environmental variation is caused.
9Aa
4 Classify different types of variation.
1 Identify examples of inherited variation.
2 Explain how inherited variation is caused.
3 Describe how information is stored in cells.
9Ab
4 Interpret graphs showing continuous variation.
1 Describe the structure of chromosomes.
2 State the number of chromosomes found in different human cells.
3 Describe the structure of genes.
9Ac
4 Recognise the names of some different scientists in the discovery of the structure of genes.
1 Explain how changes in an ecosystem cause endangerment and extinction.
2 Suggest methods of conservation.
3 Explain why organisms have certain adaptations.
9Ad
4 Explain what gene banks are.
1 Describe why there are variations in adaptations.
2 Describe what happens to variations in a population over time.
3 State what evolution is.
9Ae
4 Recall who explained evolution in terms of natural selection.
Quick Quiz: /20
At the start:
0–5 = I didn’t know much; 6–10 = I knew something; 11–15 = I knew a fair bit; 16–20 = I already knew a lot
genus jeen-ous A group of similar organisms. The genus name is the first word in the scientific name for a species (the second word is the ‘species name’). Different closely-related species belong to the same genus.
journal A scientific magazine in which scientists publish their findings by writing articles called scientific papers.
scientific paper An article written by scientists and published in a science magazine called a journal. It is like an investigation report but usually shows the results and conclusions drawn from many experiments. Scientific papers are often just called papers.
species spee-shees or spee-sees
A group of organisms that can reproduce with each other to produce offspring that will also be able to reproduce.
variation vair-ee-ay-shun The differences between things.
9Aa – Environmental variation
Word Pronunciation Meaning
characteristic kar-ack-ter-iss-tick A feature of an organism.
classification Sorting things into groups.
continuous Continuous data can take any value between two limits. Examples include length, mass, time.
continuous variation When the value of a variable is continuous, it shows ‘continuous variation’.
discontinuous Data values that can only have one of a set number of options are discontinuous. Examples include shoe sizes and blood groups.
discontinuous variation
When the value of a variable is discontinuous, it shows ‘discontinuous variation’.
environment The conditions in a habitat caused by physical environmental factors and living organisms.
environmental factor Anything that can change the conditions in a habitat or the organisms that live there.
environmental variation
Differences between organisms caused by environmental factors.
physical environmental factor
A non-living factor that can change the conditions in a habitat (e.g. amount of light, rainfall).
resource rez-ors Something needed by an organism. For example, plants need light as a resource and animals need food as a resource.
fertilisation fert-ill-I-zay-shun Fusing of a male gamete with a female gamete.
fuse fewz When two things join together to become one.
gamete gam-meet A cell used for sexual reproduction.
genetic information jen-et-tick The inherited instructions that control your characteristics.
inherit A feature that an organism gets from a parent is inherited.
inherited variation Differences between organisms that are passed on to offspring by their parents in reproduction.
normal distribution If the value of a variable changes in a continuous way, it will often show a normal distribution. This means that the middle values of the data range are most common and values at the highest and lowest extremes are least common. This sort of data forms a bell shape on charts and graphs.
nucleus new-clee-us The ‘control centre’ of a cell, where genetic information is found.
offspring The new organisms produced by reproduction.
parent An organism that has produced offspring.
sexual reproduction Reproduction that needs two individuals to produce a new organism of the same type.
zygote zY-goat Another term for ‘fertilised egg cell’.
9Ab WS – Probability
Word Pronunciation Meaning
correlation cor-al-lay-shun A relationship between two variables. If an increase in one variable appears to cause an increase in the other, it is a ‘positive’ correlation. An increase in one variable linked with a decrease in the other is a ‘negative’ correlation.
estimate An approximate answer, often calculated from a sample or using rounded values.
probability The likelihood of something happening.
sample To take a small part of something to investigate. You use a sample to draw conclusions about what the larger whole is like.
cell division The splitting of a parent cell to form two identical daughter cells. The daughter cells both contain the same genetic information as the parent cell.
chromosome krow-mO-sOwm A structure found in the nuclei of cells. Each chromosome contains one enormously long DNA molecule.
DNA A substance that contains genetic information. Short for deoxyribonucleic acid.
gene jeen Section of the long strand of DNA found in a chromosome, which contains instructions for a characteristic.
sex chromosome krow-mO-sOwm Chromosome that determines the sex of an organism. In humans, males have one X sex chromosome and one Y sex chromosome, while females have two Xs.
9Ad – Genes and extinction
Word Pronunciation Meaning
adapted If something has adaptations for a certain job or for survival in a particular place, it is said to be adapted to that job or place.
biodiversity bI-O-die-ver-sit-ee The range of different species of organisms in an area.
competition com-pet-ish-un There is competition between organisms that need the same resources as each other. We say that they compete for those resources.
ecosystem All the physical environmental factors and all the organisms that are found in a habitat.
endangered en-dayn-jerd When a type of organism is in danger of ceasing to exist.
extinct An organism that no longer exists is extinct.
food web Many food chains linked together, showing the flow of energy through organisms in a habitat.
gene bank Any facility that stores genetic material from different organisms (e.g. seeds, gametes, tissue samples).
native Naturally found in a certain area.
9Ae – Natural selection
Word Pronunciation Meaning
evolution A change in one or more characteristics of a population over a long period of time.
natural selection A process in which an organism is more likely to survive and reproduce than other members of the species because it possesses a certain inherited variation.
Draw a ring around a number of stars for each statement. If you are very confident about a statement, draw your ring around all the stars. If you do not know anything about a statement do not draw a ring.
Topic At the end of the unit:
9Aa
Identify and give examples of environmental variation. * * * * * Explain how environmental variation is caused. * * * * * Tell the difference between continuous and discontinuous variation. * * * * * Explain why environmental variation can make classification and
identification difficult. * * * * *
9Ab
Identify and give examples of inherited variation. * * * * * Explain how inherited variation is caused. * * * * * Describe where genetic information is stored and what it does. * * * * * Identify normal distribution. * * * * * 9Ab Working Scientifically
Describe what probability is. * * * * * Calculate probabilities from experimental data. * * * * * Express probabilities as percentages, decimals and fractions. * * * * * 9Ac
State what chromosomes are made of. * * * * * State the number of pairs of chromosomes in most human cells. * * * * * Describe where genes are found and what they do. * * * * * Describe the roles played by Watson, Crick, Franklin and Wilkins in the
discovery of the structure of DNA. * * * * *
Use a model to illustrate the relationship between cells, cell nuclei, DNA, chromosomes, genetic information and genes. * * * * *
9Ad
Explain how changes in an ecosystem can cause endangerment and extinction. * * * * *
Suggest methods of conservation. * * * * * Explain how particular adaptations affect the chances of survival in a habitat. * * * * * Explain why preserving biodiversity is important and the role of gene banks. * * * * * 9Ae
Explain how natural selection determines the survival of certain variations of adaptations within a population. * * * * *
Explain how natural selection can lead to evolution. * * * * *
A species is a group of organisms that are able to reproduce to give offspring that are also able to reproduce. Members of the same species have very similar characteristics (features). However, there is variation in these characteristics.
All tigers have stripes but there is variation in the stripes between each tiger.
Environmental variation Some characteristics vary due to environmental factors in an organism’s surroundings (its environment). There are living environmental factors (other organisms) and physical (non-living) environmental factors, such as the amount of sunlight. Variation caused by environmental factors is environmental variation.
All the organisms and physical environmental factors in an area form an ecosystem.
Inherited variation Offspring inherit characteristics from their parents and these characteristics can vary (e.g. brown eyes and blue eyes). This is inherited variation.
Chromosomes, genes and DNA An organism’s characteristics are controlled by genetic information contained in a code in DNA. James Watson and Francis Crick discovered the structure of DNA by making use of the data of other scientists, such as Rosalind Franklin and Maurice Wilkins.
Each chromosome contains a long molecule of DNA. Certain sections of that DNA molecule contain the genetic information and are called genes.
In humans there are 23 different types of chromosome. Most cells have two copies of each type. Gametes, however, only have one copy of each type of chromosome. When two gametes fuse during fertilisation, they form a zygote that contains the chromosomes from both gametes.
For some characteristics, scientists can work out the probability that a child will inherit that characteristic. Probabilities are shown as percentages, decimals or fractions.
Adaptation Animals and plants are adapted to where they live; they have characteristics that allow them to survive in that habitat.
Jack rabbits are adapted to living in a desert habitat.
Natural selection All characteristics vary slightly amongst the members of a species. We can often draw a bell curve (normal distribution) to show variation in a characteristic.
If conditions in a habitat change, then variation in a characteristic may help some members of a species to survive better than others. Imagine a new predator moves into the area in which jack rabbits live. By chance, some jack rabbits will have slightly longer hind legs that allow them to run faster. These are the jack rabbits that are more likely to survive and reproduce. So, the next generation of jack rabbits will have slightly more rabbits with longer hind legs.
This process is known as natural selection. Charles Darwin and Alfred Russel Wallace both came up with the idea that it is natural selection happening over and over again, over a long period of time, that causes evolution.
Endangerment and extinction Changes in an ecosystem can cause species to become endangered or extinct. This is usually due to:
● changes in physical environmental factors
● competition from other organisms
● disease
● human activities (e.g. hunting, clearing habitats, using poisons).
We can try to stop this happening and preserve biodiversity (the number of species) by:
● protecting areas and setting up nature reserves
● setting up breeding programmes in zoos
● banning the hunting of some animals or the collecting of wild plants
● setting up gene banks (to store parts of organisms, such as seeds and gametes).
We should preserve biodiversity because:
● organisms depend on one another (they are interdependent)
● we won’t be able to make use of organisms if they become extinct
● more biodiverse areas recover better from natural disasters.
Triceratops lived in forests full of plants with large, tough leaves such as tree-ferns and palms. It became extinct 65 million years ago. It may have evolved from a dinosaur called Titanoceratops. A museum wants to explain what Triceratops looked like and why it looked like that. They are going to hang labels on a life-size model of Triceratops, which people can then read to find out more about its adaptations and variation. What should the labels say, and where should they be tied?
Name Class Date
Now that you have completed the activity, circle the number of stars next to each of these sentences to describe how well you did.
I have…
stated what variation is. * * * * * outlined why dinosaurs became extinct. * * * * * used the information given above to explain how we know something about Triceratops (e.g. that it was food for T. rex). * * * * *
identified examples of environmental variation. * * * * * identified examples of inherited variation. * * * * * explained how changes in an ecosystem cause extinction. * * * * * identified the adaptations of Triceratops in the labels above. * * * * * explained how environmental variation is caused. * * * * * outlined how some variation is caused by inheriting characteristics from parents. * * * * * explained the adaptations of Triceratops in the labels above. * * * * * identified and explained some adaptations of Triceratops that are not in the labels. * * * * * described the relationship between cells, nuclei, chromosomes, genes, DNA and genetic information. * * * * * described how variations in adaptations can change how likely an organism is to survive compared with other members of the same species. * * * * * stated what evolution is. * * * * * explained how natural selection works. * * * * *
2 The drawings show a mother and her two daughters.
a Give one characteristic that both sisters have in common with their mother.
[1 mark]
b Give one characteristic that both sisters have in common with each other but not with their mother.
[1 mark]
3 The drawing shows a camel and some of its adaptations for living in a desert. In the desert it can be very cold at night and very hot during the day.
a What characteristic of the camel allows it to be hidden from predators?
[1 mark]
b What characteristic of the camel allows it go without food for some time?
[1 mark]
c Suggest why camels have wide feet.
[1 mark]
d What physical environmental factor is described in the introduction to this question?
4 The drawings show two different sorts of bird feet, X and Y.
a Suggest what foot X is adapted for.
[1 mark]
b Suggest what foot Y is adapted for.
[1 mark]
5 The bar chart shows the lengths of five carrots from each of two different types of carrot plant.
a Give two ways in which the carrot types are different.
[2 marks]
b Suggest how the differences may have been caused.
[1 mark]
c If you collected many more length measurements for Type A carrots and plotted them all on a bar chart, the chart would show a bell shape. What is this bell shape called?
Blowflies lay their eggs on the bodies of dead animals. The ‘maggots’ that hatch out of the eggs then feed on the meat. Ten blowfly maggots were put in the centre of a piece of apparatus called a choice chamber. They were observed for 15 minutes to see which conditions they preferred. The final positions of the maggots are shown in the diagram.
a Use the results to complete the bars on the bar chart.
[1 mark]
b Suggest how this adaptation of blowfly maggots helps them to survive.
[1 mark]
6 The table shows three different types of variation in humans. Put ticks () in the boxes to describe the type of variation for each.
7 Dinosaurs died out 65 million years ago. Many scientists think that this was caused by a meteorite hitting the Earth and sending huge amounts of dust up into the atmosphere, affecting plants. Explain how this could have caused the dinosaurs to die out.
8 a A fish called the blue pike used to live in the Great Lakes of Canada. It became extinct in 1975. This was partly due to new species of fish being introduced to the lakes. Describe how this could have reduced the population of blue pike.
[1 mark]
b There were several reasons why the blue pike became extinct. Apart from the introduction of new species, suggest another reason.
[1 mark]
9 a The red panda is an endangered mammal that lives up in the trees in forested parts of China. Explain one way of making sure that this mammal does not become extinct.
[1 mark]
b Give one reason why we should try to preserve biodiversity.
[1 mark]
10 a Where in a cell would you expect to find chromosomes?
[1 mark]
b Name one substance that you would find in a chromosome.
[1 mark]
c Explain how chromosomes cause inherited characteristics.
[2 marks]
11 Deer mice are found in part of the USA. The deer mice live on the ground. In hills containing a lot of sand, the mice are mainly a pale colour, whereas those living elsewhere are mainly a dark colour. Explain this observation.
2 Blowflies lay their eggs on the bodies of dead animals. The ‘maggots’ that hatch out of the eggs then feed on the meat. Ten blowfly maggots were put in the centre of a piece of apparatus called a choice chamber. They were observed for 15 minutes to see which conditions they preferred. The final positions of the maggots are shown in the diagram below.
a Write a conclusion for the investigation.
[1 mark]
b Suggest how this adaptation of blowfly maggots helps them to survive.
[1 mark]
3 a A fish called the blue pike used to live in the Great Lakes of Canada. It became extinct in 1975. This was partly due to new species of fish being introduced to the lakes. Describe how this could have reduced the population of blue pike.
[1 mark]
b There were several reasons why the blue pike became extinct. Apart from the introduction of new species, suggest another reason.
4 a The red panda is an endangered mammal that lives up in the trees in forested parts of China. Explain one way of making sure that this mammal does not become extinct.
[1 mark]
b Give one reason why we should try to preserve biodiversity.
[1 mark]
5 Dinosaurs died out 65 million years ago. Many scientists think that this was caused by a meteorite hitting the Earth and sending huge amounts of dust up into the atmosphere, affecting plants. Explain how this could have caused the dinosaurs to die out.
[2 marks]
6 Deer mice are found in part of the USA. The deer mice live on the ground. In hills containing a lot of sand, the mice are mainly a pale colour, whereas those living elsewhere are mainly a dark colour. Explain this observation.
[1 mark]
7 a Where in a cell would you expect to find chromosomes?
[1 mark]
b Name one substance that you would find in a chromosome.
[1 mark]
c Explain how chromosomes cause inherited characteristics.
d Complete the diagram below to show the total number of chromosomes in some different human cells. Write the numbers in the white squares. Make sure you write a number in each of the white squares.
[9A_EOUT_FigG – fertilisation diagram]
[2 marks]
e Look at the diagram again. What is the name of ‘process X’?
[1 mark]
f Explain why two brothers (who are not identical twins) look similar to one another but do not look the same as each other.
[1 mark]
8 The charts show the beak sizes of a species of Galapagos finch. The top chart shows the beak size during normal rainfall (counted in 1976). The lower chart shows the beak size of the birds after a drought that occurred in 1977. During the drought, plants produced fewer seeds. The seeds that they did produce were larger than during times of normal rainfall.
a What do we call the overall shape shown on the upper graph?
lack of photosynthesis (caused by dust stopping sunlight reaching the Earth)
not enough food/energy to support the other organisms in the food chains
Accept answers based on other hypotheses based on their merits (e.g. dust blocking sunlight and causing a fall in temperatures resulting in slow photosynthesis, or plants being covered in dust and so slowing photosynthesis).
2 marks – 1 mark for each point
a 6 the other fish competed with the blue pike for resources/food/shelter
Accept: other fish could have been predators of the pike, or other fish gave the pike diseases
1 mark 9
b 5 overfishing, pollution
Accept: acid rain (although this was not a factor)
1 mark
a 5 One of:
setting up a nature reserve to ensure that the habitat is preserved
banning the hunting of red pandas to make sure that more of them survive and reproduce
setting up a breeding programme to increase their numbers
Accept: other answers on their merits but each idea must be accompanied by an explanation.
1 mark – an idea must be accompanied by an explanation for the mark
10
b 6 One of: to stop other animals/plants/organisms in the same food web becoming endangered/extinct; to ensure the survival of species that may be of use to humans in the future; to keep a habitat looking beautiful
1 mark
a 6 nucleus 1 mark
b 6 One of: DNA; deoxyribonucleic acid; protein 1 mark
11
c 6 they come from an organism’s parents
they contain genetic information/genes for characteristics
2 mark – 1 mark for each point
12 6 Deer mice that happen to be darker but live in sandy-coloured soils are more likely to be spotted and eaten by predators.
Accept: explanations in terms of the dark mice surviving better on the darker-coloured soils.
2 marks – 1 mark for each fully correct row in the table
a 5 Maggots move to/prefer dark and damp places 1 mark 2
b 6 It helps them to find their food (inside bodies of dead animals)
Accept: their food is found in these conditions; to prevent them from becoming dehydrated; to hide from predators
1 mark
a 6 the other fish competed with the blue pike for resources/food/shelter
Accept: other fish could have been predators of the pike, or other fish gave the pike diseases
1 mark 3
b 5 overfishing, pollution
Accept: acid rain (although this was not a factor)
1 mark
a 5 One of:
setting up a nature reserve to ensure that the habitat is preserved
banning the hunting of red pandas to make sure that more of them survive and reproduce
setting up a breeding programme to increase their numbers
Accept: other answers on their merits but each idea must be accompanied by an explanation.
1 mark – an idea must be accompanied by an explanation for the mark
4
b 6 One of: to stop other animals/plants/organisms in the same food web becoming endangered/extinct; to ensure the survival of species that may be of use to humans in the future; to keep a habitat looking beautiful
lack of photosynthesis (caused by dust stopping sunlight reaching the Earth)
not enough food/energy to support the other organisms in the food chains
Accept answers based on other hypotheses based on their merits (e.g. dust blocking sunlight and causing a fall in temperatures resulting in slow photosynthesis, or plants being covered in dust and so slowing photosynthesis).
2 marks – 1 mark for each point
6 6 Deer mice that happen to be darker but live in sandy-coloured soils are more likely to be spotted and eaten by predators.
Accept: explanations in terms of the dark mice surviving better on the darker-coloured soils.
1 mark
a 6 nucleus 1 mark
b 6 One of: DNA; deoxyribonucleic acid; protein 1 mark
c 6 they come from an organism’s parents
they contain genetic information/genes for characteristics
2 marks – 1 mark for each point
d 7 46 chromosomes found in sperm and egg-making cells and zygote
23 or half the number of chromosomes in the sperm and egg cells as there are in the sperm- and egg-making cells and zygote (even if starting number of chromosomes is incorrect).
2 marks – 1 mark for each point
e 6 cell division
Accept: mitosis (although students are unlikely to have come across this term)
1 mark
7
f 7 each gamete that is made contains a slightly different mix of chromosomes/genes/genetic information from the parent
1 mark
a 6 normal distribution 1 mark
b 7 bigger beaks are better for crushing bigger seeds
during the drought there was less to eat and so greater competition between the birds
birds that by chance had a slightly bigger beak would be able to get more food than those that had slightly smaller beaks
the larger beaked birds would be more likely to survive and reproduce and their offspring would inherit the bigger beaks (the result of which we see in the lower chart)
Across: 3 Something required by an organism from its habitat to survive.
6 Another word for ‘features’.
9 A term for ‘non-living’.
Down: 1 Variables that affect organisms.
2 Sorting organisms into groups.
4 Differences between organisms.
5 Having values that can only be chosen from a certain set of values.
6 Having values that can be any number between two limits.
7 The surroundings of an organism.
8 A group of organisms that can breed with one another to produce offspring that can also reproduce.
9Ab 1 L5–7 Students’ own responses.
2 a L5 Samantha: features from mother – unlobed ears, upturned nose; from father – protruding chin, curly hair. Jeffrey: features from mother – straight hair, receding chin; from father – lobed ears, straight nose.
b L5 environmental variation
c L5 discontinuous (you can’t have half a filling)
d L6 normal distribution
e L7 another bell-shape but shifted to the right
9Ab WS 1 L4 The likelihood of something happening.
2 a L5–6 1/2 0.5 50%
b L5–6 1/6 0.17 17%
c L5–6 1/52 0.019 (or 0.2) 1.9% (or 2%)
d L5–6 11/100 0.11 11%
9Ac Students’ own concept maps.
9Ad 1 L5 Students’ own flowcharts showing one theory for why the dinosaurs became extinct in a series of clear steps, for example:
meteorite hits Earth
dust goes into atmosphere
plants die due to lack of sunlight for photosynthesis
2 a L5–7 One of: to preserve organisms we might need in the future; to preserve ecosystems; to preserve food webs; to keep the world beautiful.
b L5–7 One of: nature reserves to protect organisms; zoos to make sure organisms survive; gene banks to preserve cells, such as gametes, that could be used to regenerate organisms if they become extinct. Note the need for an explanation.
3 a L6 Examples include: eyes on the side of its head – gave it all-round vision to spot danger; long neck – allowed it to find leaves in a wide area around its body and/or to see predators (when lifted) and/or to eat leaves higher off the ground; large feet – to stop it sinking into the ground; long tail – for balance.
b L5–7 The numbers may go down because the animals would lose too much heat. The flaps of skin on the necks would transfer too much ‘heat’ to the surroundings. Note the need for a fuller explanation of how the animals would lose too much heat for the higher level.
c L5–7 The numbers may go down because the animals wouldn’t get enough to eat. The animals can only digest the soft leaves and the tougher leaves would not be digested. Note the need for a fuller explanation of why the new plants are unsuitable for the higher level.
9Ae L5–7 Students’ own responses. Ensure that students phrase things in such a way that it is obvious that the genetic variation already exists and is being selected by changes in the environment. The environment does not cause the variations. Note also that the ‘selection’ process usually occurs to a greater extent when there is an increase in the death rate of an organism (e.g. due to a shortage of a resource, increase in disease or predators) and some animals are by chance naturally better adapted to get that resource (e.g. food) than others. They are then more likely to survive and reproduce.
9Ae Lit L6 We can see natural selection occurring.
Peppered moths are usually pale but occasional ones are dark. In the nineteenth century, the dark form became much more common in polluted areas because the pale ones could easily be seen by birds and eaten.
The dark form was always there and people just hadn’t noticed that there were so many of them.
But studies in the twentieth century have shown that the pale form has become much more common again, since factories have stopped producing so much pollution.
Changes in the numbers of the different forms of moth are clearly matched to changes in conditions in the ecosystem, and so this remains an excellent example of natural selection.
We can see natural selection occurring in the Galapagos Islands.
Finches with medium-sized beaks generally eat medium-sized seeds. In 1977 there was a drought on the islands. The plants produced fewer but larger seeds. Finches with slightly bigger beaks than other members of the species survived better because they could eat these larger seeds.
The finches with the slightly bigger beaks were just those that had become fatter than the others in a previous year and so could survive the lack of food. They may just have become fatter because they had found a really good source of seeds that other birds hadn’t found.
But in 1984 and 1985, there were heavy rains and the seeds produced by the plants were small and soft. The numbers of birds with the slightly bigger beaks decreased but those with slightly smaller beaks were better able to feed on these smaller seeds and so survived.
The fact that changes in the finch population are clearly matched to changes in conditions in the ecosystem shows that natural selection is occurring.