CCEA GCSE Biology - Denmour Boyd, James Napier 2017
Unit 2
Variation and natural selection
Specification points
This chapter covers sections 2.5.1 to 2.5.4 of the specification. It is about the types of variation, natural selection and selective breeding.
Variation
Living organisms that belong to the same species usually vary from each other in many ways. There are two main types of variation.
Types of variation — continuous and discontinuous variation
Variation in a particular characteristic can be either continuous or discontinuous. In continuous variation there is gradual change in a characteristic across a population. Height is an example of continuous variation in humans. While people can be described as being tall or short, there is not a distinct boundary that separates short and tall people.
Figure 12.1 shows a typical set of values for height in human males. Note that the histogram produced shows a normal distribution. A normal distribution is where most individuals are around the average or mean value and relatively few are found at either extreme.
Mass in humans and length, for example of hand span, will also show continuous variation.
In discontinuous variation the population can be clearly divided into discrete groups or categories. A common example used is the ability, or inability, to roll the tongue in humans. In this example, individuals will fit into one of two categories — there are no intermediates
Hand dominance — being right or left-handed — is another example of discontinuous variation in humans. In other examples of discontinuous variation, there can be more than two categories, for example ABO blood groups, but again all individuals can be clearly identified as belonging to a particular blood group (A, B, AB or O).
You should be able to represent information on variation graphically. Continuous variation should be represented as a histogram and discontinuous variation as a bar chart (with spaces between the bars).
Tip
If you see an exam question that has variation shown as a histogram you should be able to identify it as continuous variation. Similarly, if it is represented as a bar chart, it will be discontinuous variation.
Causes of variation
Variation in living organisms involves genetic variation and/or variation due to the environment.
Genetic variation
This is variation as a result of changes to chromosomes or genes (DNA).
Mutations are random changes in the number of chromosomes or type of gene. We have already come across a number of examples:
• Down’s Syndrome is caused by an extra chromosome.
• Cystic fibrosis and Huntington’s disease are each caused by a faulty gene.
Tip
The harmful alleles in genetic conditions such as cystic fibrosis and Huntington’s disease were formed by mutations many years ago and have remained in the population ever since, passing down through the generations.
Tip
Most mutations are harmful — in Chapter 10, you learnt about a number of harmful genetic conditions caused by mutations. However, they are not all harmful — look back at question 4 on page 116 about widow’s peak.
Genetic variation is also caused by the process of sexual reproduction. Meiosis mixes up the chromosome arrangements in gamete formation — this results in all gametes being genetically different. Therefore when any two gametes fuse, the genes in that zygote will be different to the genes in any other zygote. This is why siblings in any one family often show considerable variation in phenotypes.
Tip
Environmental variation can produce variation (different phenotypes) even if the genotypes are the same — think about how plant cuttings (clones) can differ if placed in different light levels.
Environmental variation
The environment can also provide variation. While to a large extent height in humans is genetically controlled, the actual height a human grows to will depend on the quality of nutrition.
Test yourself
1 State the two different types of variation.
2 State the two causes of variation.
Show you can
Table 12.1 gives information about five students in a class.
a) Which two characteristics show discontinuous variation?
b) What percentage of students are able to roll their tongues?
c) What type of graph would you use to represent the data for hand span?
Example
Using the photograph, give two adaptations to the environment in polar bears.
Answer
They have thick fur to reduce heat loss and their white fur gives them camouflage and allows them to blend in with the snow.
Natural selection and evolution
Natural selection
In nature, adaptations in living organisms are essential for survival and success in all habitats. You should be able to work out the main adaptations in organisms if provided with enough information.
Adaptations in organisms are even more important when organisms compete with each other for resources. This competition ensures that the best adapted individuals will survive. For example, the larger seedlings growing in a clump of plant seedlings will be able to obtain vital resources such as light, nutrients and water more easily than the smaller seedlings. As a result of this competition, the stronger individuals will survive, often at the expense of the weaker ones.
This competition for survival, with the result that the better equipped individuals survive, summarises Charles Darwin’s theory of natural selection.
Charles Darwin and the theory of natural selection
Charles Darwin (1809—1882) was a naturalist who devoted much of his life to scientific research. As part of his research he spent five years as a ship’s naturalist on the HMS Beagle as it travelled to South America. Darwin was greatly influenced by the variety of life he observed on his travels and, in particular, by the animals of the Galapagos Islands. Darwin’s famous account of natural selection, On the Origin of Species, was published in 1859.
Darwin’s main conclusions about natural selection can be summarised as:
• There is variation among the phenotypes (individuals) in a population.
• If there is competition for resources, there will be a struggle for existence.
• The better-adapted phenotypes survive this struggle or competition. This leads to survival of the fittest and these (fittest) individuals are more likely to pass their genes on to the next generation.
It is useful to look at a modern day example to highlight the key features of natural selection.
Tip
Natural selection is the process in which the better-adapted individuals survive (at the expense of the less well adapted individuals) and pass on their genes.
Tip
Natural selection depends on the environment (context). If antibiotics were not used, then the antibiotic resistant bacteria wouldn’t be better adapted and therefore wouldn’t increase in number.
Antibiotic resistance in bacteria
When bacteria are treated with an antibiotic such as penicillin, most of them are killed. However, as Figure 12.6 shows, a small number (the best adapted phenotypes) may survive, probably because they have a gene (caused by a mutation) that provides resistance. Very soon, the resistant bacteria are the only ones remaining, as they are the only ones surviving and passing their beneficial mutations on to their offspring.
The link between natural selection and evolution
Darwin used the theory of natural selection to explain the process of evolution. Evolution shows that species have changed gradually through time in response to changes in the environment and that evolution is a continuing (ongoing) process. As well as species changing over time, evolution can result in the formation of new species.
Tip
Evolution is change in species over time as a consequence of natural selection favouring different phenotypes over time and also the formation of new species.
Fossils
There is so much evidence for evolution now that most scientists accept that evolution has taken place and that it is still taking place. One of the best types of evidence is fossil evidence. Fossils are the remains of living organisms that have been preserved (usually in rocks) for millions of years.
The fossil of an Archaeopteryx in Figure 12.7 is one of the most important fossils ever found. This and other similar fossils provide clues allowing scientists to work out how birds evolved from reptiles (dinosaurs).
Therefore fossils not only show what a particular animal or plant looked like millions of years ago but often show how the organism (species) changed over a long period of time.
For example, the fossil record of horses is extensive, and because of this, it shows how horses evolved over time in a changing habitat; the horse’s fossil record shows the various stages, not just the ’before’ and ’after’.
Tip
As it is possible to date rocks to when they were formed (and when the organism was fossilised), it is possible to accurately date the age of the fossil.
Tip
Natural selection acted on hoof size and animal size favouring the adaptations that suited the habitat best at any particular time. As the habitat changed, what was the best adapted phenotype also changed.
Tip
It is important to understand that the phenotype of the horse changed in many very small steps over a very long period of time — as is typical of evolutionary change.
The fossil record also shows that some species have changed very little over a long period of time. A good example is the coelacanth. This ancient species of fish has barely changed over many millions of years.
Tip
If a species remains unchanged over many millions of years what does this suggest? It suggests that the environment was very stable and barely changed over this time period.
Extinction
Species are extinct if there are no living examples left. Many species have become extinct and often we only know they did exist in the past due to the discovery of fossils. Examples of extinct species include the dodo, dinosaurs and the woolly mammoth.
Tip
Species can become extinct if they fail to adapt to environmental change.
Species that are not quite extinct but are at risk are called endangered species. For example, a number of big cat species are endangered.
Species can become extinct for many reasons. These include:
• climate change or natural disasters, for example, the dinosaurs probably became extinct when a meteor hit the Earth and caused the climate to change
• hunting by humans — the dodo was hunted until it became extinct
• hunting by animals introduced by humans to areas where they are not normally found
• the spread of diseases
• loss of habitat — this is causing a lot of species to become extinct today. The loss of habitat is often caused by human activities, for example deforestation and clearing land for towns and cities.
Tip
The list above shows that man is responsible for many of the extinctions that take place today — in the context of most extinctions, the activities of man bring about ’environmental change’.
We can slow down the rate of extinctions in a number of ways. These include:
• legislation preventing the hunting of endangered species
• international agreements, such as those that plan to limit climate change
• special programmes such as creating nature reserves to protect habitats
• education to encourage people to do their part in protecting the environment.
Test yourself
3 Describe what is meant by antibiotic resistance.
4 What is evolution?
5 What is a fossil?
6 State two ways in which man has caused extinctions of other species.
Show you can
Explain how fossils can provide very good evidence for evolution.
Selective breeding
For centuries, people have manipulated the course of natural selection by deliberately selecting particular characteristics or traits in many plants and animals that are of use to us. This is the process of selective breeding (artificial selection).
Characteristics that are advantageous to us include increased crop yield or quality, appearance, hardiness, disease resistance and longer shelf life.
The selective breeding of cereals such as wheat in Figure 12.9, show the principles of selective breeding. Wheat has been bred over many years to produce a shorter stalk length which is less likely to suffer wind damage and the uniform size is easier to harvest. The modern wheat plant has a much larger head of grain (higher yield) than the ancient variety too.
Tip
Selective breeding in wheat shows one of the key characteristics of selective breeding: it takes many generations (reproductive cycles) and a long time to reach the stage where all the animals or plants have the desired characteristics.
Tip
The selective breeding of dogs shows some of the problems of selective breeding if it is carried out to extremes (excessive inbreeding). For example, many modern breeds have limited life spans, joint issues and other problems just because they have been bred to have appearances and ’personalities’ that are attractive to pet owners.
Test yourself
7 Name one desirable characteristic that could be selectively bred into tomato plants.
Show you can
Suggest the main stages involved in selectively breeding cattle for high milk yield.
Selective breeding has been very important in the development of farm animals and pets. For example, cattle can be bred to have excellent beef properties when used to provide beef for human consumption, for example Aberdeen Angus, or to produce large quantities of high quality milk, for example Friesians. Dogs have been bred for appearance, ’personality’, their ability to act as guard dogs and for many other features.
Practice questions
1 Figure 12.10 shows some Friesian cattle grazing in a field.
a) Name the type of variation shown by coat colour in the cattle.
Choose from:
discontinuous
continuous
environmental
(1 mark)
b) Define this type of variation.
(1 mark)
c) Suggest one other example of variation shown by the cattle in the field.
(1 mark)
2 a) Figure 12.11 shows how leaf width varies in a particular type of plant leaf.
i) What is the most common leaf width in these plant leaves?
(1 mark)
ii) How many leaves were sampled in total?
(1 mark)
iii) Name the type of variation shown by leaf width.
(1 mark)
b) In a particular class of Year 11 students, 12 could roll their tongue and 8 could not.
i) What percentage of students could roll their tongue?
(2 marks)
ii) Why is tongue rolling described as discontinuous variation?
(1 mark)
iii) Name the type of graph used to show discontinuous variation.
(1 mark)
3 Scientists cultured two types of bacteria (A and B) in a beaker. Figure 12.12 shows how the numbers of the two types changed after an antibiotic was added to the beaker.
a) Describe fully the changes in numbers of bacteria A and B after the antibiotic was added.
(2 marks)
b) Explain the change in number of bacteria B.
(3 marks)
c) Name the process that this investigation demonstrates.
(1 mark)
4 Most fossils are found in rock. For example, the Archaeopteryx fossils described earlier in this chapter were found in quarries in Germany.
a) Suggest why many fossils are found in quarries or where new roads are being built.
(1 mark)
b) Describe fully how fossils provide evidence for evolution.
(3 marks)
5 Table 12.3 shows the number of extinctions that have occurred in a country over the last 100 years.
a) Copy the graph axis in Figure 12.13 below and complete a line graph of the information in Table 12.3.
(3 marks)
b) In which 20 year period was there the largest increase in extinctions?
(1 mark)
c) Predict how many extinctions there will be by the year 2020.
(1 mark)