17. Selection and evolution

17.1 Variation

Learning outcomes Candidates should be able to:

1. 1 explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors

2. explain what is meant by discontinuous variation and continuous variation

3. explain the genetic basis of discontinuous variation and continuous variation

4. use the t-test to compare the means of two different samples (the formula for the t-test will be provided, as shown in the Mathematical requirements)

17.2 Natural and artificial selection

Learning outcomes Candidates should be able to:

1. explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation

2. explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection

3. explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations

4. outline how bacteria become resistant to antibiotics as an example of natural selection

5. use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (the two equations for the Hardy–Weinberg principle will be provided, as shown in the Mathematical requirements)

6. describe the principles of selective breeding (artificial selection)

7. outline the following examples of selective breeding:

• the introduction of disease resistance to varieties of wheat and rice

• inbreeding and hybridisation to produce vigorous, uniform varieties of maize

• improving the milk yield of dairy cattle

17.3 Evolution

Learning outcomes:

1. outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

2. discuss how DNA sequence data can show evolutionary relationships between species

3. explain how speciation may occur as a result of genetic isolation by:

4. geographical separation (allopatric speciation)

5. ecological and behavioural separation (sympatric speciation).