Variation and evolution: the sources of genetic variation; natural selection and types of selection; the Hardy-Weinberg principle; genetic drift; and speciation.

What this dot point is asking

Eduqas wants you to describe the sources of variation, explain natural selection and its types, apply the Hardy-Weinberg principle, explain genetic drift, and explain speciation. This is the evolution heart of Component 2.

The sources of variation

Variation in a population arises from:

• Mutation: random changes in DNA, the only source of completely new alleles;
• Meiosis: crossing over (prophase I) and independent assortment (metaphase I) shuffle existing alleles into new combinations;
• Random fertilisation: any gamete can fuse with any other, mixing parental alleles;
• the environment, which affects phenotype but is not inherited.

Only genetic variation can be acted on by natural selection over generations.

Natural selection and its types

The Hardy-Weinberg principle

The Hardy-Weinberg principle predicts that allele and genotype frequencies stay constant from generation to generation provided there is no mutation, no selection, no migration, random mating and a large population. For a gene with two alleles:

where $p$ is the dominant allele frequency, $q$ the recessive allele frequency, $p^2$ the homozygous dominant frequency, $2pq$ the heterozygous frequency and $q^2$ the homozygous recessive frequency. It lets you estimate allele frequencies (often starting from $q^2$, the recessive phenotype). A measured change from these predictions is evidence that the population is evolving.

Genetic drift and speciation

Genetic drift is the change in allele frequencies by chance (which individuals happen to reproduce), with the biggest effect in small populations (for example after a bottleneck or founder event).

Examples in context

Example 1. Peppered moth industrial melanism. Soot darkened tree trunks, so the dark allele became favourable and its frequency rose; as pollution fell, the pale form recovered. This is the classic British example of directional natural selection changing allele frequency.

Example 2. Antibiotic resistance. An antibiotic is a strong selection pressure: resistant bacteria survive and reproduce, so the resistance allele's frequency rises rapidly, a real and current example of evolution by natural selection.

Try this

Q1. State the three main sources of genetic variation. [3 marks]

• Cue. Mutation; meiosis (crossing over and independent assortment); random fertilisation.

Q2. Write the two Hardy-Weinberg equations and state what $q^2$ represents. [2 marks]

• Cue. $p + q = 1$ and $p^2 + 2pq + q^2 = 1$; $q^2$ is the frequency of the homozygous recessive genotype (the recessive phenotype).

Q3. Explain what is meant by reproductive isolation and why it is needed for speciation. [2 marks]

• Cue. Two populations can no longer interbreed (geographically, behaviourally or by timing); this stops gene flow, so the populations can diverge into separate species.