Natural selection and the sources of variation, the Hardy-Weinberg principle and allele frequencies, types of selection, and the mechanisms of speciation.

What this dot point is asking

CCEA wants you to explain natural selection and the sources of variation, use the Hardy-Weinberg principle to calculate allele frequencies, describe directional and stabilising selection, and explain the mechanisms of speciation.

Natural selection

Directional selection favours one extreme of a range and shifts the mean (for example antibiotic resistance, or larger beak size in a drought); stabilising selection favours the average and removes the extremes (for example human birth weight, where very small and very large babies have lower survival). Disruptive selection favours both extremes over the mean.

The Hardy-Weinberg principle

In practice you usually start from the recessive phenotype frequency, which equals $q^2$ (because only homozygous recessive individuals show the recessive trait), take the square root to find $q$, then use $p = 1 - q$ and $2pq$ for the carrier frequency. Comparing real allele frequencies over time with the predicted equilibrium shows whether a population is evolving.

Speciation

A species is a group of organisms that can interbreed to produce fertile offspring. In allopatric speciation a geographical barrier (a river, mountain or sea) separates populations, which then experience different selection pressures, accumulate different mutations, and diverge until they are reproductively isolated. In sympatric speciation reproductive isolation arises within the same area, for example by behavioural, temporal or seasonal differences in mating. The result in both cases is two populations that can no longer interbreed to produce fertile offspring.

Examples in context

Example 1. Peppered moths and industrial melanism. Before industrialisation, pale peppered moths were camouflaged on lichen-covered trees and dark forms were eaten by birds. Soot from factories blackened tree bark, so the dark form became better camouflaged, survived more, and its allele frequency rose sharply in industrial areas. As air quality improved, the pale form recovered. This is a textbook case of directional selection driven by a changing selection pressure.

Example 2. Darwin's finches and allopatric speciation. Finches that colonised separate Galapagos islands faced different food sources, so beak shape was selected differently on each island. Over many generations, populations diverged so far that they no longer interbreed even where ranges overlap. This shows geographical isolation followed by divergent selection producing new species, the core of allopatric speciation.

Try this

Q1. In a population, the frequency of the recessive phenotype ($q^2$) is 0.16. Calculate the frequency of the recessive allele q. [1 mark]

• Cue. q is the square root of 0.16, so q = 0.4.

Q2. Explain how a geographical barrier can lead to speciation. [3 marks]

• Cue. Populations are isolated, face different selection pressures, diverge genetically and eventually cannot interbreed to produce fertile offspring.

Q3. In a population, the dominant allele frequency p is 0.8. Calculate the percentage expected to be homozygous dominant and the percentage that are carriers. [2 marks]

• Cue. Homozygous dominant $p^2 = 0.64$ (64 percent); carriers $2pq = 2 \times 0.8 \times 0.2 = 0.32$ (32 percent).