Gene pools and selectionEdexcel A-Level Biology B: Revision notes

A gene pool is all of the alleles of all of the genes in a population at one time. The allele frequency is how often an allele occurs in the gene pool, given as a proportion (between 0 and 1) or a percentage. Evolution is a change in allele frequencies in a population over generations.

Allele frequencies can change because of selection, which is not random, or because of chance, which is genetic drift. Both processes act on the variation that is produced by mutation and meiosis.

A selection pressure is a factor, such as predation, disease or food supply, that affects the survival and reproduction of individuals. Individuals with phenotypes that give an advantage are more likely to survive and reproduce, so they pass on their alleles more often and the allele frequencies change.

Stabilising selection favours the average phenotype and acts against both extremes. It reduces the range of variation and keeps the mean the same, so it maintains continuity in a population in a stable environment. An example is human birth mass, where babies of very low or very high mass have a higher death rate.

Disruptive selection favours both extremes of the phenotype range and acts against the average. The distribution can develop two peaks, so the frequencies of alleles for the extremes increase and those for the average decrease.

If the two groups become isolated so that they cannot interbreed successfully, the population can split into two species. This is speciation. Examples are birds on islands with only small or large seeds, where medium-beaked birds feed less efficiently.

Genetic drift is a random change in allele frequencies from one generation to the next, due to chance rather than selection. By chance, some individuals have more offspring than others, so some alleles are passed on more often.

Drift has a much bigger effect in small populations, where chance events can make an allele disappear (frequency 0) or become fixed (frequency 1). In large populations, chance effects average out.

A population bottleneck is a sharp fall in the size of a population, for example because of disease, natural disaster or hunting. The survivors carry only a sample of the alleles in the original gene pool, so genetic variation is reduced. When the population recovers, it descends from the survivors, so variation stays low and allele frequencies may be different from before.

The founder effect is when a small group leaves a population and starts a new one. The founders carry only a sample of the alleles of the original population, so the new population has lower variation and different allele frequencies. Both of these are examples of genetic drift because chance decides which alleles survive.

The Hardy-Weinberg equation is used to calculate allele and genotype frequencies in a population. For a gene with two alleles, with frequencies $p$ (dominant) and $q$ (recessive):

$p + q = 1 \qquad p^{2} + 2pq + q^{2} = 1$

$p^{2}$ is the frequency of homozygous dominant, $2pq$ of heterozygous and $q^{2}$ of homozygous recessive individuals.

It assumes a large population, random mating, no mutation, no migration and no selection. If allele frequencies change between generations, one of these conditions is not being met.

Worked example: a recessive condition affects 1 in 10 000 people. $q^{2} = 0.0001$, so $q = 0.01$ and $p = 0.99$. The carrier frequency is $2pq = 2 \times 0.99 \times 0.01 = 0.0198$.