Use the Hardy-Weinberg principle and describe the factors that change allele frequencies.

What this dot point is asking

Gene pools and allele frequencies

A population is a group of interbreeding organisms of the same species in an area. The gene pool is the total of all alleles for all genes in that population. Evolution, in genetic terms, is a change in allele frequencies in a gene pool over generations (often called microevolution).

Allele frequency is the proportion of a particular allele among all copies of that gene in the population. If 70 per cent of the alleles for a gene are the dominant form and 30 per cent are recessive, the allele frequencies are 0.7 and 0.3.

The Hardy-Weinberg principle

The Hardy-Weinberg principle describes the allele and genotype frequencies expected in a population that is not evolving. It acts as a null model: if real frequencies match the prediction, no evolutionary force is acting on that gene; if they do not, something is changing the population.

For a gene with two alleles, let p be the frequency of the dominant allele and q the frequency of the recessive allele. Then:

• p + q = 1 (the allele frequencies add to 1).
• p squared + 2pq + q squared = 1 (the genotype frequencies add to 1).

Here p squared is the frequency of homozygous dominant individuals, 2pq is the frequency of heterozygotes, and q squared is the frequency of homozygous recessive individuals.

The principle only holds under five conditions: no mutation, no natural selection, a very large population (no genetic drift), no gene flow (no migration), and random mating. Because these are rarely all true, the value of the principle is in showing when a population is changing.

Forces that change allele frequencies

Five processes can shift allele frequencies and so cause evolution:

• Mutation: introduces new alleles into the gene pool. It is the ultimate source of variation but changes frequencies slowly on its own.
• Natural selection: favours alleles that increase fitness, raising their frequency over generations.
• Genetic drift: random changes in allele frequency due to chance, strongest in small populations where chance events can remove or fix alleles. The founder effect (a new population started by a few individuals) and the bottleneck effect (a sharp population crash) are examples that reduce genetic diversity.
• Gene flow (migration): movement of alleles between populations as individuals or gametes move, which tends to make populations more similar.
• Non-random mating: when mate choice depends on traits, certain genotype combinations become more common, changing genotype frequencies.

Genetic diversity and survival

Populations with high genetic diversity have more variation for selection to act on, so they are more likely to include individuals that can survive environmental change. Small populations lose diversity through drift and inbreeding, which raises extinction risk. This is why conservation programs try to maintain large, genetically varied populations and gene flow between them.