Explain how natural selection, gene flow, genetic drift and mutation change allele frequencies in populations over time

What this dot point is asking

WACE wants you to think at the level of the population, not the individual. A population evolves; an individual does not. Your task is to define the gene pool and explain the mechanisms that change allele frequencies.

Gene pools and allele frequency

A population is a group of individuals of the same species in the same area that can interbreed. The gene pool is all the alleles of all the genes in that population. The allele frequency is the proportion of a particular allele among all copies of that gene in the gene pool.

Evolution is defined as a change in allele frequencies in a gene pool over generations. If an allele becomes more common over time, the population is evolving with respect to that gene.

Natural selection

Natural selection is the main mechanism of adaptive evolution. It follows a clear logic:

1. There is variation in a trait within the population (from mutation and recombination).
2. More offspring are produced than the environment can support, so there is a struggle to survive and reproduce.
3. Individuals with alleles that suit the environment (favourable variations) are more likely to survive and reproduce; this is differential survival and reproduction.
4. Those individuals pass on their favourable alleles, so over generations the favourable alleles become more common in the gene pool.

The fitness of an individual is its relative ability to survive and reproduce in its environment, measured by how many surviving offspring it leaves.

Gene flow

Gene flow (migration) is the movement of alleles between populations when individuals or gametes move and breed. Immigration adds alleles to a gene pool and emigration removes them. Gene flow tends to make populations more genetically similar and can introduce new alleles to a population. If gene flow is blocked, populations diverge more easily.

Genetic drift

Genetic drift is a change in allele frequencies due to random chance rather than selection. It has the biggest effect in small populations, where chance events can remove or fix alleles regardless of whether they are beneficial. Two special cases matter:

• The bottleneck effect: a population is sharply reduced (by disaster or disease), and the survivors carry only a chance sample of the original alleles, reducing variation.
• The founder effect: a few individuals start a new population, so the new gene pool reflects only the alleles those founders happened to carry.

Mutation

Mutation is the original source of new alleles and so the ultimate source of all genetic variation. On its own it changes allele frequencies only slowly, but it continually supplies the new variation that selection and drift then act on. Without mutation, the other mechanisms would eventually run out of variation to work with.

Speciation

When populations of one species are separated, their gene pools can change independently through different selection pressures, drift and mutation, with no gene flow between them to keep them similar. Over many generations they may become so different that they can no longer interbreed to produce fertile offspring. At that point they are separate species. A common pathway is geographic isolation (allopatric speciation), where a physical barrier splits a population.

How this maps to the exam

Expect questions that ask you to define evolution in terms of allele frequencies, explain a named mechanism, apply natural selection to a scenario (often resistance or a changing environment), or distinguish drift from selection and the founder from the bottleneck effect. You may be asked to outline how isolation leads to speciation.