AQA A-Level Biology: Genetic Diversity and Natural Selection

Genetic variation is the raw material of evolution, and natural selection is the process that shapes it. This guide covers what genetic diversity is, what raises or lowers it, and how natural selection changes a population over time, including the different patterns selection can take.

Genetic diversity

Genetic diversity is the number of different alleles of genes in a population. It matters because it is what makes natural selection possible: in a given environment, a new allele may give its owner an advantage, and the more alleles there are, the more likely it is that some will suit a changing environment.

Genetic diversity is increased in two main ways. Mutations create entirely new alleles. Gene flow, the movement of alleles between populations, adds diversity when individuals migrate into a population and reproduce, bringing new alleles with them.

Events that reduce genetic diversity

Some events cut genetic diversity sharply by reducing a population to a small number of individuals.

A genetic bottleneck is an event that causes a big drop in population size, for example a natural disaster or disease. Because so few individuals survive, many alleles are lost from the gene pool, so genetic diversity falls.

The founder effect occurs when a small group breaks away from the main population and starts a new one, for example by colonising a new area. The small founding group carries only a non-representative sample of the original population's alleles, so some alleles are missing from the start, and genetic diversity is lower. Because the group is small and may be closely related, inbreeding can follow, and genetic diseases can become more common.

Both processes leave a small population in which allele frequencies then change by chance, a process called genetic drift: in any small population, alleles can become more or less common simply because of which individuals happen to survive and reproduce. The difference between the two is the cause: in a bottleneck individuals are killed, while in the founder effect individuals become ecologically separated.

Evolution and natural selection

Evolution is the change in allele frequency (how common an allele is) in a population over many generations. It happens through natural selection, the process by which better-adapted organisms tend to survive and reproduce while less well-adapted ones tend not to.

Natural selection follows a clear logic:

1. Mutation produces a new allele of a gene.
2. In a particular environment, that allele gives its owner an advantage.
3. Those individuals are more likely to survive and to reproduce successfully.
4. They pass the advantageous allele on to their offspring.
5. Over many generations, the frequency of that allele increases in the population.

Types of adaptation

An adaptation is a feature that increases an organism's chance of survival, and adaptations come in three kinds. Anatomical adaptations are physical or structural features, such as a thick coat. Physiological adaptations are internal processes or chemical reactions, such as producing venom. Behavioural adaptations are the ways an organism acts, such as migrating in winter.

Types of selection

Selection does not always push a population in one direction. There are two patterns to know.

Directional selection happens when the environment changes, so that individuals with a trait at one extreme are now favoured. For example, when an antibiotic is introduced, bacteria with high resistance survive best. The frequency of the extreme trait rises, and over time the whole distribution shifts towards it, so the mean phenotype changes.

Stabilising selection happens in a stable environment, where individuals near the average are favoured and both extremes are selected against. Human birth weight is a classic example: very small and very large babies are less likely to survive, so selection favours an average weight. The mean phenotype stays the same, but variation around it is reduced.

Genetic diversity, and the ways of measuring it, connects closely to biodiversity and variation, covered in the biodiversity and variation guide.