Genetic driftAQA A-Level Biology: Revision notes

Genetic drift is a random change in allele frequencies from one generation to the next, caused by chance. In every generation only some individuals survive and breed, and each passes on only a sample of its alleles to its gametes. Which alleles are passed on is partly down to luck, so allele frequencies in the next generation will differ slightly from the parent generation.

Drift does not depend on whether an allele is advantageous. A neutral or even slightly harmful allele can rise in frequency, and a beneficial one can be lost, simply by chance.

The effect of chance depends on population size.

• In a small population, each individual carries a large proportion of the gene pool. A single death or a single failure to breed changes allele frequencies noticeably. Alleles can be lost altogether.
• In a large population, chance events affect only a tiny fraction of the gene pool and tend to cancel out. Allele frequencies change very little.

Example: one vole of genotype zz dies. In a population of 50 voles (100 gene copies) 2 copies are lost, which is 2%. In a population of 5000 voles (10 000 copies) the same death removes 0.02%.

In small populations, repeated drift can cause alleles to be lost completely. This reduces genetic variation.

A population that follows a sudden fall in numbers, for example after a flood or disease, starts again from a small random sample of the original alleles. Its allele frequencies may differ from the original, and rare alleles may be missing. A population with reduced variation is less likely to contain individuals with an advantageous allele when conditions change, so it is less able to respond to a new selection pressure.

The two forces affecting genetic variation in populations are genetic drift and natural selection.

• Natural selection: acts on phenotypes, favours advantageous alleles, so its direction is predictable.
• Genetic drift: random, acts on any allele regardless of advantage, strongest in small populations.

In a large population selection is usually the more important force. In a very small population drift can outweigh selection, so even a harmful allele can become common by chance. In real populations both act together.