The importance of genetic drift in causing changes in allele frequency in…

The idea

Genetic drift is a change in allele frequency caused by chance rather than by selection. Only a sample of each generation's alleles is passed on, and in a small population that sample can easily be unrepresentative, in the same way that ten coin tosses can give seven heads while a thousand tosses will not stray far from half. So an allele can become more common, or vanish altogether, even though it gives no advantage, and a useful allele can be lost. The smaller the population, the larger the effect, which is why drift matters after a genetic bottleneck, when a population crashes and the survivors carry only part of the original variety, and in the founder effect, when a few individuals colonise a new area so that their allele frequencies, rather than those of the parent population, become the starting point.

What earns the marks

• Genetic drift as a change in allele frequency due to chance rather than selection.
• Stating that the effect is greater in a small population.
• Explaining that only a sample of the alleles present is passed to the next generation.
• An allele becoming more or less frequent, or being lost entirely, even though it gives no advantage.
• The consequence, a reduction in the genetic diversity of the population.

Common mistakes and how to put them right

• Explaining drift as selection in disguise, by giving the allele an advantage.

• Saying drift occurs equally in large populations, when the whole point is that the effect depends on population size.

• Confusing the founder effect, a few individuals starting a new population, with a bottleneck, a crash in an existing one.

• Claiming that genetic drift creates new alleles, when it only changes the frequency of alleles already present.

• Assuming drift always removes harmful alleles.

Exam tips

• Use the word chance or random early in the answer, then add in a small population.
• Link drift to the Hardy-Weinberg condition it breaks, which is the requirement for a large population.
• In a conservation question, connect reduced genetic diversity to a reduced ability to respond to a new selection pressure.