Explain how genetic drift and gene flow change allele frequencies in populations

What this dot point is asking

You need to explain genetic drift (including the bottleneck and founder effects) and gene flow, and contrast them with natural selection as mechanisms that change allele frequencies.

Beyond natural selection

Natural selection is not the only thing that changes the genetic makeup of a population. Two other important mechanisms are genetic drift and gene flow. Both alter allele frequencies but, unlike natural selection, they are not driven by which traits are advantageous.

Genetic drift

Genetic drift is the change in allele frequencies due to random chance rather than selection. Because which individuals survive and reproduce involves an element of luck, allele frequencies can drift up or down from one generation to the next.

The key point is that drift has a much greater effect in small populations. In a large population, chance events tend to average out; in a small population, a single chance event can dramatically change allele frequencies, and alleles can even be lost entirely (reducing genetic diversity).

Two special cases:

• Bottleneck effect. A disaster (disease, fire, flood) drastically reduces the population. The survivors are a random, small sample, so the new population's allele frequencies differ from the original and genetic diversity is reduced.
• Founder effect. A small group leaves and starts a new, isolated population. Because the founders carry only a sample of the original gene pool, the new population's allele frequencies differ from the source population.

Gene flow

Gene flow (migration) is the movement of alleles between populations, when individuals (or their gametes, such as pollen) move from one population to another and breed. Gene flow:

• introduces new alleles into a population
• tends to make separate populations more genetically similar
• can counteract the genetic differences that selection or drift create between populations

If gene flow stops (for example, populations become isolated), the populations are free to diverge, which links to speciation.

How these forces compare with natural selection

A clean comparison is often worth marks. All three forces change allele frequencies, which is the definition of evolution, but they differ in cause and direction:

• Natural selection is non-random: it favours alleles that improve survival and reproduction in a given environment, so it tends to produce adaptation.
• Genetic drift is random: chance alone determines which alleles increase or are lost, so it does not produce adaptation and is strongest in small populations.
• Gene flow transfers alleles between populations: it introduces variation into a population and tends to reduce differences between populations.

Because drift is random and selection is not, the same allele could rise in frequency under selection (if advantageous) or under drift (purely by chance), so exam questions test whether you can identify which mechanism the data support.

Why conservation cares about this

Small, isolated populations are vulnerable for two linked reasons drawn from this dot point: strong genetic drift randomly loses alleles, and a lack of gene flow prevents new variation entering. The result is low genetic diversity, leaving the population poorly equipped to survive new selection pressures such as disease or climate change, and more prone to expressing harmful recessive alleles through inbreeding. This is why conservation programs deliberately maintain larger populations and sometimes move individuals between sites to restore gene flow.