What other processes change allele frequencies besides natural selection?
Explain how genetic drift and gene flow change allele frequencies in populations
Genetic drift is random change in allele frequencies, strongest in small populations (bottleneck and founder effects); gene flow is the movement of alleles between populations by migration.
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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.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 SACE Stage 21 marksAfter a storm in 1775 only about 20 people survived on an isolated Pacific island, one of whom was totally colour-blind, and they re-established the population. By 2015, 10% of the population was totally colour-blind, compared with under 0.01% worldwide. The high frequency of total colour-blindness is likely due to: adaptive radiation, mutation, genetic drift, or allopatric speciation?Show worked answer →
Genetic drift (specifically the founder effect). When a small group founds a new isolated population, the alleles they happen to carry may be present at frequencies very different from the original population, just by chance. The colour-blindness allele was over-represented among the few survivors, so its frequency rose dramatically in the small isolated population. This random change in allele frequency in a small population is genetic drift, not selection or new mutation.
2019 SACE Stage 21 marksTwo small island populations of platypus are at greater risk of dying out than mainland populations. Which combination of gene pool size, genetic diversity and potential for genetic drift is most consistent with this greater risk: small / high / low; large / high / low; large / low / high; or small / low / high?Show worked answer →
The most consistent combination is small gene pool, low genetic diversity, high potential for genetic drift. Small isolated populations have a small gene pool with low genetic diversity, which leaves them less able to adapt to change. They also experience strong genetic drift, where allele frequencies change randomly and alleles can be lost, further reducing diversity and increasing the risk of extinction.
2018 SACE Stage 23 marksEvery living black robin is descended from a single fertile female that survived in 1980. Explain why the reduced genetic diversity of the black robin species increases its risk of extinction.Show worked answer →
Three linked points earn the marks.
Low genetic diversity means there is little variation in the alleles within the population's gene pool.
With little variation, it is unlikely that any individuals carry alleles that would let them survive a new selection pressure such as a disease or environmental change.
If the environment changes, few or no individuals can survive and reproduce, so the whole population may die out. (Inbreeding also raises the chance of harmful recessive alleles being expressed.) Therefore reduced genetic diversity increases the risk of extinction.