Genetic Drift and Gene Flow

Not all changes in allele frequency are caused by natural selection. Genetic drift and gene flow are two non-selective mechanisms that can alter the genetic composition of a population. Understanding these processes is essential for explaining patterns of genetic variation, particularly in small populations.

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What Is Genetic Drift?

Key Definition: Genetic drift is the random change in allele frequencies in a population due to chance events in survival and reproduction. It is not driven by natural selection — the alleles that increase or decrease in frequency do so randomly, not because they confer any advantage or disadvantage.

Key Features of Genetic Drift

• Genetic drift affects all populations, but its effects are most significant in small populations.
• In large populations, random fluctuations in allele frequency tend to cancel out; in small populations, they can cause large, unpredictable changes.
• Genetic drift can lead to the fixation of an allele (frequency reaches 1.0 — all individuals are homozygous for that allele) or the loss of an allele (frequency reaches 0.0).
• Drift reduces genetic diversity within a population over time.
• Drift is random with respect to fitness — it can increase the frequency of harmful alleles or decrease the frequency of beneficial alleles.

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The Bottleneck Effect

Key Definition: The bottleneck effect occurs when a population is drastically reduced in size by a catastrophic event (e.g., natural disaster, disease epidemic, habitat destruction). The surviving population is a small, random sample of the original population and may have different allele frequencies.

How the Bottleneck Effect Works

1. A large population has high genetic diversity with many alleles at various frequencies.
2. A catastrophic event (e.g., earthquake, flood, hunting) kills most of the population, regardless of genotype.
3. The surviving population is small and may, by chance, have a different genetic composition from the original population.
4. As the population recovers and grows, it carries the genetic composition of the bottleneck survivors — not the original population.
5. Some alleles may have been lost entirely; others may be at much higher or lower frequencies than before.

Real-World Example — Northern Elephant Seals

• In the 19th century, hunting reduced the northern elephant seal population to as few as 20 individuals.
• The population has since recovered to over 100,000, but genetic studies show extremely low genetic diversity.
• Almost all individuals are genetically identical at many loci — the bottleneck eliminated most of the original genetic variation.
• This low genetic diversity makes the population vulnerable to new diseases or environmental changes.

Real-World Example — Cheetahs

• Cheetahs experienced a severe population bottleneck approximately 10,000 years ago.
• Modern cheetahs have extremely low genetic diversity — individuals are so genetically similar that they can accept skin grafts from unrelated individuals without immune rejection.
• Low genetic diversity contributes to poor reproductive success and high susceptibility to disease.

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The Founder Effect

Key Definition: The founder effect occurs when a small group of individuals colonises a new area and establishes a new population. The founders carry only a small, unrepresentative sample of the alleles present in the original (source) population.

How the Founder Effect Works

1. A small number of individuals migrate from a large population to colonise a new habitat (e.g., an island, a new continent).
2. The founding population carries only a subset of the original population's alleles — some alleles may be absent, and others may be present at higher or lower frequencies.
3. As the founding population grows, its allele frequencies reflect those of the founders, not the original population.
4. Over time, the new population may become genetically distinct from the source population.

Real-World Example — Amish Community and Ellis-van Creveld Syndrome