Syllabus: Paper I > 6. Anthropological Theories > Culture and Personality
Genetic Drift — 2020 Paper I
Question · 2020 · Paper I · 10 Marks
Genetic drift
Model Answer
VAID ICSApproach
- Demand of Question: Explain genetic drift as an evolutionary force and show how it alters allele frequencies, especially in small populations.
- Structuring the Response: Define drift, explain founder and bottleneck effects, and distinguish it from natural selection.
- Key Dimensions to Cover: Random allele-frequency change, small population size, Sewall Wright, founder effect, bottleneck effect and loss of heterozygosity.
Model Answer
Introduction
Genetic drift refers to random changes in allele frequencies from one generation to another due to chance sampling of gametes. Its effect is strongest in small populations and was emphasized in population genetics by Sewall Wright.
Major features
- Random process: Unlike natural selection, genetic drift does not necessarily favour alleles because they increase fitness.
- Loss or fixation of alleles: Chance alone may cause an allele to:
- disappear from a population, or
- eventually become fixed.
- Reduction of variability: Persistent drift can decrease heterozygosity within a small isolated population.
Founder effect
When a small number of individuals establish a new population, their allele frequencies may differ considerably from those of the parent population.
Parent population → Few founders → New gene pool
This is called the founder effect.
Population bottleneck
A sudden reduction in population size due to disaster, disease or migration may leave only a small, genetically unrepresentative sample of the original population.
Evolutionary significance
Drift can:
- increase differentiation among isolated populations;
- counter or reinforce selection;
- alter rare allele frequencies;
- contribute to microevolution.
Conclusion
Genetic drift demonstrates that evolution is shaped not only by adaptation but also by chance, particularly where populations are small, isolated or recently founded.
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