Genetic Drift
மரபணு நகர்வு
Evolution is not always about the survival of the fittest. Sometimes, it is just pure, blind luck. If you think Natural Selection explains every allele frequency change, you are walking into a trap.
The idea
Genetic drift is the change in allele frequency occurring purely by chance in a population. It is most pronounced in small, isolated populations. When a small group separates from a larger population to start a new colony, the new allele frequencies differ from the original, a phenomenon known as the Founder Effect.
Why it matters
It occurs due to random sampling errors in small populations, which can lead to the loss or fixation of alleles regardless of their adaptive value.
NEET lens
NEET focuses on the distinction between Natural Selection and Genetic Drift. Remember: Drift is random; Selection is directional. Expect questions linking the Founder Effect to small population size. Do not confuse this with Gene Flow, which involves migration.
Clinical significance
In human populations, genetic drift explains the high prevalence of rare genetic disorders in isolated communities, such as Ellis-van Creveld syndrome in the Amish. When a small group interbreeds, recessive deleterious alleles become homozygous and manifest as clinical disease. This is the biological cost of isolation.
Introduction
Genetic drift is a change in allele frequency that happens purely by chance. It has the biggest effect in small populations.
Core concept
When a small group separates from a population, its allele frequencies may differ by chance from the original. If the change is large enough to form a new species, the original drifted population becomes the founders — the founder effect. Genetic drift is one of the five factors that disturb Hardy-Weinberg equilibrium (along with gene migration, mutation, recombination and natural selection). Gene flow (migration) is the movement of genes into or out of a population; when it happens multiple times, it is called gene flow.
Go deeper
Unlike natural selection, drift is random — it does not make populations better adapted. Bottlenecks (sudden huge drops in population size) also cause strong drift.
From Class 10
Pick ten marbles at random from a jar of mixed colours — your handful may not match the jar's mix. Genetic drift is that same chance effect in a small population.