Genetic Drift (Or Wright’s Effect)

Theories of Evolution of Class 12

It is an evolutionary force operating mainly in small populations.

Gene frequency in small population, changes purely be chance.

In small size population, some genes may be reduced in frequency or even lost by chance and others may be increased in frequency by chance.

It upsets Hardy Weinberg equilibrium.

In small populations genetic drift fixes or preserves certain genes and eliminates the other genes completely.

A new mutation arising in a small population is either lost or fixed as a result of genetic drift.

Genetic drift tend to preserve or destroy genes without distinctions, whether favourable, neutral or unfavourable. Hence drift works in opposite to selection (Natural selection).

Heterzygous Homozygous.

It produces variation, but reduces overall genetic variability.

Helps in the origin of new species.

It results in non-adaptive trait in some population.

Heterzygous Homozygous.
It produces variation, but reduces overall genetic variability.
Helps in the origin of new species
It results in non-adaptive trait in some population.

Genetic drift Natural selection

Operates in only small population.

Operates in all populations bsoth large and small

Genetic drift fixes or eliminates any Natural selection fixes up only adaptive characters and character by chance irrespective of its Eliminates non-adaptive trait adaptive values.

Founder Effect : New colony formed in split population by fixing only few genes is called Founder. After settling in new habitat the phenotype quickly changes from parental type due to genetic drift forming a new species, this phenomenon is called founder effect.

Bottle Neck Effect : Sometimes population size is drastically reduced (= population crash) causing change in allele frequencies. If in this case the gene pool in already limited then population can not regain its former richness. Such reduced allele frequency is called Genetic bottle Neck - that may cause extinction.

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