Genetic drift

revision notes on genetic drift

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  • Created on: 11-05-13 16:43
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What is genetic drift and why can it cause large changes in small populations?
Genetic drift is also known as the Sewell-Wright effect.
Genetic drift refers to the random changes in allele frequency that occurs in all
populations.
Evolution occurs due to genetic drift, chance dictates which alleles are passed on to
the next generation.
Example:
1. Individuals within a population show variation in their genotypes e.g. A and B.
2. By chance the allele for one genotype (B) is passed on to the offspring more than
the other genotype.
3. The number of individuals within allele B increases.
4. If by chance the allele is passed on more again and again then it will lead to evolution
because that allele will become more common within the population.
Textbook definition: Also called allelic drift. The change in allele frequency in a
population, as some alleles pass to the next generation and some disappear. This
causes some phenotypic traits to become rarer or more common.
Genetic drift is where for various reasons not all individuals are able to contribute
their genes to the next generation.
Genetic drift is much more pronounced in small populations.
In a small population, the effect of a few individuals not contributing their alleles to
the next generation can have a great effect on allele frequencies.
Evolution by genetic drift usually has a greater effect in smaller populations because
chance has a greater influence.
For various reasons not all individuals contribute alleles to the next generation, the
random loss of these alleles causes a change in the allele frequency from one
generation to the next.
The change in frequency is directionless, there is no selecting force.
In a small population alleles may even become lost from the gene pool altogether,
when the frequency becomes 0%.
Fluctuations are more severe in smaller breeding populations because random
changes in a few alleles can cause a greater percentage change in allele frequencies.
In a very small breeding population fluctuations are so extreme that the allele can
become fixed (frequency of 100%) or lost from the gene pool altogether (frequency
of 0%).
The allele combination for each generation is determined by how many alleles of each
type are passed on.
Genetic drift can lead to the chance elimination of one allele from the population.
Genetic drift could lead to the extinction of a population or to the production of new
species.

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