Genetic drift

Refers to the change in the frequency of an existing gene variant, allele, in a population due to random sampling .

A population’s allele frequency is the fraction of the copies of one gene that share a particular form.

Genetic drift may cause gene variants to disappear completely and then reduce genetic variation.

It can also cause rare alleles to become much more frequent and even fixed.

When there are few copies of an allele, the effect of genetic drift is larger.

The Hardy–Weinberg principle:within sufficiently large populations, the allele frequencies remain constant from one generation to the next unless the equilibrium is disturbed by migration, genetic mutations, or selection.

In finite populations, no new alleles are gained from the random sampling of alleles passed to the next generation, but the sampling can cause an existing allele to disappear.

Genetic drift drives a population towards genetic uniformity over time.

When an allele reaches a frequency of 1 it is said to be fixed in the population and when an allele reaches a frequency of 0 it is lost.

Smaller populations achieve fixation faster.

The limit of an infinite population, fixation is not achieved.

Once an allele becomes fixed, genetic drift comes to a halt, and the allele frequency cannot change unless a new allele is introduced in the population via mutation or gene flow.

It is a random, directionless process, but it acts to eliminate genetic variation over time.

The probability that an allele will eventually become fixed in the population is simply its frequency in the population at that time.

The effective population size, which is smaller than the total population, is used to determine these probabilities.

If an allele is lost by mutation much more often than it is gained by mutation, then mutation, as well as drift, may influence the time to loss.

In natural populations, genetic drift and natural selection are always at play, together with mutation and migration.

Natural selection guides evolution towards heritable adaptations to the environment.

Genetic drift has no direction and is guided only by the mathematics of chance.

Genetic drift acts upon the genotypic frequencies within a population without regard to their phenotypic effects.

In contrast to genetic drift, natural selection favors the spread of alleles whose phenotypic effects increase survival and/or reproduction of their carriers, lowers the frequencies of alleles that cause unfavorable traits, and ignores those that are neutral.

When the absolute number of copies of the allele is small as in small populations, the magnitude of drift on allele frequencies per generation is larger.

A population bottleneck occurs when a population contracts to a significantly smaller size over a short period of time due to some random environmental event.

A bottleneck can result in changes in allele frequencies, completely independent of selection.

The loss of variation leaves populations vulnerable to any new selection pressures such as disease, climate change or shift in the available food source, because adapting in response to environmental changes requires sufficient genetic variation in the population for natural selection to take place.

A founder effect is a special case of a population bottleneck, that occurs when a small group in a population splinters off from the original population and forms a new one.

The difference in gene frequencies between an original population and colony may also trigger the two groups to diverge significantly over the course of many generations.

As the difference, or genetic distance, increases, the two separated populations may become distinct, both genetically and phenotypically.

Genetic drift but also natural selection, gene flow, and mutation contribute to this divergence.

Genetic drift is one of four factors that cause gene pools to change over time, and genetic drift is at the heart of several recent theories of evolution: mutation, migration, genetic drift, and natural selection.

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