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Genetic drift

Genetic drift is the change in allele frequency due to random events (or random sampling). It occurs alongside natural selection and leads to evolution.

Unlike natural selection, genetic drift is not an adaptive process (i.e. it does not make typical members of the species more adapted to their environment). It can influence all alleles regardless of whether they experience selection pressure.

When the population is large, genetic drift has little effect on allele frequencies. However, in a small population, random sampling can have a significant effect on the change in allele frequencies between generations.

Genetic drift is a random process: it is the random change in allele expression frequencies.

For example, an organism may die before reaching reproductive age for reasons unrelated to its fitness.

An organism may be hit by lightning bolt or caught up in a natural disaster. Such incidents are chance events that could hit any member of a population equally.

Genetic drift explains how disadvantageous alleles can become common in small populations or in a population that grew from a very small number of individuals.

In a population bottleneck the number of members of a species suddenly falls to a very low level due to an event such as a drought, or the migration of a small group.

After an event like this it is not uncommon to see disadvantageous alleles spread through the population. A common case study is the population of Pingelap.

On this small Micronesian island, about 10% of the population suffer from complete colour blindness (they can only see in black and white). This is significantly higher than the normal average of 0.003%.

This is because a typhoon swept through the island in 1775, killing all but 20 of its inhabitants.

One of the survivors carried the recessive allele for colour blindness. As the population grew, genetic drift meant that the allele spread rapidly.

There are three main mechanisms that drive evolution: natural selection, genetic drift and mutations.

  • Natural selection occurs when two groups that have different alleles also have different rates of survival and reproduction.

    Giraffes with longer necks have a higher chance of survival during periods of food shortage than giraffes with shorter necks. Natural selection favours alleles leading to longer necks.

  • Genetic drift causes random changes in the allele frequency of a population. This is particularly relevant for small populations.

    There may only be a single carrier of an allele for white fur in a population of mammals. If that individual dies before reproducing, the white fur allele will be extinguished from the population.

  • Mutations cause new alleles to form. Different mutations may arise separately in two populations. If there is no gene flow between the populations, the two populations evolve with distinct sets of alleles.

Genetic drift and natural selection are both fundamental processes in evolution. There is a significant difference between the two.

Genetic drift is a random process: it is the random change in allele frequencies.

There is a certain degree of chance involved in determining whether alleles are passed on.

Genetic drift is not an adaptive process, meaning that it plays no role in making the species more suited to its environment.

Natural selection is non-random. It alters only the frequency of alleles that pose a selective advantage to the organism. Natural selection is an adaptive process.

In bacteria, natural selection has led to an adaptation that increases their survival rates.

There has been an increase in the frequency of alleles that confer antibiotic resistance. These genes directly increase the survival and reproductive success of the organism that carries them.

This is a non-random adaptive change.

Genetic divergence is the independent change in allele frequencies in two separated populations.

Genetic divergence can be due to random mutations, natural selection or genetic drift, or a combination of all three.

This divergence can cause new species to form if the genes become sufficiently different, or it may simply lead to different subpopulations.

Genetic divergence has led to the formation of several species of finch.
Genetic divergence has led to the formation of several species of finch.

The evolutionary theory of punctuated equilibrium states that for the most part, species evolve very little. When they do speciate, it occurs in a very short time period due to strong selection pressure.

The theory tries to explain why there is little evidence for gradual changes in the fossil record.

The theory of punctuated equilibrium was introduced by the evolutionary biologist Stephen Jay Gould, but is not accepted by all biologists.

Periods of rapid evolution and speciation can be seen on this diagram.
Periods of rapid evolution and speciation can be seen on this diagram.

Genetic variation ( the diversity of alleles in a population) is essential for evolution.

However, genetic drift and natural selection may reduce the diversity of genetic material in a species.

Genetic drift can cause the loss of some alleles, particularly from small populations. If these particular alleles 'drift out', they cannot be replaced unless a mutation reintroduces them.

Natural selection is far less effective than genetic drift at removing low levels of genetic variation in the environment.

Natural selection acts on the phenotype but not the genotype. It cannot remove alleles that are not expressed in the phenotype.

Low levels of disadvantageous recessive alleles can remain in a population as natural selection cannot select against them unless they are expressed in the phenotype.

The neutral theory of molecular evolution states that the vast majority of evolution that occurs on a molecular level (e.g. evolution of DNA sequence) is due to genetic drift and random mutations.

Therefore most molecular-level evolution is selectively neutral. In other words, it confers no survival advantage to the organism and is not impacted by natural selection.

This theory can be used to estimate relatedness of species and how long ago they diverged.

Mutations occur at random but at a constant rate. It is possible to count the number of differences in equivalent strands of non-coding DNA (i.e. DNA that is assumed not to affect natural selection).

The number of differences indicates the number of mutations that have occurred and can give an estimate of how long ago the two species diverged.