Genetic drift facts for kids

Genetic drift is a random change in how common certain genes are in a group of living things, called a population. It's like a game of chance that can make a population lose some of its genetic variety.

Think of it this way: In nature, there are things that add new differences to a population, like mutations (tiny changes in DNA) and recombination (when genes mix during reproduction). But there are also things that take away these differences. Genetic drift is one of those things.

What is Genetic Drift?

In any group of living things that can breed together, there's usually a lot of genetic variety. This variety comes from different versions of genes, called alleles. These alleles are found at specific spots (called loci) on the chromosomes.

Changes in how often an allele appears in a population are mostly caused by natural selection. This means some individuals survive and have more offspring because they have helpful traits. But in smaller populations, something else happens. Over time, each new generation tends to become more alike, with less variety of alleles. This is similar to inbreeding, and it makes the population more genetically uniform.

Genetic drift is the name for this loss of variety. It can have a surprisingly big effect on evolution, but only when populations are quite small. What "small" means depends on the type of organism. For example, 50 individuals is small, 5000 is not, and 500 might be. The main idea of drift is that genetic variety is reduced by chance. This makes individuals more similar and potentially more vulnerable.

• Genetic drift reduces the genetic variety in a population. This can make it harder for the population to survive new challenges.
• Genetic drift works faster and has bigger effects in smaller populations. Small populations often die out.
• If a small group survives, genetic drift might even help create new species.

Population Bottlenecks

A "bottleneck event" happens when a large population suddenly becomes much smaller. This can be caused by things like infections, extreme weather, or even invasions by other species. When the population shrinks so much, it loses a lot of its genetic variety.

• For example, in the late 1800s, hunting reduced the Northern elephant seal population to only about 20 individuals. Even though their numbers have grown back, their genetic variety is much less than that of the Southern elephant seal.
• Cheetahs also have very little genetic variety. Scientists think their species was once reduced to a very small number. Because they lack genetic differences, cheetahs are more at risk from diseases.

Founder Events

A "founder event" happens when a small group breaks away from a larger population. This small group then lives separately.

• The human species is often thought to have gone through such stages. For instance, when groups left Africa to settle in other parts of the world (see human evolution). It seems we have less genetic variety than you might expect for a species spread worldwide.
• Groups that arrive on islands far from the mainland are also good examples. Because these groups are small, they cannot carry all the different alleles found in the original population.

Related pages

• Evolution
• Natural selection
• Hardy–Weinberg law
• Population genetics

Images for kids

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  Ten examples of random genetic drift for one allele. This is shown over 50 generations in three populations of different sizes. In these examples, alleles are lost or become fixed (meaning they are the only allele left) only in the smallest population.

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  Changes in a population's allele frequency after a population bottleneck. The quick and big drop in population size has reduced the population's genetic variation.

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  When very few members of a population move to form a new, separate population, it's called the founder effect. For a while after it starts, the small population experiences strong drift. In this picture, it makes the red allele the only one left.

See also

In Spanish: Deriva genética para niños