What does genetic drift cause?

Define genetic drift in biology

In wild populations, there are always relatively common and relatively rare alleles. Small populations are at risk of losing genetic variation much faster than large populations.

While common alleles generally tend to remain common, rare alleles are likely to be randomly lost in subsequent generations. Consider how each parent only passes on half of their genetic code to each child; this means that the ability of the persistence of the rare allele depends on how many individuals carry it, which individuals produce offspring, and how many offspring these individuals produce.

Another important factor is population size: in any small population, only a limited number of individuals can carry a single allele, so the smaller the population, the greater the likelihood of alleles being lost to the next generation. This loss of alleles is called genetic drift.

How does genetic drift affect a population?

While genetic drift causes a loss of genetic diversity, there are instances where populations show no obvious negative effects. This may have been the case with female elephants in Addo Elephant National Park in South Africa.

Hunting killed the entire population from time to time; when they were properly protected in 1931, only 11 animals remained, including eight females. Of these eight females, at least four were tuskless, while only two or even three females had both tusks.

Over the following decades, female Addo elephants showed increasing degrees of tusklessness; in 2002, only 2% of females had tusks (in comparison, 96-98% of female elephants would normally develop tusks.

We can therefore postulate that the allele responsible for the development of tusks in female elephants has become rare and that the gradual loss of female fangs is a sign of genetic drift.

While female Addo elephants show no known boundaries for being tuskless, the loss of alleles can also be devastating to the suffering population, if, for example, the lost alleles encode traits that would have enabled a species to adapt to a changing environmental condition.

It is important to note that genetic drift differs from natural selection. That is, genetic drift involves random changes in allele frequency, while natural selection involves changes in traits in response to sexual selection or specific environmental conditions.

For example, small tusk size in some heavily hunted elephants in Africa is a selective pressure in response to hunting favoring large fangs, distinct from female Addo elephants that have lost their tusks even in the absence of selective hunting pressures.