Genetic drift is defined as the changing of the number of available alleles in a population by chance events. Also called allelic drift, this phenomenon is usually due to a very small gene pool or population size. Unlike natural selection, it is a random, chance event that causes genetic drift and it depends solely on statistical chance instead of desirable traits being passed down to offspring. Unless the population size increases through more immigration, the number of available alleles gets smaller with every generation.
Genetic drift happens by chance and can make an allele disappear completely from a gene pool, even if it was a desirable trait that should have been passed down to offspring. The random sampling style of genetic drift shrinks the gene pool and therefore alters the frequency the alleles are found in the population. Some alleles are completely lost within a generation due to genetic drift.
This random change in the gene pool can affect the speed of evolution of a species. Instead of taking several generations to see a change in allele frequency, genetic drift can cause the same affect within a single generation or two. The smaller the population size, the greater the chance of genetic drift occurring. Larger populations tend to function through natural selection much more than genetic drift due to the sheer number of alleles that are available for natural selection to work on as compared to smaller populations. The Hardy-Weinberg equation cannot be used on small populations where genetic drift is the main contributor to diversity of alleles.
One specific cause of genetic drift is the bottleneck effect, or population bottleneck. The bottleneck effect occurs when a larger population shrinks significantly in size in a short amount of time. Usually, this decrease in population size is generally due to a random environmental affect like a natural disaster or spread of disease. This rapid loss of alleles makes the gene pool much smaller and some alleles are completely eliminated from the population.
Out of necessity, populations that have experienced population bottleneck increase the instances of inbreeding to build the numbers back up to an acceptable level. However, inbreeding does not increase diversity or numbers of possible alleles and instead just increases the numbers of the same types of alleles. Inbreeding can also increase the chances of random mutations within DNA. While this may increase the number of alleles available to be passed down to offspring, many times these mutations express undesirable traits such as disease or reduced mental capacity.
Another cause of genetic drift is called founders effect. The root cause of founders effect is also due to an unusually small population. However, instead of a chance environmental effect reducing the numbers of available breeding individuals, the founders effect is seen in populations who have chosen to stay small and do not allow breeding outside of that population.
Often, these populations are specific religious sects or offshoots of a particular religion. The mate choice is significantly reduced and is mandated to be someone within the same population. Without immigration or gene flow, the number of alleles is limited to only that population and often the undesirable traits become the most frequently passed down alleles.
An example of founders effect happened in a certain population of Amish people in Pennsylvania. Since two of the founding members were carriers for Ellis van Creveld Syndrome, the disease was seen much more often in that colony of Amish people than the general population of the United States. After several generations of isolation and inbreeding within the Amish colony, the majority of the population became either carriers or suffered from Ellis van Creveld Syndrome.