Mechanisms of Evolution
Genetic Drift
Genetic drift is a non-adaptive mechanism of evolution in which chance/random events results in a change in allele or genotypic frequencies in a population. Imagine having a bowl of skittles, where each color skittle represented a different genotype. If you were to reach into the bowl and take out a handful of skittles, it's entirely possible, and depending on how many skittles you take out, even likely, that the proportion of colors in your hand is different than the proportion in the bowl; that is essentially how genetic drift works. This process has a stronger effect on small populations than it does on large populations.
There are two major types of genetic drift.
The Bottleneck Effect
The bottleneck effect occurs when there is a serious decrease in population size that results in a decrease in genetic variation. The variation that is lost is lost due to chance, not because it had a lower fitness. It is possible for a bottleneck to change the prevalence of alleles, and even for it to cause alleles to be lost. Even if the population quickly rebuilds, the effect on genetic diversity can last for a very long time (often for thousands of generations).
The Founder Effect
Allelic variation can be lost by chance without the original population shrinking as well. Something known as the founder effect occurs when a new population is founded by a small sample from a much larger population. This can result in the new population having a fraction of the variation and very different frequencies of alleles and genotypes when compared to the original population.
