What is genetic drift?

Genetic drift is any change to the gene pool of a population (the total set of genes they have as a group) that is NOT a caused by selection. They can be thought of as 'random' changes, unlike natural selection, where some alleles increase in frequency/percentage in the gene pool because they are more fit.An example of genetic drift is a population bottleneck, where a population suddenly gets smaller, such as after a random environmental event like a volcanic eruption. In this case, the remaining individuals may not have all the alleles that were present in the bigger starting population (some genes are lost from the pool). Also, by random chance, the surviving individuals may not be representative of the starting population. For example, we may start with 100 green snakes and 100 blue snakes (equal numbers), but by chance, after the eruption there are 30 green snakes and only 5 blue snakes remaining (many more green snakes).