Genetic drift, Founder effect, Bottleneck Effect?

Wikipedia is your friend:

Genetic drift is the accumulation of random events that change the makeup of a gene pool slightly, but often compound over time. More precisely termed allelic drift, the process of change in the gene frequencies of a population due to chance events determine which alleles (variants of a gene) will be carried forward while others disappear.

Founder effect refers to the loss of genetic variation when a new colony is established by a very small number of individuals from a larger population. As a result of the loss of genetic variation, the new population may be distinctively different, both genetically and phenotypically, from the parent population from which it is derived. In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species.

Bottleneck effect is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing, and the population is reduced by 50% or more, often by several orders of magnitude. Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the population size. They also increase inbreeding due to the reduced pool of possible mates.