Genetic Drift: random changes in allele frequencies in a population from generation to generation.

founder effect: random changes in allele frequencies in a population during colonization.

Fact: founder effect occurs within the same generation.

bottleneck effect: random changes in allele frequencies in a population due to dramatic reduction of population size.

Fact: Reduction in size is caused either by some catastrophic events (density- independent factors) or only the frequencies of those loci are considered that are not under selection.