SMALL POPULATION SIZE
If the population is large, it is less likely that random fluctuations will change the allele frequencies within the population, resulting in evolutionary change. Small populations, which result from the physical isolation of a small group of individuals from the larger group or from bottlenecks (that is, the rapid reduction of a large population) are characterized by small subsets of the original varied gene pool. Influences as simple as the accidental loss of specific individuals results in shifts away from the parental population allele frequencies (genetic drift) and increases the likelihood that evolution will occur.