Greater geographic Isolation, lower migration, more divergence, esp. Random effects also occur, particularly at small population sizes . Values ranging from 0.25 to 0.75 for the frequencies. In two generations an allele can be essentially eliminated at this minimum population size. Many individuals don't reproduce, and frequently the number of males that reproduces is different than the number of females.
Genetic drift is a very significant factor in changing gene frequencies at small Ne.
Over many generations, drift can also significantly effect gene frequencies in large populations. A special type of bottleneck is the Founder Effect , which occurs when a small group 'founds' a new population in an area geographically isolated from the main population, like on an island. Effects of bottlenecks are unpredictable . They are usually accompanied by selection . Bottlenecks can result in rapid appearance of new combinations of gene frequencies, and are a likely mechanism for speciation.