What influences the effective population size?

1. Sex ratio. If one sex is rarer, the population size of the rarer sex will dominate the changes in gene frequencies. It is much more likely that identical genes will be drawn from the rarer sex, because fewer individuals are contributing genes to the next generation.

2. Population fluctuations. If population size fluctuates, homozygosity will increase more rapidly while the population goes through a 'bottleneck' of small size. N_e is disproportionately influenced by N during the bottleneck, and a formula can be derived for N_e in terms of the harmonic mean of N.

3. Small breeding groups. If most breeding takes place within small groups, then the effective population size will be much smaller than if there is population-wide random mating (called 'panmixis'). The degree of homozygosity will be more that predicted from the size of the breeding groups than from the total population size. Breeding groups can be small either because of geographic subdivision of the population or because of non-random mating. Either way, the chance that copies of the same gene will be 'drawn' together is increased relative to the case with a large population.

4. Variable fertility. If the number of successful gametes varies between individuals the more fertile individuals will accelerate the march to homozygosity. Again, the chance that copies of the same gene will combine in the same individual in the production of the next generation is increased and the effective population size decreased relative to the total number of adults.