Wild populations are smaller than we think: a commentary on 'Effective population size/adult population size ratios in wildlife: a review' by Richard Frankham.

where H0 is the heterozygosity of the population before the bottleneck, Ht is the heterozygosity after t generations of maintenance with 2N individuals and F is the inbreeding coefficient (Falconer & Mackay, 1996). Clearly heterozygosity decreases, and inbreeding increases, as N decreases ; and these effects accumulate over time. However, no real population fits the ideal model on which this theory is based, which includes self-fertilization in random amounts. The concept of effective population size enables us to utilize this expression by replacing the N in the equation with Ne, where Ne, the effective population size, is the number of individuals that would give rise to the same variance in gene frequency or rate of inbreeding as an ideal population of that size (Falconer & Mackay, 1996). Major departures from the ideal population model that affect Ne are unequal numbers of males and females, unequal numbers of individuals in different generations, non-random distribution of family size, and overlapping generations. Analytical expressions relating the census size of the population (N) to the effective population size have been derived for each of these cases (Frankham, 1995; Falconer & Mackay, 1996) ; under most scenarios Ne