Ecologists and evolutionary biologists often census population size to asses population health and to identify processes that shape evolution. For example, genetic drift plays a larger role in smaller populations. Population size, however, may not always be a good measure. For example, a population may consist of 95% sexually immature and 5% sexually mature individuals. In such cases, effective population size (N_e ) may be computed based on the number of individuals that actually take part in reproduction.

The southern elephant seal. Photo credit: Photo Rebecca Shoop, National Science Foundation.

Effective population size and census population size may be drastically different when the number of breeding males and breeding females in a population are drastically different (e.g. polygamous species and social insects). In this case, we can express effective population size using a rational function as,

where N_f and N_m are the number of breeding females and breeding males, respectively. We will also assume that,

N_f + N_m = N.