Abstract
In population genetics idealized Wright-Fisher (WF) populations are generally considered equivalent to real populations with regard to the major evolutionary processes that influence genotype and allele frequencies. As a result we often model the response of populations by focusing on the effective size N (e) . The Diversity Partitioning Theorem (DPT) shows that you cannot model the behavior of a system solely on the basis of a diversity (accounting for unevenness among items) without taking richness into account. I show that the census population size (the number of adults, N (c) ) is equivalent to a richness, and that the effective size N (e) is equivalent to a true diversity. It follows logically from the DPT that we require both N (e) and N (c) to understand how drift, selection, mutation, and gene flow interact to shape the course of evolution of populations. Here I review evidence that both N (c) and N (e) affect evolutionary trajectories of populations for neutral and adaptive processes. This also influences how we should consider evolutionary potential and genetic criteria for conservation of populations. The effective size of a population is of huge importance in evolutionary biology, but it should not be the sole focus when population size is concerned. Applied evolutionary studies need to integrate N (c) in the equation more consistently when modeling the response to selection, mutation, migration, and drift.