Population Size in Evolutionary Biology Is More Than the Effective Size

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.