For mutations on which natural selection can act (i.e., those
with s != 0, Box 2), the NeRR depends on the fitness effects of
mutations (s, Figure 1). As Ne increases, natural selection
becomes more effective at fixing advantageous mutations
and removing deleterious mutations, but larger populations
also produce more of both types of mutation. Theory sug-
gests that as Ne increases the power of natural selection
increases faster than the production of new mutations (see
 for a recent review). This results in lower deleterious
substitution rates as Ne increases (a negative NeRR,
Figure 1B,D), and higher advantageous substitution rates
as Ne increases (a positive NeRR, Figure 1A,C). However,
these predictions can sometimes be altered when the sim-
plifying assumptions of the underlying theory are not met.
Trends in Ecology & Evolution Volume 29, Issue 1, January 2014, Pages 33–41
Population size and the rate of evolution
Robert Lanfear et al.
Does evolution proceed faster in larger or smaller populations? The relationship between effective population size (Ne) and the rate of evolution has consequences for our ability to understand and interpret genomic variation, and is central to many aspects of evolution and ecology. Many factors affect the relationship between Ne and the rate of evolution, and recent theoretical and empirical studies have shown some surprising and sometimes counterintuitive results. Some mechanisms tend to make the relationship positive, others negative, and they can act simultaneously. The relationship also depends on whether one is interested in the rate of neutral, adaptive, or deleterious evolution. Here, we synthesize theoretical and empirical approaches to understanding the relationship and highlight areas that remain poorly understood.