Fig. 6.

Variation in selection efficiency. Purifying selection is increasingly inefficient for alleles of small selection coefficient s within species of relatively small effective population sizes N _e , leading to an increasing rate of accumulation of deleterious changes. The graph shows the probability of fixation of a new variant relative to the neutral expectation (Y-axis) as a function of N _e s for s = −10⁻⁵ (modified from [10]). For larger values, such as N _e  = 10⁵, the probability of fixation relative to the neutral expectation is small at approximately 7%. Nevertheless, in a population with a tenfold smaller N _e this probability rises to 81%