Brock Harpur > Consider a new allele acted on by strong positive selection. Selection increases the frequency of this allele within a population over time, fixing it in the population (assuming that the population size is sufficiently large and that the stochastic influence of genetic drift on allele frequencies is sufficiently small).
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Population Size Matters
The classification of mutations into three distinct types—deleterious, neutral, and advantageous—is of course an oversimplification. In reality, there is a continuum from highly deleterious to weakly deleterious, nearly neutral, neutral, weakly advantageous, and strongly advantageous mutations. It is important to note that the effectiveness of selection on a mutation depends both on the fitness effect of this mutation (the selection coefficient S) and on the effective population size (Ne).
Specifically, when the product NeS (Ne x S) is much less than 1, the fate of mutations is essentially determined by random genetic drift. In other words, in small populations, the stochastic effects of random genetic drift overcome the effects of selection. Thus, all mutations for which NeS is much less than 1 can be considered effectively neutral. This implies that the proportion of neutral mutations is expected to inversely vary with a taxon's effective population size.
Empirical data are consistent with this prediction. For example, in Drosophila species (where Ne is about 10,000,000), the proportion of nonsynonymous substitutions that have been fixed by positive selection is about 50%. Contrast this with the data for hominids (with Ne around 10,000 to 30,000), where this proportion is close to zero. Similarly, the proportion of nonsynonymous mutations that are effectively neutral is less than 16% in Drosophila, whereas it is about 30% in hominids (Eyre-Walker & Keightley, 2007).
Duret, L. (2008) Neutral theory: The null hypothesis of molecular evolution. Nature Education 1(1):218
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