Figure 1

Amplification of expected selection differentials in small populations. Fitness distributions for two phenotypes; the size of the dot indicates the probability of that fitness value. In (A) and (B), Individuals with phenotypic value 0 leave either 0 or 1 descendant with equal probability, and those with phenotype 1 leave either 1 or 2 descendants with equal probability. In (A), there is one individual with each phenotype. The lines show the four possible (and equally probable) combinations of fitness values, with the corresponding Δ$\overline{\varphi }$. The average change is E(Δϕ) = $\frac{7}{24}$. (B): In an infinite population evenly divided between the two phenotypes, the total contribution of individuals with phenotype ϕ = 1 will always be 3 times greater than the total contribution of individuals with ϕ = 0, yielding $\text{E}(\Delta \overline{\varphi })=\frac{1}{4}=\frac{6}{24}$, which is the prediction of classical theory. (C): Another example of fitness distributions leading to directional selection. Numbers adjacent to dots are probabilities. (D): Results of monte-carlo simulations using the fitness distributions in (C). The dashed line is the value for N = ∞.