Figure 2.

Selection in large plankton populations. Results from a model derived from standard population genetics theory (Crow and Kimura 42) showing that substitution rate is sensitive to small selection coefficients in large populations. For slightly beneficial mutations with selection coefficient (s), the fixation probability (P) can be approximated by:

where N is diploid effective population size. When s converges to 0 (i.e., mutations are neutral) P is 1/2N and as s grows larger P becomes approximately 2s. For simplicity we assume that the substitution rate can be described as the number of mutations arising times the fixation probability. The substitution rate, relative to the neutral substitution rate, is plotted as a function of effective population size for various immeasurably small selection coefficients (ranging from 10⁻⁹ to 10⁻¹⁵). This model ignores clonal interference, that is, competition between beneficial mutations, which is expected to slow down the response to selection in asexual species (e.g., Gerrish and Lenski 59).