6 - Directional Selection, Stabilizing Selection, and Random Drift

Summary

Response to artificial or natural selection can be expressed in terms of a selection differential in measurement units, or a selection intensity in standard deviation units. Heritability determines the proportional relationship of response to selection in both cases. Truncation is the most efficient form of directional selection. The response is independent of population size and the response curve increases indefinitely. Gradient selection is less efficient. The response is independent of population size but the response curve for a linear gradient is limited at R < 0.2 ∙ h2 ∙ σ. Stabilizing selection requires a balance of truncation or gradient selection (or both). Stability in an evolutionary time series can also be achieved through a succession of directional corrections, with the response in each generation determined by the difference between the mean at the time and some optimal value. Random drift is acutely sensitive to sample size and inherently self-balancing. Consequently, random drift has much less power to move a mean or to constrain it than any form of mass selection.