7 - Phenotypic Change in Experimental Lineages

Summary

Analysis of 14 selection experiments in 8 published studies yields 672 step rates quantifying high- and low-line selection from one generation to the next. The median step rate for this sample, in haldanes, is h0 = 0.33 standard deviations per generation on a time scale of one generation. High-line experiments selecting for increases in trait value yield virtually the same rates as low-line experiments selecting for decreases in trait value, with median h0 values of 0.32 and 0.35, respectively. Both are near the selection median of h0 = 0.33. Control lineages not subject to experimental selection have a median h0 = 0.23. Control lineages with a substantial number of generations have temporal scaling slopes showing that they are stationary rather than random, which is attributable to stabilizing natural selection. What separates stationary lineages responding to stabilizing selection from directional lineages responding to directional selection is the pattern of change in their signs. Step rates observed in selection experiments (high lines, low lines, and control lines) exceed ‘null’ rates expected for purely random change by factors ranging from 2 to 100.