8 - Phenotypic Change Documented in Field Studies

Summary

Analysis of 57 field studies yields a total of 814 independent step rates quantifying change from one generation to the next. The median step rate for field studies is h0 = 0.15 standard deviations per generation on a time scale of one generation. Step rates following directional selection in a laboratory setting have the same range of values as step rates observed in field studies. This consistency shows: (a) experimental selection is not artificial, but represents what we see in field studies; and (b) the change we see in field studies is what we expect for experimental and hence natural selection. Rapid change in a field study is change lying above a line fit to an empirical LRI distribution of step rates and base rates: change lying above Y = −0.784 ∙ X − 0.787, where Y is log10 of the rate r and X is log10 of the corresponding interval i. Empirically, the proportion of zero rates in field studies is 1.34%, mostly due to rounding, and zero rates have negligible effect on the rate statistics presented here. The anthropologist’s ‘secular trend’ of increasing human stature has rates in the range found here for evolution by natural selection.