Figure 7.9 Stabilizing selection. (a) Stabilizing selection favors intermediate phenotypes and selects against both extremes. (b) In the sociable weaver bird, body size has a normal distribution. (c) The bird experiences stabilizing selection on body size because birds with intermediate body sizes experience high survival while birds with low and high body sizes have low survival. This selection for intermediate phenotypes would cause the next generation of birds to have a narrower distribution of phenotypes.

Data from R. Covas et al., Stabilizing selection on body mass in the sociable weaver Philetairus sociu., Proceedings of the Royal Society of London Series B 269 (2002): 1905–1909.