When disruptive selection operates, individuals at opposite extremes of a character distribution contribute more offspring to the next generation than do individuals close to the mean, which increases variation in the population (see Figure 20.12C).

The strikingly bimodal (two-peaked) distribution of bill sizes in the black-bellied seedcracker (Pyrenestes ostrinus), a West African finch (Figure 20.15), illustrates how disruptive selection can influence populations in nature. The seeds of two types of sedges (marsh plants) are the most abundant food source for these finches during part of the year. Birds with large bills can readily crack the hard seeds of the sedge Scleria verrucosa. Birds with small bills can crack S. verrucosa seeds only with difficulty; however, they feed more efficiently on the soft seeds of S. goossensii than do birds with larger bills. Young finches whose bills deviate markedly from the two predominant bill sizes do not survive as well as finches whose bills are close to one of the two sizes represented by the distribution peaks. Because there are few abundant food sources in the finches’ environment, and because the seeds of the two sedges do not overlap in hardness, birds with intermediate-sized bills are less efficient in using either one of the species’ principal food sources. Disruptive selection therefore maintains a bimodal bill size distribution.

439