How full is the evolutionary fuel tank?

In 1898, Hermon Bumpus gathered 136 house sparrows immobilized by an ice storm, noting that the averages of several morphological traits differed between survivors and nonsurvivors. This was one of the first attempts to measure the phenotypic selection component of Charles Darwin’s thesis, that adaptation is driven by heritable traits that affect fitness. Since then, a vast literature on quantifying associations between trait values and fitness has emerged (1). The quantification of Darwin’s second evolution component—that such traits are heritable—required the development of quantitative genetics by Ronald Fisher in 1918 (2). Although the selection and genetics components can be combined to determine the expected change in any trait, of greater interest is the general adaptive potential of a population. On page 1012 of this issue, Bonnet et al. (3) present a meta-analysis of 19 studies showing the abundance of heritable variations in fitness and the potential for adaptation.