How strong is phenotypic selection on quantitative traits in the wild? We reviewed the literature from 1984 through 1997 for studies that estimated the strength of linear and quadratic selection in terms of standardized selection gradients or differentials on natural variation in quantitative traits for field populations. We tabulated 63 published studies of 62 species that reported over 2,500 estimates of linear or quadratic selection. More than 80% of the estimates were for morphological traits; there is very little data for behavioral or physiological traits. Most published selection studies were unreplicated and had sample sizes below 135 individuals, resulting in low statistical power to detect selection of the magnitude typically reported for natural populations. The absolute values of linear selection gradients |beta| were exponentially distributed with an overall median of 0.16, suggesting that strong directional selection was uncommon. The values of |beta| for selection on morphological and on life-history/phenological traits were significantly different: on average, selection on morphology was stronger than selection on phenology/life history. Similarly, the values of |beta| for selection via aspects of survival, fecundity, and mating success were significantly different: on average, selection on mating success was stronger than on survival. Comparisons of estimated linear selection gradients and differentials suggest that indirect components of phenotypic selection were usually modest relative to direct components. The absolute values of quadratic selection gradients |gamma| were exponentially distributed with an overall median of only 0.10, suggesting that quadratic selection is typically quite weak. The distribution of gamma values was symmetric about 0, providing no evidence that stabilizing selection is stronger or more common than disruptive selection in nature.

Directional selection is a major force driving adaptation and evolutionary change. However, the distribution, strength, and tempo of phenotypic selection acting on quantitative traits in natural populations remain unclear across different study systems. We reviewed the literature (1984 -1997) that reported the strength of directional selection as indexed by standardized linear selection gradients (␤). We asked how strong are viability and sexual selection, and whether strength of selection is correlated with the time scale over which it was measured. Estimates of the magnitude of directional selection (ͦ␤ͦ) were exponentially distributed, with few estimates greater than 0.50 and most estimates less than 0.15. Sexual selection (measured by mating success) appeared stronger than viability selection (measured by survival). Viability selection that was measured over short periods (days) was typically stronger than selection measured over longer periods (months and years), but the strength of sexual selection did not vary with duration of selection episodes; as a result, sexual selection was stronger than viability selection over longer time scales (months and years), but not over short time scales (days).ow strong is selection on quantitative traits in nature? Is strong directional selection common? Is viability selection typically stronger or weaker than sexual selection? Is selection strength correlated with the time period over which it is measured? The answers to these and similar questions are fundamental to understanding how selection determines evolutionary change and adaptation. Whereas theoretical analyses have typically modeled selection as a weak evolutionary process (1, 2), some recent empirical studies demonstrated that strong selection (3, 4) and rapid evolution (5-8) of discrete and quantitative traits may sometimes occur in natural populations. Reconciling these apparent differences through knowledge of the distribution of selection strengths will allow us to more accurately explore both the theoretical and practical consequences of selection.An early synthesis by Endler (4) revealed two important patterns about the distribution of selection strengths measured by selection differentials. First, selection differentials exhibited a roughly exponential frequency distribution, suggesting that the strength of selection in the wild varied substantially such that strong selection was not necessarily rare. Second, nonmortality and mortality components of selection on quantitative traits yielded similar selection differentials, but nonmortality components yielded higher selection coefficients (S) for discrete polymorphic traits (4). Although provocative, Endler's work used data from just 25 species and, for quantitative traits, relied on selection differentials that did not correct for indirect or correlative selection. Quantitative estimates of the strength of selection using selection gradients, which are standardized and correct for phenotypic correlations among traits (9), have become available only durin...

The light color of mice that inhabit the sandy dunes of Florida's coast have served as a textbook example of adaptation for nearly a century, despite the fact that the selective advantage of crypsis has never been directly tested or quantified in nature. Using plasticine mouse models of light and dark color, we demonstrate a strong selective advantage for mice that match their local background substrate. Further our data suggest that stabilizing selection maintains color matching within a single habitat, as

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