The Strength of Phenotypic Selection in Natural Populations

Abstract

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.

Individual fitness and phenotypic selection in age‐structured populations with constant growth rates

“…Selection gradients and differentials are popular comparative measures for studying natural selection for phenotypes (Kingsolver et al 2001). Another metric for the strength of selection is the opportunity for selection, often identified by I , defined as the variation in relative fitness among individuals in a population (or the mean-standardized variance - Crow 1958, O’Donald 1970, Wade 1979, Wade and Arnold 1980).…”

Spatially varying natural selection in a fish hybrid zone

“…This lack of knowledge results partly from the extreme difficulty of measuring the strength and describing the pattern of selection acting in natural populations (Kingsolver et al, 2001). This problem is particularly acute in studies of fishes, where estimates of fitness components (among them survival) in wild populations are notoriously difficult to obtain.…”

A New Way to Integrate Selection When Both Demography and Selection Gradients Vary over Time

“…Three reviews of the published literature have addressed this question. Kingsolver et al (2001) concluded that directional selection was typically weak, although the frequency distribution of selection gradients displayed a long right tail, indicating that selection was sometimes strong. In contrast, Hereford et al (2004) concluded that selection in nature was typically strong.…”

“…They calculated univariate variance-standardized selection gradients (which they called i , although what they calculated is the same as what Kingsolver et al [2001] call β and Hereford et al [2004] call β σ ) for selection acting through reproduction. Values across three years ranged from −0.26 standard deviation units to +0.08 standard deviation units.…”

“…However, they did not propose a method to integrate such variability across time. The invaluable comprehensive review of selection in the wild by Kingsolver et al (2001) summed up a decade and a half of studies that had been carried out since the mid 1980’s, when seminal papers by Lande (1979), Lande and Arnold (1983) Arnold and Wade (1984a, b) made measuring selection in the wild accessible to field ecologists and evolutionary biologists. This extensive review did not contain many studies with temporal replication and it contained only a handful of studies that used total fitness rather than fitness components.…”