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.

Geographic variation in the G matrices of wild populations of the barn swallow

“…Selection on morphological components has been demonstrated to occur in a wide range of organisms (Endler, 1986;Fairbairn and Reeve, 2001;Kingsolver et al, 2001) and in various species of birds (Holland and Yalden, 1995;Brown and Brown, 1998;Larsson et al, 1998;Merila et al, 1999;Balmford et al, 2000;Barbraud, 2000;Nowakowski, 2000;Przybylo et al, 2000), including barn swallows (M ller, 1993;M ller and Tegelstrom, 1997;M ller et al, 1998;Brown and Brown, 1999). In Nebraska, a period of severe weather resulted in size-related mortality of barn swallows (Brown and Brown, 1999) indicating that, as with other bird species natural selection acts upon at least some morphological components.…”

“…Effective population sizes are frequently small enough that significant drift can be expected to occur, particularly over long periods of time (Lande, 1976;Lynch, 1990; Chapter 8 in Roff, 1997). Estimates of selection coefficients in wild populations show that the strength of selection varies widely from weak to very strong (Endler, 1986;Kingsolver et al, 2001). Thus, these data indicate that the assumption that the G or P matrix will be invariant cannot be justified on theoretical grounds alone but must be verified empirically (Turelli, 1988;Arnold, 1992).…”

Towards a unified science of cultural evolution

“…Selection gradients have become a common currency within evolutionary biology for estimating the strength and mode of selection, and for making comparisons between populations (e.g., Endler 1986;Hoekstra et al 2001;Kingsolver et al 2001). The actual methods used to obtain these measures are varied.…”

The evolution of social learning rules: Payoff-biased and frequency-dependent biased transmission

“…This assumption is often employed for mathematical convenience, yet in reality, cultural dynamics can take many generations to reach a stable state (CavalliSforza and Feldman, 1981;Boyd and Richerson, 1985) while biological evolution can be fast (Kingsolver et al, 2001). Moreover, the trajectory of genetic evolution may be influenced by the frequency of the learned trait prior to equilibrium.…”