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 |β| were exponentially distributed with an overall median of 0.16, suggesting that strong directional selection was uncommon. The values of |β| 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 |β| 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 |γ| were exponentially distributed with an overall median of only 0.10, suggesting that quadratic selection is typically quite weak. The distribution of γ values was symmetric about 0, providing no evidence that stabilizing selection is stronger or more common than disruptive selection in nature.

Selection DatabaseThe excel file has both the data, and a worksheet explaining each of the columns/fields. We would appreciate it if you would bring to our attention any errors or ambiguities that you find in the database (no doubt there are some). We will periodically update the database as needed. In addition, we are very interested in new insights you may have about selection from your own analyses of this database and relate sources. We’d appreciate it if you would tell us what you are finding, or papers that have resulted from it. Any questions or comments may be directed to: Joel Kingsolver, Department of Biology, CB-3280, University of North Carolina, Chapel Hill, NC 27599-3280, 919 843-6291 (phone) 919 962-1625 (fax) jgking@bio.unc.eduKingsolveretal.SelnDatabase.norepdur.3.2001.xls