Phenotypic selection in natural populations: what limits directional selection?

Studies of phenotypic selection document directional selection in many natural populations. What factors reduce total directional selection, and the cumulative evolutionary responses to selection? We combine two datasets for phenotypic selection, representing more than 4600 distinct estimates of selection from 143 studies, to evaluate the potential roles of fitness trade-offs, indirect (correlated) selection, temporally varying selection, and stabilizing selection for reducing net directional selection and cumulative responses to selection. We detected little evidence that trade-offs among different fitness components reduced total directional selection in most study systems. Comparisons of selection gradients and selection differentials suggest that correlated selection frequently reduced total selection on size, but not on other types of traits. The direction of selection on a trait often changes over time in many temporally-replicated studies, but these fluctuations have limited imp...