Inheritance and natural selection on functional traits

We surveyed the literature published since 1985 for evidence of natural selection and heritability in vegetative functional traits and performance. Our goals were to (1) review patterns of selection on specific functional traits and (2) assess general evolutionary questions about selection and heritability for broad classes of traits. While generalizations about the functional significance of specific traits are premature, several functional hypotheses are supported. For example, herbivores can exert strong selection on secondary chemistry and mechanical defenses, but costs of resistance and negative correlations between defense traits may constrain their evolution. Competitive interactions select for early germination and favor stem elongation and shifts in flowering time where such responses actually minimize competitive effects. In the very few studies of physiology, selection on gas exchange and leaf size is clearly environment dependent. More generally, in reciprocal transplant experiments, populations often are locally adapted, and selection favors the native phenotype. These results suggest that selection is important in functional trait evolution and population differentiation. At the same time, selection often varies in space and time and across life-history episodes. This variation could slow the rate of evolutionary change, maintain genetic variation within populations, and select for plasticity. Analyses of general questions revealed that indirect selection through correlated characters accounts for a substantial portion of total selection on traits and often appears to reinforce the pattern of direct selection. This could be due to environmental effects on multiple phenotypic traits and fitness. Alternatively, indirect selection could contribute to the rapid evolution of suites of traits. We found only weak evidence that traits under strong selection have low heritability, a pattern that has been reported for animals and predicted by some theory. Thus, the rate of evolutionary change may well differ among traits. The strength of selection also depended on the fitness measure, being stronger selection through cumulative fitness than fertility or vegetative performance. Attributes of species' biology and experimental design affected selection and heritability estimates. Heritability was lower in inbreeding species relative to outbreeders, as expected. Heritabilities in controlled environments substantially overestimate estimates from the wild and should not be used as reliable predictors of the rate of adaptive evolutionary change in natural populations. Likewise, broad-sense heritability overestimates narrow-sense heritability and is thus unreliable for predicting evolutionary change in outbreeding species. Future studies of functional trait evolution should focus on physiology and a broader array of phenological and developmental traits. Long-lived species are severely underrepresented in microevolutionary studies, no doubt for practical reasons. Finally, an emphasis should be placed on exploring the nature and effect of trait interactions on fitness, since these are likely to be very important in shaping the course of evolution.