Population differences in leaf traits in Quercus oleoides: Effects of natural selection and neutral evolutionary processes

Background/Question/Methods

Widely distributed tree species often harbor considerable genetic variation both within and among populations. Adaptation to the local environment by natural selection is considered one of the most important evolutionary forces leading to population diversification. Nevertheless, neutral evolutionary processes such as genetic drift, bottlenecks or founder effects can also drive observed population divergence. In this study, we examine inter-population and intrapopulation differentiation in a long-lived, evergreen tree species, Quercus oleoides for a set of functional traits and phenology. Previous studies have shown high genetic differentiation among populations in neutral makers of DNA within this species. Specifically, southern populations are known to be genetically differentiated from the rest of the species. We investigated whether population differences in quantitative traits have been caused by adaptation to different climates or are due to neutral evolutionary processes. To test these alternatives, we collected acorns from ninety three mother trees from three populations with contrasting precipitation regimes. Acorns were planted in a common garden experiment under three water treatments (xeric, mesic, hydric) in a greenhouse experiment using a randomized block design.

Results/Conclusions

Our preliminary results showed high inter- and intrapopulation differentiation in chlorophyll content (among other traits) but not in phenological patterns. Maternal families originating from the driest places had the lowest values of chlorophyll content and shortest leaf life spans. Population differences in the latter trait were markedly higher than those observed in neutral markers, suggesting that a deciduousness strategy has evolved in response to adaptation to drought conditions. Contrary to our expectation, the population from the climate with highest rainfall (Rincon de la Vieja, Costa Rica) had an intermediate value for this trait. Other leaf morphological traits are also distinct for this population. The geographically isolated location of the population in the southern extreme of the distribution range of the species could have prevented gene exchange with other populations, leading to neutral differentiation in traits.