Climate-driven convergent evolution on sky islands

Abstract

Climate-induced evolution will determine population persistence in a changing world. However, finding natural systems in which to study these responses has been a barrier to estimating the impact of global change on a broad scale. Here we hypothesize that isolated sky islands (SI) and adjacent mountain chains (MC) can serve as natural laboratories for studying the impact of long-term climatic pressures on natural populations. We used greenhouse common garden trees to test whether populations on SI exposed to hot and dry climates since the Pleistocene have diverged from populations on MC, and if populations on SI have converged in their evolutionary responses. We show: (1) in the southwestern U.S., isolated SI are significantly hotter and drier than adjacent MC, (2) populations of Populus angustifolia from SI have diverged from MC in reproductive and productivity traits, (3) these traits (cloning and aboveground biomass, respectively) are significantly correlated, suggesting a genetic linkage between the traits, and (4) that the observed phenotypic change is driven both by natural selection and genetic drift. These results suggest that long-lived tree populations on distantly related SI have evolved in response to long-term selective pressures and genetic drift by converging on similar phenotypes and diverging from phenotypes on MC. These shifts are towards potentially beneficial phenotypes for population persistence in a changing world. These results also suggest that the SI-MC comparison is an ideal natural laboratory, as well as predictive framework, for studying responses to climate change across the globe.