No abstract

Sky islands are unique geologic formations, home to populations of organisms that have weathered climate change since the Pleistocene. Long-term isolation and climatic differences between sky islands and adjacent mountain chains result in natural laboratories well suited for examining the direct effects of climate change. Here, we review the global sky island literature to examine how taxa have responded to climate change. Results show lineage formation, reduced genetic variation, and trait evolution across taxa driven by genetic drift and natural selection. These effects continue today due to ongoing habitat reduction and steep selective gradients on sky islands relative to mountain chains. Comparative studies and experimental manipulations are needed to build broad inference into how past climate change has shaped the structure and function of whole ecosystems. The next era of sky island research is poised to create a model for climate change responses and eco-evolutionary dynamics, with profound conservation implications. Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 54 is November 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.