A view of the lagoon in the center of Duck Cay, a particularly hot islet that sits about 25 km off the coast of the big island of Great Exuma. Brown anole lizards on this island are adapted to these warm conditions despite being regularly inundated with immigrants from other, much cooler, islets.
Just as populations occur within communities of other species, they also occur within a broader geographic network of populations that interact through the emigration and immigration of individuals. As the Earth's climate warms, this metapopulation structure could, in principle, both increase or decrease a species' probability of extinction. For example, if the primary source population for emigrating individuals is adapted to a cooler environment, then their arrival in warmer areas may decrease the heat tolerance of the populations that are receiving these individuals. This would increase their extinction risk. However, it is possible, maybe even likely, that local selection is strong enough to overwhelm gene flow from other populations. If local adaptation occurs despite high rates of gene flow between populations, genetic variation in thermal physiology can be maintained at the species level. We have coined this the "adaptive reservoir hypothesis," which proposes that spatial variation in selection generates genetic and phenotypic diversity which can then be distributed to vulnerable populations through gene flow. This phenomenon, if common, would increase the resilience of many species to rapid environmental change.
Ryan Calsbeek (Dartmouth College), M. C. Duryea (University of Lund), Orsolya Molnar (University of Brazil), Benji Kessler (University of California - Berkeley), and are studying several island-dwelling populations ofAnolis sagreifrom The Bahamas. These islands differ dramatically in temperature such that we might expect each lizard population to be locally adapted with some being relatively heat-tolerant and others being relatively cold-tolerant. However, gene flow is extremely high across the archipelago---so high, in fact, that it may overwhelm the effects of selection on any one island, homogenizing the thermal tolerance of all the populations. Interestingly, we have found that each island population is very nicely adapted to its local thermal environment despite being inundated each year with immigrants from other islands. Strong local selection appears to be counteracting the effects of gene flow, weeding out maladapted genotypes as they arrive, and maintaining phenotypic diversity across a broad geographic region. Selection on thermal performance appears to be so strong, in fact, that the magnitude of gene flow between any two islands is controlled by the difference in their climates, rather than by how geographically isolated they are from one another.