Award: OCE-1419986

One of the central goals of ecology is to find out why species live where they do.  Understanding adaptations that allow species to survive and reproduce in their native habitats helps us understand the processes that generate new species, and sheds light on how environmental change can disrupt species’ ranges and cause populations to differentiate and potentially form new species.   This research focused on Pollicipes elegans, an intertidal marine barnacle with an unusual geographic distribution in the tropical eastern Pacific ocean.  The distribution of P. elegans is “antitropical”: abundant at the edges of the warmest waters of the tropical eastern Pacific but rare within this warm region, an area known as the Intertropical Convergence Zone (ITCZ).  These northern (Mexico: cool tropical), southern (Peru: cool tropical), and central (El Salvador: warm tropical) populations of P. elegans live in regions that differ substantially with respect to temperature and seasonality. 

We used a combination of genetic and physiological data to test hypotheses about the origins of P. elegans and its ability to adapt to different temperatures.  Adults are stationary and cannot move, so colonization of new habitats and exchange of genes between populations happen at the larval stage; larvae are generally thermally sensitive, so high water temperatures may may isolate adult populations from each other, potentially leading to local adaptation or even speciation.  We first used microsatellite frequencies and DNA sequence data to estimate the timing of separation between populations of P. elegans in Mexico, El Salvador, and Peru, as well as to measure the amount of larval dispersal and genetic connectivity between these three distant populations.  We found that the antitropical distribution of P. elegans did not form in a stepping-stone-like manner, as would have been expected. Instead, P. elegans used to be more abundant in the warm ITCZ but expanded its range north and south in the last several hundred thousand years, likely in response to climate change.  The northern population is genetically very distinct from the other two, suggesting that P. elegans is in the process of separating into at least two species. 

Our larval experiments showed that larvae of the three populations were physiologically adapted to the different temperatures in their respective regions.  We showed this by describing the optimum, deteriorating, and lethal temperature ranges for larval metabolic performance.  We found that all three populations live within their respective optimal ranges, but larvae from Mexico are close to the upper limit of their optimum during warm months and therefore have limited capacity to disperse through warmer waters that currently isolate them from central populations.  Larvae from the southern population (Peru) also would not be likely to tolerate tropical temperatures, so for the southern and northern P. elegans populations, high tropical temperatures are likely to be a direct physiological barrier to equatorward dispersal.  Only a handful of studies have explicitly examined geographic variation in thermal performance of larvae, even though larvae are often the life stage that allows species to exchange genes between populations and to colonize new areas.   Our study is also one of the first to show that tropical populations are living further from their thermal limits than conspecific extratropical populations.

Overall, our study indicates that repeated cyclical global warming and cooling, which causes expansion and contraction of ancestral tropical species’ ranges into adjacent subtropical regions, is punctuated by periodic geographic isolation and adaptation of pop...