Mechanisms of population divergence along elevational gradients in the Tropics
Date
2016
Authors
Vacas, Mónica Isabel Páez, author
Funk, W. Chris, advisor
Angeloni, Lisa M., committee member
Crooks, Kevin R., committee member
Ghalambor, Cameron K., committee member
Guayasamin, Juan Manuel, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Elucidating the mechanisms that give rise to population divergence and eventually initiate speciation is a key step for understanding the evolution of biodiversity. Most theories of differentiation and speciation have traditionally focused on geographically isolated populations. However, there is growing evidence that speciation can occur due to divergent selection despite initially high gene flow. My doctoral dissertation project investigates the effects of environmental heterogeneity and geography in promoting phenotypic and genetic divergence along elevation gradients in natural populations of a poison frog, Epipedobates anthonyi, across the landscape with a focus on environmental variation along elevational gradients. I studied populations distributed along a broad elevational gradient (0–1800 m above sea level) on the western slope of the Andes of southern Ecuador. First, I examined the relative roles of geographic distance and environmental gradients on genetic and phenotypic divergence. I found that populations are phenotypically divergent in size, color, and male advertisement calls, but they exhibit low genetic divergence at neutral loci. There is substantial gene flow between populations throughout the lowlands, but populations at higher elevations are relatively isolated. This is mainly due to a mountain ridge acting as a physical and possibly environmental barrier between northern and southern populations. Within elevational gradients, geographic distance corrected for topography is the main factor explaining both genetic and phenotypic divergence. However, when controlling for the effect of topographic distance, environmental conditions, such as temperature and precipitation between sites best explain observed patterns of genetic divergence, whereas environmental conditions at a given site best explains differences in phenotypic traits, presumably due to divergent selection pressures. To study the effect of temperature variation along elevational transects on adaptive divergence, I measured thermal tolerance of tadpoles across elevation. I found that populations from higher elevation had higher cold tolerance, suggesting that changes in temperature along elevation may cause divergent selection in thermal tolerance. Additionally, tadpoles from all sites have the ability to shift their thermal tolerance in response to previous exposure to different temperatures. Finally, to examine the degree of local adaptation to environmental conditions at high and low elevations, I conducted a reciprocal transplant experiment. I evaluated populations from high and low elevations from two elevational transects. Overall, I found that all populations have higher reproduction rates at low elevation. In fact, at high elevation, populations had very low reproductive rates or did not reproduce at all. However, variation in life-history traits differed between transects. Populations from one transect revealed a pattern that was consistent with the expectation under local adaptation, namely, low elevation frogs had higher reproduction than high elevation frogs at the low elevation site. In contrast, populations from the other transect had a pattern that would be expected under countergradient variation, namely higher elevation frogs had higher reproduction at the low elevation site. Intriguingly, low elevation frogs had overall higher reproduction rates than high elevation frogs, suggesting that frogs from low elevation have higher fecundity than their counterparts at high elevation. Overall, the findings of my dissertation suggest that (i) phenotypic divergence occurs in the face of gene flow, (ii) environmental variation along elevation, particularly temperature, is a force that drives population divergence, and (iii) the influences of environmental conditions on populations are variable at the intraspecific level.