Adaptation is the basis of biodiversity and is an important mechanism that allows species to exploit novel environmental conditions. This study will generate a detailed analysis of the ecological and evolutionary factors that cause and limit adaptation for a group of fishes. The project will measure the relative performance of many shape and size variants from three closely related species of pupfish and their hybrids, following the growth and survival of these fish in pens in their natural environment. Because the species are closely related, they can be readily hybridized to generate a wide variety of shapes and sizes in their offspring that are not found in the wild. These hybrid offspring allow measuring the complete adaptive performance profile for these fish in their natural environment.
Understanding of the ecological and evolutionary basis of variation in performance in nature supports prediction of such significant societal issues as which environments are more vulnerable to invasive species, which species are more likely to become invasive, and which species are likely to readily accommodate to environmental changes. The project includes research-based training opportunities for undergraduates and diverse public outreach.
The adaptive landscape is the most powerful unifying concept in evolutionary biology for understanding biodiversity: the species we observe should correspond to fitness peaks spaced along a continuous landscape of varied organismal forms. Nonetheless, the adaptive landscape is poorly understood because it is rarely measured beyond a single species or trait. Here we measured a large portion of the adaptive landscape within a 10,000-year-old radiation of Cyprinodon pupfishes endemic to the 11-mile San Salvador Island in the Bahamas. Although pupfishes are found from Massachusetts to Venezuela and throughout the Caribbean, they have started rapidly evolving new species only on this tiny island, with jaw shape changing up to 50 times faster than other pupfish species. We do not understand why this rapid evolution occurs on San Salvador Island and no where else across thousands of pupfish populations in the Caribbean. Direct measurement of the adaptive landscape for pupfish on San Salvador helped answer this quesiton. We measured the growth and survival of 2,077 F2 hybrids placed in field enclosures in their native lake habitats on San Salvador. This experiment essentially simulates the past colonization of this island by an ancestral population of pupfish in order to better understand what forces drive explosive evolutionary diversification in the wild. We observed a complex adaptive landscape with fitness peaks corresponding nearly exactly to wild species. Unexpectedly, this landscape also provides a fascinating explanation for the rarity of rapid evolution. We found that abundant generalist phenotypes sit atop an isolated fitness peak separated by a valley from a higher fitness peak corresponding to trophic specialization. Thus, widespread stabilizing selection on generalist founding populations could constrain their diversification across the Caribbean, whereas if they are able to escape this fitness peak and specialize, the higher neighboring peak could explain their 51-fold increase in rate of jaw evolution. Overall, we confirmed experimentally the presence of multiple fitness peaks in sympatry driven by increased competition at high densities, strongly supporting the early burst model of adaptive radiation. These results provide a better foundation for understanding the spread of invasive species, rapid evolution in response to anthropogenic disturbances like catastrophic climate change, and the stable coexistence of biodiversity in the tropics. In addition, this project contributed to one PhD dissertation and the training of 4 undergraduate students.
