According the character displacement hypothesis, similar species that use similar resources and come to live in the same area should evolve differences that allow them to coexist. Otherwise, one species will drive the other species extinct. This research tests for character displacement in the lizard Anolis carolinensis in response to the recent invasion of a closely related species, A. sagrei. The two species compete for food and space, so the researchers expect them to evolve to use different resources now that their geographic ranges overlap. The researchers are using a replicated island experiment to test this hypothesis, comparing habitat use patterns and physical characteristics of A. carolinensis on islands where A. sagrei is present to control islands where A. sagrei is absent.
This research, the first replicated experimental test of the character displacement hypothesis in nature, will serve as a clear example of evolution in action, useful to teach evolutionary principles to the general public and to help dispel the popular notion that evolution by natural selection is a slow process. This research examines the role that small-scale evolutionary processes may play in generating large-scale evolutionary patterns, and the investigators will describe the results in popular scientific articles, public museum exhibits, and high-school science curricula. As a study of rapid evolution in response to environmental change, this research will inform conservation biologists and policy makers who are developing measures to conserve species and ecosystems in the face of an increasing human population, climate change, invasive species, and habitat destruction.
The goal of this project was to test whether strong interactions between two species could lead to evolution in habitat use and, subsequently, body shape and size (morphology). Such evolution, called character displacement, is expected to lessen the strength of those interactions. Character displacement is thought to be an important process driving the evolution of morphological and species diversity, however, the evidence for character displacement is mostly equivocal. Thus, we wished to provide a more definitive test of the hypothesis using a replicated natural experiment. We studied the habitat use and morphology of populations of the lizard Anolis carolinensis on man-made dredge-spoil islands in Mosquito Lagoon, Florida. We compared seven A. carolinensis populations whose islands had been recently invaded by the lizard Anolis sagrei to five populations whose islands had not been invaded, to ask whether the presence of a novel, interacting species could drive the evolution of character displacement. The interactions between the two species should be strong because they are closely related and similar in size, diet, and habitat use. Thus, in accordance with character displacement theory, we predicted that A. carolinensis would change its habitat use, and subsequently adapt its morphology, in the presence of A. sagrei. We found that our predictions were met. Anolis carolinensis populations shift their habitat use to higher parts of the tree in the presence of A. sagrei; we expected this because the latter is abundant, aggressive, and prefers the lower parts of trees and the ground. We also found that A. carolinensis populations in the presence of A. sagrei have relatively larger toepads with more scales on them. Moreover, we were able to determine, using a random habitat surveys and a common garden experiment, that the larger toepads with more scales in A. carolinensis are an evolved response to the presence of A. sagrei. Why might A. carolinensis evolve larger toepads with more scales? These scales, called lamellae, are covered with thousands of microscopic hairs that allow the lizards to cling to surfaces, and notably, lizards with relatively larger toepads have greater clinging ability. Since perches high in trees are likely to be smaller, more slippery, and more flexible, and the consequences of falls more severe, it is probable that better clinging ability would be favored in tree-dwelling lizards. Indeed, this is borne out by the observation that across the ~400 species of Anolis, tree dwelling species tend to have larger toepads with more lamellae. Thus, our observation that A. carolinensis has evolved larger toepads with more lamellae suggests that it is responding adaptively to the habitat use shift in the presence of A. sagrei. This work provides one of the strongest field tests of the character displacement hypothesis to date. Importantly, our replicated, natural experimental framework allowed us to rule out many of the alternative processes that could have explained our result, like differences in environment between invaded and un-invaded islands. We also determined that the differences we observed were due at least in part to genetic evolution between populations, rather than plastic responses to experience during development and growth. Often, studies of character displacement cannot rule out alternative processes such as these. This work also contributes to the cannon of literature documenting rapid evolutionary change. We found that evolution took place in less than 20 generations (~20 years) in our system. Thus, we find that character displacement is measurable in real time, providing impetus for future tests of character displacement to take such an experimental approach. In sum, we found that A. carolinensis evolves rapidly in response to interactions with an invasive species. This case-study for character displacement is intuitive – the organism is familiar, the predictions are clear, and the findings are concrete. Thus, the work is likely to be useful to educators for elucidating the processes of natural selection and evolution. The work has been discussed on a well-read science blog (Anole Annals), will be highlighted in a widely distributed newsletter from the Merritt Island National Wildlife Refuge, and will be distributed in press release and article form to newspapers. This research provided training in lab and field settings for seven undergraduate students, six of which are under-represented in science. Two of those students are now pursuing Ph. D. degrees in science.