Interacting evolutionary mechanisms during island introductions in brown anole lizards

Background/Question/Methods

Founding events are a necessary component of both natural colonization and human-mediated introductions; yet, they are rarely observed in nature, thus the evolutionary significance of these events is difficult to evaluate. Whether random processes such as founder events contribute substantially to patterns of evolutionary divergence is controversial. We conducted an experiment in nature to determine the respective contributions of founder effects and natural selection to evolutionary divergence among lizard populations established on small islands in the Bahamas. We introduced opposite-sex pairs of brown anoles (Anolis sagrei) sampled from the same population on a large island to each of seven unoccupied islands. We characterized the vegetation (height and diameter) and repeatedly sampled lizards from the source population, from the seven experimental founder islands, and from 12 nearby reference islands. We took tissue samples for genetic analyses, measured body size and hindlimb length, and recorded perch use (height and diameter) throughout the experiment to track changes over time. We predicted that the founding events would generate genetic and morphological differences among the experimental islands due to random sampling of the source population. By contrast, the shift from the more forested habitat of the source area to the sparse, narrow-diameter vegetation on the small experimental islands should result in selection for shorter hindlimbs. 

Results/Conclusions

The founding events generated significant among-island genetic and morphological differences unrelated to environments on the experimental islands. This founder effect persisted throughout the course of the experiment despite all populations adapting in the predicted direction—shorter hindlimbs—in response to the narrower vegetation on the small islands. Thus both founder effects and natural selection jointly determine trait values in these populations over the first four years of the experiment. By measuring founder attributes at the start of the study and repeatedly sampling populations on the colonized islands, we were able to evaluate the relative contributions of founder effects, population bottlenecks, natural selection, and gene flow to the pattern of evolutionary divergence among these island populations. Persistence of the founder effect signal in this experiment suggests this evolutionary process may be more important than previously thought.  Furthermore, the recent founding of populations via human-mediated introduction may increase the relative importance and detectability of founder effects in relation to other evolutionary mechanisms.