Rapid evolution of great kiskadees on Bermuda: an assessment of the ability of the island rule to predict the direction of contemporary evolution in exotic vertebrates
To determine whether an exotic bird species, the great kiskadee (Pitangus sulphuratus), has diverged in morphology from its native source population, and, if so, has done so in a manner predicted by the island rule. The island rule predicts that insular vertebrates will tend towards dwarfism or gigantism when isolated on islands, depending on their body size. For birds, the island rule predicts that species with body sizes below 70–120 g should increase in size. The great kiskadee has a mean mass of c. 60 g in its native range, therefore we predicted that it would increase in size within the exotic, and more insular, Bermudan range. Location
The islands of Bermuda (exotic population) and Trinidad (native source population). Methods
We took eight morphological measurements on 84 individuals captured in the exotic (Bermudan) population and 62 individuals captured in the native source (Trinidadian) population. We compared morphological metrics between populations using univariate and principal components analyses. We assessed whether the effects of genetic drift could explain observed differences in morphology. We calculated divergence rates in haldanes and darwins for comparison with published examples of contemporary evolution. Finally, we used mark–recapture analysis to determine the effects of the measured morphological characters on survivorship within the exotic Bermudan population. Results
Individuals in the exotic Bermudan population have larger morphological dimensions than individuals in the native source population on Trinidad. The degree of divergence in body mass (g) and bill width (mm) is probably not due to genetic drift. This rate of divergence is nearly equal to that observed amongst well-documented examples of contemporary bird evolution, and is within the mid-range of rates reported across taxa. There is no clear effect of body size on survivorship as only one character (bill width) was found to have an influence on individual survivorship. Main conclusions
Exotic species provide useful systems for examining evolutionary predictions over contemporary time-scales. We found that divergence between the exotic and native populations of this bird species occurred over c. 17 generations, and was in the direction predicted by the island rule, a principle based on the study of native species.