Species that have been introduced to islands experience novel and strong selection pressures after establishment. There is evidence that exotic species diverge from their native source populations; further, a few studies have demonstrated adaptive divergence across multiple exotic populations of a single species. Exotic birds provide a good study system, as they have been introduced to many locations worldwide, and we often know details concerning the propagule origin, time of introduction, and dynamics of establishment and dispersal within the introduced range. These data make them especially conducive to the examination of contemporary evolution. Island faunas have received intense scrutiny, therefore we have expectations concerning the patterns of diversification for exotic species. We examine six passerine bird species that were introduced to the Hawaiian archipelago less than 150 years ago. We find that five of these show morphological divergence among islands from the time since they were established. We demonstrate that some of this divergence cannot be accounted for by genetic drift, and therefore we must consider adaptive evolution to explain it. We also evaluate evolutionary divergence rates and find that these species are diverging at similar rates to those found in published studies of contemporary evolution in native species.

Aim 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.

There is increasing evidence that exotic populations may rapidly differentiate from those in their native range and that differences also arise among populations within the exotic range. Using morphological and DNA‐based analyses, we document the extent of trait divergence among native North American and exotic Hawaiian populations of northern cardinal (Cardinalis cardinalis). Furthermore, using a combination of historical records and DNA‐based analyses, we evaluate the role of founder effects in producing observed trait differences. We measured and compared key morphological traits across northern cardinal populations in the native and exotic ranges to assess whether trait divergence across the Hawaiian Islands, where this species was introduced between 1929 and 1931, reflected observed variation across native phylogeographic clades in its native North America. We used and added to prior phylogenetic analyses based on a mitochondrial locus to identify the most likely native source clade(s) for the Hawaiian cardinal populations. We then used Approximate Bayesian Computation (ABC) to evaluate the role of founder effects in producing the observed differences in body size and bill morphology across native and exotic populations. We found cardinal populations on the Hawaiian Islands had morphological traits that diverged substantially across islands and overlapped the trait space of all measured native North American clades. The phylogeographic analysis identified the eastern North American clade (C. cardinalis cardinalis) as the most likely and sole native source for all the Hawaiian cardinal populations. The ABC analyses supported written accounts of the cardinal's introduction that indicate the original 300 cardinals shipped to Hawaii were simultaneously and evenly released across Hawaii, Kauai, and Oahu. Populations on each island likely experienced bottlenecks followed by expansion, with cardinals from the island of Hawaii eventually colonizing Maui unaided. Overall, our results suggest that founder effects had limited impact on morphological trait divergence of exotic cardinal populations in the Hawaiian archipelago, which instead reflect postintroduction events.

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