Predictability in evolution: Adaptation of the Bonaire anole (Anolis bonairensis) to an extreme environment

Phylogeographic and phenotypic outcomes of brown anole colonization across the Caribbean provide insight into the beginning stages of an adaptive radiation

Some of the most important insights into the ecological and evolutionary processes of diversification and speciation have come from studies of island adaptive radiations, yet relatively little research has examined how these radiations initiate. We suggest that Anolis sagrei is a candidate for understanding the origins of the Caribbean Anolis adaptive radiation and how a colonizing anole species begins to undergo allopatric diversification, phenotypic divergence and, potentially, speciation. We undertook a genomic and morphological analysis of representative populations across the entire native range of A. sagrei, finding that the species originated in the early Pliocene, with the deepest divergence occurring between western and eastern Cuba. Lineages from these two regions subsequently colonized the northern Caribbean. We find that at the broadest scale, populations colonizing areas with fewer closely related competitors tend to evolve larger body size and more lamellae on their toepads. This trend follows expectations for post-colonization divergence from progenitors and convergence in allopatry, whereby populations freed from competition with close relatives evolve towards common morphological and ecological optima. Taken together, our results show a complex history of ancient and recent Cuban diaspora with populations on competitor-poor islands evolving away from their ancestral Cuban populations regardless of their phylogenetic relationships, thus providing insight into the original diversification of colonist anoles at the beginning of the radiation. Our research also supplies an evolutionary framework for the many studies of this increasingly important species in ecological and evolutionary research.

Evolution of larval competitiveness and associated life-history traits in response to host shifts in a seed beetle

Resource competition is frequently strong among parasites that feed within small discrete resource patches, such as seeds or fruits. The properties of a host can influence the behavioural, morphological and life-history traits of associated parasites, including traits that mediate competition within the host. For seed parasites, host size may be an especially important determinant of competitive ability. Using the seed beetle, Callosobruchus maculatus, we performed replicated, reciprocal host shifts to examine the role of seed size in determining larval competitiveness and associated traits. Populations ancestrally associated with either a small host (mung bean) or a large one (cowpea) were switched to each other's host for 36 generations. Compared to control lines (those remaining on the ancestral host), lines switched from the small host to the large host evolved greater tolerance of co-occurring larvae within seeds (indicated by an increase in the frequency of small seeds yielding two adults), smaller egg size and higher fecundity. Each change occurred in the direction predicted by the traits of populations already adapted to cowpea. However, we did not observe the expected decline in adult mass following the shift to the larger host. Moreover, lines switched from the large host (cowpea) to the small host (mung bean) did not evolve the predicted increase in larval competitiveness or egg size, but did exhibit the predicted increase in body mass. Our results thus provide mixed support for the hypothesis that host size determines the evolution of competition-related traits of seed beetles. Evolutionary responses to the two host shifts were consistent among replicate lines, but the evolution of larval competition was asymmetric, with larval competitiveness evolving as predicted in one direction of host shift, but not the reverse. Nevertheless, our results indicate that switching hosts is sufficient to produce repeatable and rapid changes in the competition strategy and fitness-related traits of insect populations.