However, recent studies have highlighted the importance of “nonadaptive” or minimally adaptive divergence in the early stages of adaptive radiation. In terms of species accumulation alone, this may occur through the joint processes of immigration and speciation, with successful establishment the result of differing mechanisms of competitive exclusion on the one hand acting to limit access of similar species to a site, and ecological character displacement on the other, in which traits evolve through selection acting to reduce resource competition between species. In particular, where multiple close relatives co-occur within a rapidly multiplying lineage, the processes that characterize early differentiation are almost entirely unknown. As a consequence, the mechanism through which diversity originates in the course of adaptive radiation is very poorly understood. Because most cases of adaptive radiation in nature necessarily unfold over extended time periods, comparative inference is often the only way to infer evolutionary and ecological underpinnings.
The phenomenon involves the interplay of ecological and evolutionary processes, and as such is central to understanding mechanisms of evolution through natural selection. More importantly, the ability of close relatives of the same ecomorph to interact, without admixture, may provide the conditions necessary for ecological divergence and independent evolution of ecomorphs associated with adaptive radiation.Īdaptive radiation is the rapid diversification of an ancestral species into several ecologically different species, associated with adaptive morphological or physiological divergence. The observation of co-occurrence of ecologically equivalent species on the young volcano of Maui provides a missing link in the process of adaptive radiation between the point when recently divergent species of the same ecomorph occur in allopatry, to the point where different ecomorphs co-occur at a site, as found throughout the older islands. Further, that multiple genetic lineages exist on a single volcano on Maui suggests that there are no inherent dispersal barriers and that the observed limited distribution of taxa reflects competitive exclusion. The major outcomes from the current study are first that closely related species within the same green ecomorph of spiny leg Tetragnatha co-occur on the same single volcano on East Maui, and second that there is no evidence of genetic admixture between these ecologically equivalent species.
Given that the early stages of adaptive radiation are characterized by allopatric divergence between populations of the same ecomorph, the question is, what are the steps towards subsequent co-occurrence of different ecomorphs? Using a transcriptome-based exon capture approach, we focus on early divergence among close relatives of the green ecomorph to understand processes associated with co-occurrence within the same ecomorph at the early stages of adaptive radiation. Hawaiian spiders in the genus Tetragnatha have undergone adaptive radiation, with one lineage (“spiny legs”) showing four different ecomorphs ( green, maroon, large brown, small brown) one representative of each ecomorph is generally found at any site on the older islands. A key question is, at what point do closely related species interact, setting the stage for competition and ecological specialization? The Hawaiian Islands provide an ideal system to explore the early stages of adaptive radiation because the islands span ages from 0.5–5 Mya. The processes through which populations originate and diversify ecologically in the initial stages of adaptive radiation are little understood because we lack information on critical steps of early divergence.