Co-occurrence of ecologically similar species of Hawaiian spiders reveals critical early phase of adaptive radiation

Darko D. Cotoras,Ke Bi,David R. Lindberg,Stefan Prost,Michael S. Brewer,Rosemary G. Gillespie

doi:10.1186/s12862-018-1209-y

Abstract

BackgroundThe 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. 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. 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. 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.ResultsThe 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. 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.ConclusionsThe 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. 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.

Highlights

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
The current study sets out to elucidate the processes involved in the early stages of adaptive radiation in a lineage of long-jawed spiders within the Hawaiian Islands in which diversification has led to the presence of non-random, parallel sets of ecological forms, or ecomorphs [7]
The question is, if early diverging taxa are inevitably the same ecomorph and allopatric, what might be the selective pressure for divergence, and subsequent co-occurrence, of different ecomorphs? In particular, can, and if so when do, closely related species of the same ecomorph co-occur in the process, providing the opportunity for competition and ecological specialization? Here we focus explicitly on the early divergence within close relatives of the green spiny form to assess evidence for co-occurrence within the same ecomorph at the early stages of adaptive radiation, and the possible outcomes of co-occurrence events, whether competitive exclusion, hybridization and genetic admixture, or haphazard accumulation of genetic diversity within the ecomorph

Summary

Introduction

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. 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? The current study sets out to elucidate the processes involved in the early stages of adaptive radiation in a lineage of long-jawed spiders (genus Tetragnatha) within the Hawaiian Islands in which diversification has led to the presence of non-random, parallel sets of ecological forms, or ecomorphs [7]. We focus explicitly on the early divergence within close relatives of the green spiny form to assess evidence for co-occurrence within the same ecomorph at the early stages of adaptive radiation, and the possible outcomes of co-occurrence events, whether competitive exclusion, hybridization and genetic admixture, or haphazard accumulation of genetic diversity within the ecomorph The question is, if early diverging taxa are inevitably the same ecomorph and allopatric, what might be the selective pressure for divergence, and subsequent co-occurrence, of different ecomorphs? In particular, can, and if so when do, closely related species of the same ecomorph co-occur in the process, providing the opportunity for competition and ecological specialization? Here we focus explicitly on the early divergence within close relatives of the green spiny form to assess evidence for co-occurrence within the same ecomorph at the early stages of adaptive radiation, and the possible outcomes of co-occurrence events, whether competitive exclusion, hybridization and genetic admixture, or haphazard accumulation of genetic diversity within the ecomorph

Results

Discussion

Conclusion