Abstract
A powerful approach to address the general factors contributing to ecological speciation is to compare distantly related taxa that inhabit the same selective environments. In this design, similarities among taxa can elucidate general mechanisms of the process whereas differences may uncover specific factors important to the process for individual taxa. Herein, we present evidence of parallel patterns of morphological and behavioral variation among host-associated populations of two species of cynipid gall wasps, Belonocnema treatae and Disholcaspis quercusvirens, that each exhibit a life cycle intimately tied to the same two host plant environments, Quercus geminata and Q. virginiana. Across both gall-former species we find consistent differences in body size and gall morphology associated with host plant use, as well as strong differences in host plant preference, a measure of habitat isolation among populations. These consistent differences among taxa highlight the important role of host plant use in promoting reproductive isolation and morphological variation among herbivorous insect populations–a prerequisite for ecological speciation.
Highlights
Throughout the modern synthesis, biologists described a central role for ecological adaptation in the speciation process [1,2,3]
For each cynipid species we examine a main prediction of ecological speciation theory: allopatric populations inhabiting different host plant species will exhibit greater differences in ecologically important traits and express greater reproductive isolation than will allopatric populations inhabiting the same host plant
In the present study we provide evidence from two cynipid gall wasp species representing different genera that differences in host plant use within each species are associated with parallel differences in adult and gall morphology between cynipid species
Summary
Throughout the modern synthesis, biologists described a central role for ecological adaptation in the speciation process [1,2,3]. The majority of studies addressing parallel ecological speciation have compared geographically separate populations of individual species in different versus similar environments [7,9,13,22,23] These studies have suggested a central role for divergent selection in promoting speciation [4]. Evidence for parallel ecological speciation produced by comparing more distantly related taxa is rare but includes studies of multiple lizard species inhabiting two soil color habitats [24] and multiple herbivorous insect species inhabiting similar sets of host plants [25,26,27,28]. Across the southeastern United States a diverse community of host-specific gall formers (Hymenoptera: Cynipidae) inhabits two sister species of live oak, Quercus virginiana and Q. geminata These oak species overlap in geographic range (Figure 1) but occupy different microhabitats. By comparing multiple allopatric pairs of populations within each gall wasp species that occur in ecologically similar habitats (i.e., host plant species), we can control for host plant independent divergence (i.e., due to genetic drift, sexual conflict, and certain forms of sexual selection) whereas pairs of populations in ecologically different habitats may diverge due to any of these processes as well as from host plant associated divergent selection [7,22]
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