Abstract
BackgroundReproductive isolation (RI) is widely accepted as an important "check point" in the diversification process, since it defines irreversible evolutionary trajectories. Much less consensus exists about the processes that might drive RI. Here, we employ a formal quantitative analysis of genetic interactions at several stages of divergence within the ring species complex Ensatina eschscholtzii in order to assess the relative contribution of genetic and ecological divergence for the development of RI.ResultsBy augmenting previous genetic datasets and adding new ecological data, we quantify levels of genetic and ecological divergence between populations and test how they correlate with a restriction of genetic admixture upon secondary contact. Our results indicate that the isolated effect of ecological divergence between parental populations does not result in reproductively isolated taxa, even when genetic transitions between parental taxa are narrow. Instead, processes associated with overall genetic divergence are the best predictors of reproductive isolation, and when parental taxa diverge in nuclear markers we observe a complete cessation of hybridization, even to sympatric occurrence of distinct evolutionary lineages. Although every parental population has diverged in mitochondrial DNA, its degree of divergence does not predict the extent of RI.ConclusionsThese results show that in Ensatina, the evolutionary outcomes of ecological divergence differ from those of genetic divergence. While evident properties of taxa may emerge via ecological divergence, such as adaptation to local environment, RI is likely to be a byproduct of processes that contribute to overall genetic divergence, such as time in geographic isolation, rather than being a direct outcome of local adaptation.
Highlights
Reproductive isolation (RI) is widely accepted as an important “check point” in the diversification process, since it defines irreversible evolutionary trajectories
Our results indicate that the isolated effect of ecological divergence between parental populations does not result in reproductively isolated taxa
Sampling and study units most of the 20 genetically distinct populations interact in zones of secondary contact around the Central Valley [22], there are a few gaps around the ring that either represent real distributional gaps (e.g. Mohave Desert) or possible sampling deficits (e.g. Lassen Peak, Pajaro river)
Summary
Reproductive isolation (RI) is widely accepted as an important “check point” in the diversification process, since it defines irreversible evolutionary trajectories. In a Darwinian sense, species formation is an outcome of a continuum of gradual evolution, from ecological races and biotypes, to hybridizing taxa and, to “good” biological species that no longer cross [1]. The different biological properties of species upon which several of the alternative concepts are based, such as distinct adaptive zones, fixed character traits or reproductive isolation (RI), arise at different times during the process of species formation and do not necessarily occur in a predictable order [3]. Once RI is complete (establishment of post-zygotic mechanisms barring successful genetic interactions [7]), the evolution of closely related taxa is no longer reversible and they are bound to follow independent evolutionary trajectories, even in the face of environmental change
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