Abstract
Gene flow tends to impede the accumulation of genetic divergence. Here, we determine the limits for the evolution of postzygotic reproductive isolation in a model of two populations that are connected by gene flow. We consider two selective mechanisms for the creation and maintenance of a genetic barrier: local adaptation leads to divergence among incipient species due to selection against migrants, and Dobzhansky–Muller incompatibilities (DMIs) reinforce the genetic barrier through selection against hybrids. In particular, we are interested in the maximum strength of the barrier under a limited amount of local adaptation, a challenge that many incipient species may initially face. We first confirm that with classical two-locus DMIs, the maximum amount of local adaptation is indeed a limit to the strength of a genetic barrier. However, with three or more loci and cryptic epistasis, this limit holds no longer. In particular, we identify a minimal configuration of three epistatically interacting mutations that is sufficient to confer strong reproductive isolation.This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers’.
Highlights
Understanding the mechanisms that drive speciation remains a challenge of evolutionary research [1,2,3,4]
We here show that interactions between three loci can be sufficient to confer strong reproductive isolation between two populations in parapatry, and the evolution of this barrier is possible in the presence of ongoing gene flow
We first establish that in the absence of epistasis or under a large number of ‘simple’ epistasis schemes, the amount of local adaptation between well-adapted types in both populations is a hard limit for the strength of a genetic barrier
Summary
Understanding the mechanisms that drive speciation remains a challenge of evolutionary research [1,2,3,4]. The dissociation between local adaptation and the strength of a genetic barrier to gene flow is key for the evolution of strong reproductive isolation and for completing the speciation process In this manuscript, we address when and how strong reproductive isolation can evolve between two parapatric populations with limited ecological differentiation. (a) AbC (b) abc aBc abc (c) history 1 continent abc island continent abc aBc abc aBc. In addition, we define the maximum amount of local adaptation that can occur in the model over the course of the differentiation process that results in a given genetic barrier as Λmax. The fitness difference between the optimal island genotype and a hybrid (or any maladapted genotype) may be much larger, since these genotypes are not part of any evolutionary trajectories
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