Abstract
Reinforcement, the increase of assortative mating driven by selection against unfit hybrids, is conditional on pre-existing divergence. Yet, for ecological divergence to precede the evolution of assortment, strict symmetries between fitnesses in niches must hold, and/or there must be low gene flow between the nascent species. It has thus been argued that conditions favouring sympatric speciation are rarely met in nature. Indeed, we show that under disruptive selection, violating symmetries in niche sizes and increasing strength of the trade-off in selection between the niches quickly leads to loss of genetic variation, instead of evolution of specialists. The region of the parameter space where polymorphism is maintained further narrows with increasing number of loci encoding the diverging trait and the rate of recombination between them. Yet, evolvable assortment and pre-existing assortment both substantially broaden the parameter space within which polymorphism is maintained. Notably, pre-existing niche preference speeds up further increase of assortment, thus facilitating reinforcement in the later phases of speciation. We conclude that in order for sympatric ecological divergence to occur, niche preference must coevolve throughout the divergence process. Even if populations come into secondary contact, having diverged in isolation, niche preference substantially broadens the conditions for coexistence in sympatry and completion of the speciation process.This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.
Highlights
Darwin observed that many closely related species occupied the same habitat
Figure S2 we show that for low recombination, certain cost is robustly tolerated in a large region of the parameter space: in an example with s 1⁄4 0:1, c 1⁄4 0:46 and e 1⁄4 À0:5s, cost g 1⁄4 0:05s is tolerated for a2 ! 0:1, cost g 1⁄4 0:1s for a2 ! 0:2 and cost g 1⁄4 0:2s for a2 ! 0:3
An arguable assumption frequently made in many sympatric speciation models is their restriction to one or rarely several loci, whereas ecological adaptation of a population often involves a gradual change in a polygenic trait [36]
Summary
Darwin observed that many closely related species occupied the same habitat. he considered the sympatric origin of species by ecological divergence due to an advantage of specialists over generalists rather unlikely [1]. Because regulation is independent within niches (soft selection, [26]), the frequency of a type well adapted to its niche rises faster when this type is rare than when it is common This protects the polymorphism in the ecological loci (coexistence of the specialists) via negative frequency-dependence [26]. Sympatric speciation under strong (convex) tradeoff is implausible as variation is quickly lost (cf [7,11,27]) Such an assortment only enables evolution of coexisting specialists when trade-offs are weak (concave) [16]. Since we focus on the effects of disruptive selection on sympatric divergence, we use a haploid biallelic version of Levene’s [29] model with mating within niches, which is more favourable to speciation [30].
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