Testing alternative models for sexual isolation in natural populations of Littorina saxatilis: indirect support for by-product ecological speciation?

Abstract

Two ecotypes of the rough periwinkle Littorina saxatilis occur at different shore levels, showing assortative mating for size and partial reproductive isolation when they meet at the mid‐shore. This system represents a putative case of incomplete speciation in sympatry. Two processes contribute to the assortative mating: morph‐specific microhabitat aggregation and mate choice. The estimation of mate choice coefficients in nature and a simulation of the aggregation effects on sexual isolation were used to disentangle these processes as well as to test alternative mechanisms of mate choice. Mate choice significantly increased the frequency of within‐morph pairs and significantly decreased the frequency of between‐morph pairs, whereas those pairs including at least one hybrid morph mated randomly. These results allow us to reject a discriminant mate choice and support a model of evolution of sexual isolation as a side‐effect of size‐assortative mating in a context of divergent natural selection for size in the population. This mechanism is more compatible with a model of incomplete by‐product ecological speciation, as suggested by previous evidence.

Fucus vesiculosus and spiralis species complex: a nested model of local adaptation at the shore level

“…Such ecotype differences that are strongly correlated to different habitat type along the shore were extensively studied in Littorina saxatilis (see for critical review Butlin et al 2008). In this marine gastropod, repeated experimental work has demonstrated that strong divergent natural selection acting across an environmental gradient is the mechanism that maintains ecotypic divergence in spite of ongoing gene flow (Rolán-Alvarez et al 1997, Cruz et al 2004a. However, demonstrating speciation with gene flow is far from easy due to the difficulty of separating the effects of time since divergence from gene flow (but see review of Nosil 2008).…”

Strong assortative mating between allopatric sticklebacks as a by-product of adaptation to different environments

“…However, the by-product mechanism is still implicated when differentially adapted populations produce fully functional hybrids, as there is then no incentive to avoid hybridization. For example, hybrids between the lower and upper shore morphs of the marine snail Littorina saxatilis are fitter than either parent in the midtidal zone, yet there is still some assortative mating between the parental morphs (Rolan-Alvarez et al 1997;Cruz et al 2004).…”

Repeated evolution of reproductive isolation in a marine snail: unveiling mechanisms of speciation

“…That is, snails crossing habitat borders (either by creeping or by rafting) reduce their chance of survival to about 10 -20% of their earlier value ( Janson 1983;Rolán-Alvarez et al 1997). In addition, parental ecotype snails disperse non-randomly at least in the Spanish hybrid zone, contributing to the gene flow barrier Cruz et al 2004). The presence of a postzygotic component of isolation has been suggested from observations from a hybrid zone in the UK, where five hybrid females out of seven sampled had strongly elevated levels of aborting embryos (Hull et al 1996).…”