Case studies and mathematical models of ecological speciation. 3: Ecotype formation in a Swedish snail

Abstract

Formation of partially reproductively isolated ecotypes in the rough periwinkle, Littorina saxatilis, may be a case of incipient nonallopatric ecological speciation. To better understand the dynamics of ecotype formation, its timescale, driving forces and evolutionary consequences, we developed a spatially explicit, individual-based model incorporating relevant ecological, spatial and mate selection data for Swedish L. saxatilis. We explore the impact of bounded hybrid superiority, ecological scenarios and mate selection systems on ecotype formation, gene flow and the evolution of prezygotic isolation. Our model shows that ecotypes are expected to form rapidly in parapatry under conditions applicable to Swedish L. saxatilis and may proceed to speciation. However, evolution of nonrandom mating had complex behaviour. Ecotype evolution was inhibited by pre-existing mating preferences, but facilitated by the evolution of novel preferences. While in many scenarios positive assortative mating reduced gene flow between ecotypes, in others negative assortative mating arose, preferences were lost after ecotype formation, preferences were confined to one ecotype or the ancestral ecotype became extinct through sexual selection. Bounded hybrid superiority (as observed in nature) enhanced ecotype formation but increased gene flow. Our results highlight that ecotype formation and speciation are distinct processes: factors that contribute to ecotype formation can be detrimental to speciation and vice versa. The complex interactions observed between local adaptation and nonrandom mating imply that generalization from data is unreliable without quantitative theory for speciation.

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

“…colonization is likely to be initiated by a single mated female and a new population will expand rapidly to population sizes of 10 000 snails or more (as shown by empirical observations, Johannesson & Johannesson (1995), and by modelling, Sadedin et al (2009)). Following the initial colonization, the species probably quite rapidly colonized additional areas along the coast, a process that would possibly have been completed over some 100 years ago, assuming a step-wise pattern and Nm 0.01 between adjacent sites.…”

Ecological Speciation and the Intertidal Snail Littorina saxatilis

“…With the evidence for gene flow between the ecotypes in each system they were all considered to belong to one species, namely, L. saxatilis. Given the high fitness of hybrids (in their habitat) and the smooth transition between the extreme ecotypes in the Swedish system it was considered unlikely that this represented a situation in which reproductive isolation might arise [33,34], and see Sadedin et al [35]. A more likely candidate system for the evolution of isolation might be the Galician ecotypes, which show greater genetic differentiation on average [7].…”

High-Dimensional Fitness Landscapes and Speciation

“…Some of them are based on a model of adaptive radiation which is intended to be more abstract and general Vose 2005, 2009). Other attempts use models tailored for particular case studies such as cichlids in a crater lake (Barluenga et al 2006;), palms on an oceanic island Savolainen et al 2006), snails on seashores (Hollander et al 2005(Hollander et al , 2006Sadedin et al 2008), and butterflies in jungles (Duenez-Guzman et al 2009;Mavárez et al 2006). The general setup in all these models is similar.…”