Relative Contribution of Dispersal and Natural Selection to the Maintenance of a Hybrid Zone in Littorina

Abstract

Abstract. Habitat preference behavior may play an important role in nonallopatric speciation. However, most examples of habitat preference contributing to differentiation within natural populations correspond to parasites or herbivores living in the discrete environments constituted by their animal or plant hosts. In the present study we investigated migration guided by habitat preference in the intertidal snail Littorina saxatilis in a hybrid zone associated with an ecotone across the shore, which is therefore a continuously varying environment. First, we found evidence for this behavior in one of the two locations studied. Second, we made reciprocal transplants to suppress the phenotypic gradient observed across the hybrid zone and measured the relative contributions of selection and migration to its regeneration. Selection played an important role at the two locations studied, but migration was only important at one, where it accounted for between a third and a half of the regenerated gradient. This overall minor effect of migration was relevant for theoretical models dealing with nonallopatric speciation, because it suggested that variation for habitat preference did not have an important role in the initiation of the differentiation process. The preference behavior observed in the hybrid zone would have evolved secondarily, as a consequence of habitat-dependent fitness differences between phenotypes.

Adaptation to a Steep Environmental Gradient and an Associated Barrier to Gene Exchange in Littorina Saxatilis

“…However Cruz et al (2004) considered that while migration guided by habitat choice was evident in this system, it played a minor role in the re-establishment of experimentally manipulated phenotypic gradients. They suggested that habitat choice may be in the process of evolving as a result of habitat-related fitness differences of different morphs.…”

Habitat Choice and Speciation

“…Experimental evidence for nonrandom dispersal in L. saxatilis ecotype populations has been obtained in both Spain and Sweden [78,82,83]. In the Swedish populations, displaced snails exhibited greater average dispersal distances than nondisplaced ones and dispersal differed between E and S ecotypes, in addition to a tendency to recapture snails in their own habitat more often than expected from random dispersal [83].…”

“…In avoiding the hazards of crab predation by their position on the shore (low in Spain, high in Britain and Sweden), these ecotypes are free to develop larger shell apertures, increasing foot area and thus grip on the substrate to minimise dislodgement. Nonrandom mating is also observed in each population of ecotype pairs [78][79][80][81], primarily due to assortative mating by size (see [72] for a review). L. saxatilis lacks a pelagic larval stage, instead exhibiting direct development where females retain their brood internally and release fully formed young [71].…”

“…On the Galician shore in Spain, Cruz et al [78] tried to separate survival and habitat preference in the two ecotypes of L. saxatilis using two reciprocal transplant experiments. In the first experiment, sample groups of each ecotype were transplanted both to their native and nonnative shore level at each of two sites.…”

Parallel Ecological Speciation in Plants?

“…Although we only discuss one here, there are several regional cases of this divergence, with ecotypes adapted to different microhabitats created by tidal and substrate variation. The best-studied ecotype pair has evolved at least three times independently in Galician Spain, where the two types specialize in and prefer either the high intertidal barnacle belt or the low intertidal mussel belt and occasionally hybridize in the intermediate area between those environments [134,135]. The main axes of morphological differentiation are in shell size and shape, which aid in resistance either to dislodging by wave action (thin, small shells with large apertures for the muscular foot) or to crab predation and desiccation stress (thick, large shells with small apertures), and there are additional differences in shell ornamentation [19].…”