Ecological Speciation in Sticklebacks: Environment-Dependent Hybrid Fitness

Abstract

Abstract.-"Ecological" speciation occurs when reproductive isolation evolves as a consequence of divergent selection between populations exploiting different resources or environments. We tested this hypothesis of speciation in a young stickleback species pair by measuring the direct contribution of ecological selection pressures to hybrid fitness. The two species (limnetic and benthic) are strongly differentiated morphologically and ecologically, whereas hybrids are intermediate. Fitness of hybrids is high in the laboratory, especially F) and F 2 hybrids (backcrosses may show some breakdown). We transplanted F 1 hybrids to enclosures in the two main habitats in the wild to test whether the distribution of resources available in the environment generates a hybrid disadvantage not detectable in the laboratory. Hybrids grew more slowly than limnetics in the open water habitat and more slowly than benthics in the littoral zone. Growth of F 1 hybrids was inferior to the average of the parent species across both habitats, albeit not significantly. The contrast between laboratory and field results supports the hypothesis that mechanisms of F 1 hybrid fitness in the wild are primarily ecological and do not result from intrinsic genetic incompatibilities. Direct selection on hybrids contributes to the maintenance of sympatric stickleback species and may have played an important role in their origin.

Genetics of ecological divergence during speciation

“…1f). This pattern corroborates the results from transplant experiments in the native lake showing a growth disadvantage in intermediate hybrids, relative to the two parental species in their native habitats 14,21,22 . Thus, our results on the genetics of divergence in niche use and whole-organism performance suggest that the underlying genetic basis of extrinsic postzygotic reproductive isolation between benthic and limnetic stickleback is largely additive.…”

“…Each species pair likely arose from a double lake invasion via the sea 12 , followed by further divergence with gene flow 16,20 . Hybrids are intermediate in morphology and are outperformed by each parental species in the preferred parental habitats 14,21-23 . Little intrinsic postzygotic isolation has evolved between the species, as laboratory-reared hybrids are viable and fertile 16,21 .…”

“…Hybrids are intermediate in morphology and are outperformed by each parental species in the preferred parental habitats 14,21-23 . Little intrinsic postzygotic isolation has evolved between the species, as laboratory-reared hybrids are viable and fertile 16,21 .…”

Low levels of hybridization between sympatricArctic char (Salvelinus alpinus) andDollyVarden char (Salvelinus malma) highlights their genetic distinctiveness and ecological segregation

“…Studies of hybrid zones have suggested that phenotypically intermediate hybrid individuals often perform poorly in parental niches (e.g., Hatfield and Schluter 1999; Godoy-Herrera et al. 2005), and DeLacy and Morton (1943) indicated that the two species have very different habitat use and migratory life history in sympatry (see above).…”

Genetic Coupling of Female Mate Choice with Polygenic Ecological Divergence Facilitates Stickleback Speciation

“…These species are morphologically adapted to their contrasting food sources: benthic stickleback primarily feed on invertebrates inhabiting the substrate or attached to vegetation, whereas limnetics specialize on zooplankton [10–12]. Although hybrids exist in the wild [13–15] and there are no strong intrinsic incompatibilities [14,16], benthics and limnetics show nearly complete assortative mating in experimental trials [17]. Previous no-choice mating trials suggested that benthic and limnetic females prefer mates with similar body size [18–20] and shape [20].…”