Systematics, ecology and ecological genetics of some Northeastern Pacific Littorina (original) (raw)

1990

Abstract

ABSTRACT Thesis (Ph. D.)--University of Washington, 1990 A new species Littorina tatooshensis from the Northeastern Pacific is described on the basis of morphological and behavioural differences, hybridization studies, and allozyme electrophoresis. Genetic differences in shell morphology were identified among (1) the new species L. tatooshensis, (2) Littorina sitkana which occurs in contiguous habitats in the Northeastern Pacific, and (3) Littorina kurila from further west in the Northern Pacific, by culturing them in a common environment. There was considerable morphological overlap among the three species. L. tatooshensis cultured at low density had faster growth rates, higher spires, and narrower apertures than their siblings cultured at high density; but L. sitkana cultured at low density had the same shape as L. sitkana grown more slowly in the field but were smooth rather than heavily ridged.L. tatooshensis was found only on wave-exposed shores and survived poorly when transplanted to protected shores containing predatory crabs. The larger, thick-shelled, protected shore species, L. sitkana, survived moderately well when transplanted onto exposed shores but would be unlikely to reach maturity before being washed away in winter storms.Heritabilities of some shell form traits were estimated for a L. tatooshensis population in order to predict the short term response to selection on these traits. Quantitative genetic analysis was done using a half-sib design with each brood of full sibs split between two environments. Significant heritability estimates were obtained for shell size and shape and for aperture shape. Significant genetic correlations were obtained between shell shape parameters. Such correlations can constrain the response of a trait to directional selection unless selection is strong.A simulation model was constructed for a polygenic, quantitative trait that experiences different selective pressures on exposed and sheltered shores; genetic differences were maintained between the two populations despite high intermigration rates even though the model did not include mutation. Changing the phenotypic plasticity from random to adaptive reduced the ability of selection to maintain genetic differentiation between the two populations. Genetic differences between exposed and protected populations can persist for significant periods of time and may have sometimes been the first step in speciation.

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