Genetic Differentiation without Isolation in the American Eel, Anguilla rostrata. II. Temporal Stability of Geographic Patterns

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Abstract
During 1972 through 1975, elver and 'adult' (recruit and resident, respectively) specimens of the American eel, Anguilla rostrata, were sampled at several North American localities. Three enzyme loci (Sdh, Phi and Adh) were studied to assess the temporal stability of patterns of spatial differentiation first described by Williams et al. (1973) from samples taken in 1971. Because of the universal panmixia of this species, assuming the life cycle as understood is correct, differentiation among resident continental populations must be due to natural selection and not isolation, unless there is temporal instability of the variation or absence of universal panmixia. Spatial variation, as latitudinal variant frequency clines, occurred in both residents and recruits at the Sdh locus, consistent with the findings of Williams et al. (1973). The cline was temporally stable. Phi variant frequencies were correlated with latitude in residents, but recruits were genetically homogeneous throughout the geographic range of Puerto Rico to St. John's, Newfoundland. Within-locality differences occur between residents and recruits in northernmost localities, reflecting the stronger clinal differentiation of residents in this area. The differentiation of variant frequencies between the two life stages, seems to occur immediately after residence in continental waters has begun, as frequencies were correlated with within-year sampling date. The weak clinal variation described for Adh variant frequencies by Williams et al. (1973) could not be substantiated. Rather, allele frequencies among recruits are homogeneous throughout North America, but vary significantly from one year to another These results strengthen and extend those of Williams et al. (1973) in that patterns of spatial differentiation are generally temporally stable, thereby implying the differentiation of each annual cohort in the same pattern and by the action of natural selection. Selection must be implicated in the temporal variation at the Adh locus, which occurs in a similar manner among subpopulations (if they exist), or in a single panmictic population, as is thought to exist in this species.

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