The genetic basis of the ecological amplitude of Spartina patens. II. Variance and correlation analysis.

Abstract

Reciprocal transplantations of Spartina patens genotypes from adjacent salt marsh, swale and dune habitats provided evidence for genetic differentiation among subpopulations, due at least in part to contrasting selection regimes. Genet survival in the different habitats was related to the amount of genetic divergence. In dune, marsh ramets showed lowest survival, swale ramets intermediate survival, and dune ramets highest survival. This relationship was not reciprocal. The marsh habitat afforded an environment where survival was maximal for all genotypes. By comparison, the dune environment appeared to impose a more intense selection pressure, and the swale an intermediate selection pressure on S. patens. In each site resident genotypes tended to show greater relative fitness than aliens. High levels of phenotypic plasticity may permit greater adaptation to the spatially and temporally heterogeneous environment occupied by S. patens than would genetic variation alone. Dune and swale genets were more phenotypically plastic across traits examined. The higher plasticity in these peripheral subpopulations may confer increased fitness among residents and compensate for observed declines in genetic variation. Evolutionary divergence among subpopulations may be retarded by negative or unfavorable correlations among characters being selected simultaneously. These negative correlations may increase extinction probabilities in small peripheral populations, such as those represented by the dune or swale, and are likely to lower fitness. Further microevolution may be retarded in peripheral dune and swale subpopulations, primarily by unfavorable genetic correlation structures among fitness components or characters under simultaneous selection. Contributing factors may include lowered genetic variance and higher levels of phenotypic plasticity. -from Author