Both gene-flow and natural selection can play an important role in determining the amount of geographic variation between populations of marine benthic organisms. Genetic similarity between populations depends upon gene-flow. Clinal and disjunct differences between spatially separated populations may result from either nongenetic adaptation or genetically determined variations. Among most coastal and estuarine benthic species gene-flow is largely limited to the dispersal that occurs during pelagic larval development. To understand the dispersal capability of sedentary bottom-dwelling invertebrate spp it is necessary, lst, to know their life history, i.e., whether development is pelagic or nonpelagic and the length of planktonic larval existence; 2nd, to have information on the estuarine and coastal circulation which can disperse the larvae; 3rd, to study behavior of larvae, particularly their vertical movement and small-scale distribution or 'patchiness' and its relationship to local hydrography; and 4th, to know from direct evidence the geographical distribution of the larvae. However, gene-flow, even with extensive dispersal, is only successfully completed if immigrants survive to reproduce. Genetic differences between populations result largely from natural selection of favored genotypes, although random loss of alleles from small populations and the 'founder effect' near the species geographic limit can account for some genetic heterogeneity. One direct way to measure selection in natural populations is to relate variation to age structure within a population, another is to demonstrate genetic variation within a population in a heterogeneous or mosaic environment. This review considers some of the evidence for gene-flow between and natural selection within estuaries and coastal regions of the continental shelf. |
