Spatial Genetic Patterns in Relation to Regional History and Structure: Gammarus minus (Amphipoda) in Appalachian Watersheds

Abstract

Populations of Gammarus minus (Amphipoda) at spring and stream sites of 54 small drainage basins in the mid-Appalachian Mountains of central Pennsylvania are highly differentiated genetically at three allozyme loci. Mean heterozygosity, H, is little affected by drainage area, stream density and mean stream gradient. However, H is sig- nificantly correlated with: (1) latitude, with a cline of increasing homozygosity running northward, and (2) distance from the regional trunk stream, with upstream populations most remote from large trunk streams lowest in H. Latitude and trunk stream influences have probably resulted from founder effect operating over recolonizations of vacated habitats. Frequency isopleth surfaces of high frequency alleles are less broadly regional than patterns of H. Some isopleths follow ridgelines, but their modal direction is transverse across valleys, especially along drainage divides. This pattern runs counter to environmental gra- dients and is inconsistent with the operation of natural selection. It supports the hypothesis that surface streams are the main avenues of migration. Spatial correlograms of H and allele frequencies in stream-connected graphs show positive autocorrelation diminishing with distance to 40-60 km. Beyond this radius, correlations are negative or absent.