Evolution from transplants between genetically distinct populations of freshwater snails

Abstract

In July, 1979, a series of 12 transplant experiments was begun using Goniobasis proxima (Say), a freshwater snail living in highly isolated populations in the southern Appalachians. At each site, 500 native snails were removed and replaced with 500 snails from a second population in which different alleles were fixed at one or two enzyme loci. The sites were resampled in 1983 to identify populations where the introduced genome had in fact become established. Only two of the twelve introductions were successful. Introduced genes have spread about 15–20 meters per year upstream and 5–10 meters per year downstream. Considering observed population sizes and age-specific survivorships, it appears that the observed frequencies of the introduced genomes at both sites are significantly greater than expected from simple diffusion. A selective advantage for the introduced snails or their F₁ progeny seems likely. This implies that the genetic differences between G. proxima populations may in some cases be due to random processes.