Loss of polymorphism in a declining population of Xiphophorus variatus

Allelic frequencies at the tailspot locus are usually stable in populations of the poeciliid fish, Xiphophorus variatus, but they changed over the course of several years at one locality. The change was correlated with a decrease in spring flow and pool size, which followed the installation of an irrigation pump nearby. Loss of genetic variability at this locus was not due to any single bottleneck incident because it was progressive over the course of years. Furthermore, histocompatibility variation remained high in the population, which shows that it had not become inbred. It is hypothesized that loss of tailspot diversity was selective and resulted from loss of habitat diversity as pool area contracted. The results are relevant to endangered species because they suggest that decreases in evolutionary potential may occur long before effects caused by small population size can be detected