POPULATION ANALYSIS OF A HYBRID SWARM BETWEEN GAMBUSIA AFFINIS AND G. HETEROCHIR

Abstract

Many natural fish hybrids have been reported by Carl L. Hubbs (1955). Most of the known hybrid combinations are based on few specimens. Some localities are known where hybrid frequency is high enough to be considered to represent a hybrid swarm. An analysis of the population of one such hybrid swarm is presented below. This hybrid swarm and degree of parental relation was reported in a previous paper (Hubbs, 1957a). One parental species, G. heterochir Hubbs, is restricted to the hybrid area in Clear Creek, Menard County, Texas. Its abundance is correlated with the distribution of a species of Ceratophyllum and the distribution of this plant is correlated with the relatively low water pH there. The low pH in turn is probably dependent upon Permian inliers in Cretaceous strata (Hubbs, 1957b). The other parental species, G. affinis (Baird and Girard), is widely distributed in the southeastern United States where it occupies a variety of habitats. In Texas and northern Mexico it is notably absent from clear spring waters inhabited by other gambusiine fishes. However, it occupies this habitat in the absence of related species. In some spring localities (i.e., in Graham Ranch Warm Springs in Brewster County, Texas) G. affinis is replacing endemic stocks of G. gaigei Hubbs and in others G. affinis is being replaced by introduced stocks of G. geiseri Hubbs and Hubbs (Hubbs and Springer, 1957). MATERIALS AND METHODS