Biochemical Evidence for Congeneric Competition as a Factor Restricting Gene Flow Between Populations of a Darter (Percidae: Etheostoma)

Echelle, A. A. , A. F. Echelle, and B. A. Taber (Biology Department, Baylor University, Waco, Tx. 76703 and Biology Department, Southwest Missouri State University, Springfield, Mo.) 1976. Biochemical evidence for congeneric competition as a factor restricting gene flow betweerw populations of a darter (Percidae: Etheostoma). Syst. Zool. 25:228-235. Electrophoretic analysis of proteins encoded by polymorphic loci demonstrates that Etheostoma spectabile populations show greater differentiation in allelic frequencies in an area of sympatry with E. radiosum than in an area of allopatry. The data are more compatible with the assumption that degree of gene-flow rather than natural selection is the primary mechanism maintaining local differentiation. Increased heterogeneity in sympatry is attributed to exclusion of spectabile from downstream areas through competition with radiosum, thereby isolating spectabile in headwater areas and restricting gene flow between populations. In sympatry, spectabile showed greater differentiation than radiosum, presumably because of greater gene flow in the latter. [Pisces; Percidae; Etheostoma; darters; proteins; electrophoresis; differentiation: gene-flow: natural selection: geograDhical isolation. ] As a group, members of the darter genus Etheostoma are primarily adapted for life in upland streams with hard substrates and clear, relatively fast-flowing waters. Downstream habitats become increasingly unsuitable for most Etheostoma. As a result, gene flow between headwater populations is restricted and marked differentiation may occur even within small geographical areas (Echelle et al., 1975). This is reflected by the high degree of speciation that has occurred in the genus. Our purpose is to demonstrate that, as a result of competition, geographical isolation and differentiation may occur even within areas of continuously suitable physical habitat. This was omitted from Mayr's (1963) survey of barriers to gene flow, but Hubbs and Springer (1957) alluded to it with their suggestion that competitive exclusion by the mosquitofish, Gambusia affinis, has contributed to speciation in the G. nobilis species group. In this paper we provide biochemical evidence for this effect. The orangethroat darter, E. spectabile, is ubiquitous in riffle habitats of streams of the Red River drainage in southcentral Oklahoma which are outside the range of the -orangebellv darter, E. radiosum. However, within the range of E. radiosum, i.e., from Blue River eastward, E. spectabile tends to be restricted to springs and headwater areas of spring-fed streams. In downstream areas E. radiosum is ubiquitous in riffle habitat whereas E. spectabile is generally rare or absent. This seems to be a clear example of competitive exclusion. In Arkansas, E. spectabile is abundant in Ozarkian areas where E. radiosum is absent, but it is much more sparsely distributed in southwest Arkansas where E. radiosum is abundant (Buchanan, 1973), further implying a competitive relationship. Our objective was to assess the degree of homogeneity in allelic frequencies among E. spectabile populations in two tributaries of the Red River, Clear Boggy River and Washita River. These are, respectively, within and outside the range of E. radiosum in southern Oklahoma. Also, Clear Boggy populations of E. radiosum were examined. We hypothesized that E. spectabile should show greater heterogeneity in the area of sympatry than in the area of allopatry, and that E. spectabile in sympatry should show greater heterogeneity than associated populations of E. radiosum.