A local hybrid swarm between Etheostoma radiosum and Etheostoma spectabile, found in June 1985 in a short reach of Little Glasses Creek, southern Oklahoma, consisted by eye of 35% phenotypic E. radiosum, 18% phenotypic E. spectabile, and 47% hybrids. In July 1985, investigation of an additional longer reach downstream of the initial discovery site showed that hybrids also were present there, although fewer. Thus, in 1985 both parental species, and substantial numbers of hybrids, were present in approximately a 300 m reach of the stream. No further collections were made until 2003, when sampling in the former hybrid reach and substantially further downstream showed E. radiosum to be the only species of Etheostoma present, and neither E. spectabile nor hybrids (by eye) were detected. From 2003 to 2013 the reach was surveyed 11 times, with a total of 363 darters identified phenotypically as E. radiosum, and none as E. spectabile or hybrids. Of 20 individuals sampled in December 2011 for molecular analyses, 19 had E. radiosum mitochondrial DNA, but one individual that had been identified phenotypically as E. radiosum exhibited a mtDNA control region sequence that was 97% similar to E. spectabile. These molecular findings are consistent with at least some genetic admixture and retention of maternal genes of E. spectabile in this now morphologically uniform population of E. radiosum. We suggest two hypotheses for the breakdown of the hybrid swarm, and the prevalence of E. radiosum over E. spectabile, including asymmetric introgressive backcrossing to largely eliminate E. spectabile, and environmental sorting by droughts or other stressors that favored E. radiosum.

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