A narrow hybrid zone between two Cottus species in Wills Creek, potomac drainage

We describe a narrow hybrid zone between the mottled sculpin (Cottus b. bai rdi) and the Blue Ridge sculpin (C. caeruleomentum). Seven characters (dors al fin rays, pectoral fin rays, caudal base band condition, male spawning c oloration, and one frequency and two fixed allozyme differences) distinguis h the two taxa in the hybrid zone. C. caeruleomentum and C. b. bairdi diver ged in these characters in allopatry as indicated by their distribution on opposite sides of the Atlantic-Ohio divide. However, a stream capture place d these two taxa in secondary contact in Wills Creek, Potomac drainage (Atl antic slope). Allozyme data indicate the presence of post-F-1 hybrids in th e zone of secondary contact. Changes in allozymes, morphology, and spawning coloration along a transect in Wills Creek reveal the hybrid zone is less than 20 river kilometers in length. Estimates of root mean square dispersal and gene flow tentatively suggest that selection is operating in the Wills Creek hybrid zone. C. b. bairdi and C. caeruleomentum are maintaining thei r identity in seven distinguishing characters on opposite ends of the hybri d zone revealing these two taxa are independent evolutionary lineages.