MORPHOLOGICAL VARIATION IN INTERGRADE PUPFISH POPULATIONS FROM THE PECOS RIVER, TEXAS, USA

In the early 1980s, sheepshead minnow Cyprinodon variegatus was introd uced into the Pecos River, Texas, U.S.A. where it hybridized with the endemic Pecos pupfish C. pecosensis. By 1985, pupfish populations thro ughout approximately 300 km of the river consisted exclusively of indi viduals of hybrid origin (intergrades). There was significant (P<0.05) geographic variation in most morphological characters; the general pa ttern of variation was of a bidirectional dine centred near Pecos, Tex as. At that site, morphology of intergrade populations resembled mostl y that of the introduced species. Upstream and downstream from Pecos, morphology shifted progressively toward that typical of the native for m. Intergrade populations were morphologically intermediate to the par ental forms, showed a rapid approach to random assortment of character s, and generally exhibited greater morphological variability than occu rred in either parent species. These observations and the consistent l ack of bimodality in frequency distributions of a morphological hybrid index support the contention that intergrade populations comprise pan mictic admixtures of C. variegatus and C. pecosensis. (C) 1997 The Fis heries Society of the British Isles.