MOLECULAR AND MORPHOLOGICAL EVIDENCE FOR HYBRIDIZATION BETWEEN 2 ECOLOGICALLY DISTINCT GRASSHOPPERS (MELANOPLUS-SANGUINIPES AND M-DEVASTATOR) IN CALIFORNIA

This paper applies the biological species concept to two ecologically distinct species of grasshopper, melanoplus sanguinipes and M. devasta tor, by testing for reproductive isolation in the field in California. Two independent techniques for assessing gene flow between species we re employed. Firstly, we examined male genitalic morphology in populat ions from the foothills of the Sierra Nevada where the two species are parapatric. Two genetically based genitalic traits that differed betw een allopatric populations of each species formed a dine in this zone. Males captured in the field from the region of parapatry resembled of fspring from interspecific laboratory hybridizations. Secondly, we sur veyed electrophoretic variation of populations from across California and used F statistics to estimate levels of gene flow within and betwe en species. Results from both morphology and F statistics suggested th at these grasshoppers are not reproductively isolated in nature but th at gene flow between species is reduced relative to within-species. Th ese field results were consistent with a laboratory study that showed partial but not complete hybrid egg inviability. Because the hybrid zo ne is centred along an ecological transition, this system offers an op portunity to investigate whether adaptive changes across the zone cont ribute to reduced gene flow between species.