Spatial structure and habitat variation in a grasshopper hybrid zone

A hybrid zone between the grasshoppers Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) in northern Spain has been analyzed for variation i n morphology and ecology. These species are readily distinguished by the nu mber of stridulatory pegs on the hind femur. Both sexes are fully winged an d inhabit disturbed habitats throughout the study area. We develop a maximu m-likelihood approach to fitting a two-dimensional cline to geographical va riation in quantitative traits and for estimating associations of populatio n mean with local habitat. This method reveals a cline in peg number approx imately 30 km south of the Picos de Europa Mountains that shows substantial deviations in population mean compared with the expectations of simple ten sion zone models. The inclusion of variation in local vegetation in the mod el explains a significant proportion of the residual variation in peg numbe r, indicating that habitat-genotype associations contribute to the observed spatial pattern. However, this association is weak, and a number of popula tions continue to show strong deviations in mean even after habitat is incl uded in the final model. These outliers may be the result of long-distance colonization of sites distant from the cline center or may be due to a patc hy pattern of initial contact during postglacial expansion. As well as cont rasting with the smooth hybrid zones described for Chorthippus parallelus, this situation also contrasts with the mosaic hybrid zones observed in Gryl lus crickets and in parts of the hybrid zone between Bombina toad species, where habitat-genotype associations account for substantial amounts of amon g-site variation.