COMPARISONS AMONG MORPHOLOGICAL CHARACTERS AND BETWEEN LOCALITIES IN THE CHORTHIPPUS-PARALLELUS HYBRID ZONE (ORTHOPTERA, ACRIDIDAE)

Two subspecies of the grasshopper, Chorthippus parallelus, meet and hybridize in the Pyrenees. The hybrid zone between the two taxa is believed to have formed following range expansion at the end of the last glaciation and to be maintained by a balance between gene flow and selection against hybrids. Laboratory F1 hybrid males are sterile. We have examined morphological characters in two cols about 200 km apart and compared the positions and widths of clines for these characters both within and between cols. Clines are neither coincident (centred in the same position), nor concordant equal in width) in either col. Furthermore clines are wider and more dispersed in the western col. The lack of concordance can be explained on several models for the maintenance of the hybrid zone and is a common observation. There are many fewer examples of lack of coincidence and this result is more difficult to interpret. We suggest that it is due to a difference in population structure between the cols: a patchy distribution of grasshoppers in the western col produces wider, more dispersed clines while a sharp break in distribution in the eastern col tends to concentrate morphological change over a shorter distance.