Population structure and the shape of a chromosomal cline between two races of Podisma pedestris (Orthoptera: Acrididae)

A survey was made to determine the shape of the chromosomal cline, between two races of the alpine grasshopper Podisma pedestris, in an area distant from previous detailed studies. The new section was in an isolated mountain block at the western end of the Alpes Maritimes. The past changes in the grasshoppers' distribution can be deduced from a consideration of the history of climatic fluctuations. These distribution changes imply that the hybrid zone has persisted on the isolated mountain block since before the last climatic optimum. The stability of the zone's location can be explained if there are persistent, wide gradients in Podisma density. An assessment of Podisma density revealed suitable gradients. Whilst the cline was generally the same width as in other areas, it was more abrupt where it crossed an inhospitable area, and less well defined in marginal populations. This suggested other interactions between the population density and the dynamics of the racial mixing. The implications of these interactions are discussed. It is suggested that population structure is important in the spread of co-adapted genomes, as well as in their establishment.