Genetical and ecological differentiation across a hybrid zone

There is a narrow hybrid zone between two chromosomal races of the grasshopper Podisma pedestris (L.) in the French Alps. There are no gross differences in habitat across the zone, and a multivariate analysis of the vegetation composition detected no significant transition either. The hatch and development rates of Podisma in hybrid populations were lower than those found in matched habitat types outside the zone. The simplest explanation for the persistence of the parapatric distribution of the two races is that hybrid disadvantage is maintaining a tension zone between them.