Environmental factors, microgeographic patterns of endemism and hybrid zones in Monobella grassei (Insecta: Collembola: Neanuridae)

Abstract

A complex pattern of narrow parapatric distributions with hybrid zones is described among subspecies of the Collembola species Monobella grassei in the eastern Pyrenees. Though hybrid zones are usually narrow, they vary from simple contact zones without recorded hybrids to relatively wide hybrid swarms, suggesting that large differences exist locally in the level of genetic divergence between subspecies. The course and structure of contact zones are analysed on morphological grounds, in relation to environmental (rainfall, vegetation, substrate) and topographical (altitude, hydrography) parameters. The results show that, unlike most cases documented so far, the courses of the contact zones in M. grassei are not connected to major ecological boundaries nor to ecological ecotones, on either the regional or the habitat scale. It is suggested that this is due to the great ecological similarity between the different parapatric subspecies of M. grassei, and their broad ecological requirements. There is no conclusive evidence that the distribution observed is linked to recent palaeoclimatic events such as glaciation. Endogenous selection could therefore be prevalent in this model to explain parapatry and the hybrid zone pattern.