ENVIRONMENTAL-FACTORS, MICROGEOGRAPHIC PATTERNS OF ENDEMISM AND HYBRID ZONES IN MONOBELLA-GRASSEI (INSECTA, COLLEMBOLA, NEANURIDAE)

A complex pattern of narrow parapatric distributions with hybrid zones is described among subspecies of the Collembola species Monobella gra ssei in the eastern Pyrenees. Though hybrid zones are usually narrow, they vary from simple contact zones without recorded hybrids to relati vely wide hybrid swarms, suggesting that large differences exist local ly in the level of genetic divergence between subspecies. The course a nd structure of contact zones are analysed on morphological grounds, i n relation to environmental (rainfall, vegetation, substrate) and topo graphical (altitude, hydrography) parameters. The results show that, u nlike most cases documented so far, the courses of the contact zones i n M. grassei are not connected to major ecological boundaries nor to e cological 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 ec ological requirements. There is no conclusive evidence that the distri bution observed is linked to recent palaeoclimatic events such as glac iation. Endogenous selection could therefore be prevalent in this mode l to explain parapatry and the hybrid zone pattern. (C) 1998 The Linne an Society of London.