Gene flow and differential mortality in a contact zone between two Albinaria species (Gastropoda; Clausiliidae)

This paper reports on the disruption of normally effective ecological and behavioural barriers, at a contact zone on Monemvasia peninsula (Peloponnese, Greece), between parapatric populations of the land snail species Albinaria discolor and Albinaria campylauchen. Detected outcomes were the increase of gene flow between these two species at the contact zone, the occurrence of rare alleles, as well as the occurrence of morphologically and ethologically intermediate A. campylauchen specimens. Furthermore, there was high genetic variability along with deficiency of heterozygotes, probably due to inbreeding. Additionally, the study of mortality of these populations, during the adverse summer aestivation period, indicated that A. campylauchen individuals with an inappropriate combination of morphological and behavioural characteristics suffered heavily. The maladaptation and disadvantage of these specimens revealed the possible action of a postzygotic isolation mechanism between these two morphologically, molecularly and electrophoretically distinct species, that can, however, mate under laboratory conditions. Finally, the above situation is discussed in terms of exogenous Versus endogenous selection. (C) 2000 The Linnean Society of London.