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 b ehavioural barriers, at a contact zone on Monemvasia peninsula (Peloponnese , Greece), between parapatric populations of the land snail species Albinar ia 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 ethologi cally intermediate A. campylauchen specimens. Furthermore, there was high g enetic variability along with deficiency of heterozygotes, probably due to inbreeding. Additionally, the study of mortality of these populations, duri ng the adverse summer aestivation period, indicated that A. campylauchen in dividuals with an inappropriate combination of morphological and behavioura l characteristics suffered heavily. The maladaptation and disadvantage of t hese specimens revealed the possible action of a postzygotic isolation mech anism between these two morphologically, molecularly and electrophoreticall y distinct species, that can, however, mate under laboratory conditions. Fi nally, the above situation is discussed in terms of exogenous Versus endoge nous selection. (C) 2000 The Linnean Society of London.