Distribution, hybridization and variation in the Israeli landsnail Levantina (Pulmonata: Helicidae)

The rock-dwelling landsnail Leuantinu is represented in Israel by two species, caesareana and hierosolyma . They are both found abundantly in Mediterranean, steppe, semi-arid and extremely arid habitats, and are probably ancient elements in the Israeli fauna. Their common borderline shows no association with altitude, rainfall, temperature, climatic zones, lithology, phytogeography, or the distribution of predators. The possiblity that one species is actively displacing the other, and that the position of the borderline is temporary, is rejected on grounds both of previous research and of fossil finds.
A narrow hybrid zone occurs in most places along the borderline. Laboratory experiments show that these hybrids are inferior to the parent species in viability and fecundity, and that assortative mating of the two parent species occurs in a mixed population. Hybridization may perhaps be maintained because selection favours the individual that will mate with its own species whenever it can tind one, but is capable of mating with other species when it cannot. In these conditions, hybridization is not a waste of reproductive energy.
Certain morphological changes occur in the vicinity of the hybrid zone. Far from the zone, caesareana resembles hierosolyma in shell-height, shell shape and genitalia; approaching the hybrid zone it diverges from hierosolyma in these characteristics and develops a distinct variant, caesareana var. werneri . This divergence may represent an accumulation of isolating mechanisms in the proximity of the hybrid zone, and thus be a case of reproductive character displacement. If indeed it is, then it differs from most described cases of character displacement in that the species are parapatric, not sympatric, and the displaced character is found not only at the common frontier but also from 2–12 km into the allopatric zone.