Convergence in the shell‐banding polymorphism of two sympatric landsnails

Xeropicta vestalis, a polymorphic helicelline as regards its shell‐banding pattern, has shells that closely resemble those of Theba pisana, a helicine. These resemblances are found mainly in Israel's coastal plain, where distribution of the two species overlaps to a certain extent, and it hardly ever occurs in Israel's hilly region where T. pisana is missing. In the Jordan Valley however (where T. pisana is missing also), some T. pisana‐like forms of X. vestalis may occur. Because X. vestalis is smaller than T. pisana, the resemblance refers mainly to juveniles and small adults of the latter, and less to large adults. We examined whether there are any interspecific relations between the frequencies of visually analogous morphs, in mixed populations. No such relations was found. Further, mixed populations are so unstable in time that the chances of building up any such relation is highly unlikely. It is difficult to accept that there are any interrelationships whatsoever (of apostatic selection as well as mimicry) between T. pisana and X. vestalis as regards their shell banding polymorphism. The convergence between the two species appears to represent parallel, but not interspecific forces of natural selection.