THE MORPHOLOGY OF A "HYBRID ZONE" IN CERION: VARIATION, CLINES, AND AN ONTOGENETIC RELATIONSHIP BETWEEN TWO "SPECIES" IN CUBA.

The pulmonate genus Cerion exhibits an exceptionally high degree of geographic variation in shell size, shape, sculpture, and coloration. This morphological differentiation has been so extreme that almost every local population is different from every other one. At least some of this extensive differentiation appears to have a genetic basis; for as Bartsch (1920) demonstrated, descendants of Cerion transplanted from the West Indies to the Dry Tortugas were still indistinguishable from the parentals after several generations. Malacologists have given more than 600 specific and subspecific names to morphologically uniform population samples. But despite this plethora of names, there is no general agreement as to the actual number of valid species (Clench, 1957). The use of conventional taxonomic categories has perhaps hindered more than facilitated an understanding of variation and evolution in this group. Although Cerion can be found on almost every island, reef and cay in the Bahamas, Cuba, the Dutch Leewards, the Turks and Caicos, Hispanola, Puerto Rico, and the Virgins, the density of snails varies from none or a few to in excess of sixty per meter squared. Although there seems to be some habitat preference by different morphotypes, all the space available to a given morphotype is not occupied. As a result, it has been argued that Cerion is "normally limited to well-defined colonies, each of which can be ecologically characterized" (Mayr and Rosen, 1956). The distribution of morphotypes has been said to resemble a "crazy quilt." For on the one hand, adjacent localities may contain populations with extremely different morphologies. The observed sedentariness of individuals within such localities has been taken to indicate a very small capacity for dispersal (Mayr and Rosen, 1956). On the other hand, localities separated by up to 500 km may contain populations with very similar morphotypes. The existence of such populations has been taken to indicate a very great capacity for long distance dispersal via hurricane transport (Clench and Aguayo, 1952). In the southern Bahamas and in Cuba, a large number of morphotypes can be found in different localities within a relatively small area. For example, on the Banes Peninsula of northeastern Cuba, seven highly distinct morphotypes replace each other geographically along a strip of coastline only 50 km long (Mayr, 1963). This unusual aspect of Cerion's pattern of distribution, as well as the others mentioned above, have been explained by the genus' contradictory capacities for dispersal (Mayr, 1963). Throughout the genus' range, morphologically distinct populations are frequently separated geographically by populations whose morphologies appear intermediate in the sense that they contain different combinations of characters seen separately in either adjacent, or endpoint, locality. The transition zone between endpoint morphologies may involve few or many measures of shell form. Transitions involving few characters generally occur over distances from hundreds of meters to tens of kilometers; whereas, transitions involving many characters typically occur over distances from tens to hundreds of meters. Within the zone connecting end-