Geographic variation and hybridization in populations of Bahama snails (Cerion). American Museum novitates ; no. 1806

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New York, N.Y. : American Museum of Natural History
"A survey was made of the Cerion snails in the Bimini group, Bahama Islands. Samples were obtained of every colony on North, East, and South Bimni, and the location and extent of the colonies were mapped. Samples from Gun Cay, North Cat Cay, and South Cat Cay were also studied. Subfossil dead shells of Cerion were collected in several areas where Cerion has become extinct. 2. Each colony has its own diagnostic characteristics of size, shape, sculpture, and coloration, but three groups of colonies are distinguishable: the lerneri group, the biminiense group, and the Cat Cay group (pillsburyi). Even though superficially each group appears to be a separate species, each is allopatric, and adjacent colonies show signs of gene exchange. Two colonies are apparently hybrid populations. 3. Hybrid colonies show high variability (C.V.) with respect to sculpture (amount of costation) and coloration, but size and shape are not more variable than in other colonies. 4. Colony size is very variable, ranging from a few hundred individuals to several hundred thousands. 5. Some colonies are isolated from one another by stretches of from 1 to 5 kilometers of unsuitable or unoccupied terrain; in other areas the colonies form an uninterrupted band parallel to the beach. 6. The colonies are similar to one another in basic ecology, but differ in preference for substrate (coral rock or sand), preferred plants, and exposure. 7. Geographic variation is pronounced, but irregular. The facts are best explained by one's assuming two antagonistic tendencies: a high degree of sedentariness and infrequent long-distance dispersal by hurricanes. 8. Differences in shell characteristics evolve more rapidly than reproductive isolation. As a consequence, some populations which had become morphologically very distinct during a period of isolation were still able to interbreed freely when they again came into contact. 9. The characteristics that adapt Cerion so superbly to its continuously changing habitat make it exceedingly difficult to classify the populations of this genus in the conventional categories of species and subspecies"--P. 45-46.
48 p. : ill., map ; 24 cm.
Includes bibliographical references (p. 46-48).