Genetics and Morphometrics of Cerion at Pongo Carpet: A New Systematic Approach to this Enigmatic Land Snail

Gould, S. J., D. S. Woodruff, and J. P. Martin (Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138; and Department of Zoology, Duke University, Durham, North Carolina 27708) 1975. Genetics and morphometrics of Cerion at Pongo Carpet: a new systematic approach to this enigmatic land snail. Syst. Zool. 23:518-535.Cerion displays an extraordinary range of phenetic diversity. This has led to the naming of several hundred species, though only a mere fraction of these are valid if, as we believe, morphological diversity does not reflect reproductive isolation in this genus. The taxonomic morass has precluded fruitful evolutionary study of these extremely plastic animals. We apply techniques of multivariate morphometrics and biochemical genetics to a local situation on the northeastern coast of Great Abaco, Bahamas. Here, a local population is semi-isolated along a strip of coastline near the settlement of Pongo Carpet; it is in contact with other populations only to the north. Its morphology is highly distinct and, on all previous criteria, it merits recognition as a separate species. We find, however, that its patterns of covariation (as revealed by factor analysis) cannot be distinguished from those of the only recognized taxon of northern Abaco, C. bendalli (while patterns in the southern Abaconian species, C. abacoense, are entirely different). Canonical analysis displays a cline towards standard morphology from the region of maximum isolation at Pongo Carpet to the northern area of potential contact with standard C. bendalli. Levels of morphological variation do not differ among samples; those of intermediate morphology show no increase in variability or other signs of hybridization. Starch-gel electrophoresis showed the pongo carpet samples to be genetically indistinguishable from standard bendalli at the 18 structural gene loci surveyed. No evidence for selfing or inbreeding was detected. The proportion of polymorphic loci in each population ranged between 16.6-22.2%; genic heterozygosity per individual was 5.25-6.79%. The only genetic "anomaly" within the area contrasts some standard and pongo carpet samples with C. bendalli from elsewhere on Abaco and Grand Bahama Island. We conclude that the pongo carpet population is a well-marked geographic variant within Cerion bendalli. The coordinated application of biochemical genetics and multivariate morphometrics can resolve many issues in the systematics of highly variable organisms. [Geographic variation; Cerion; morphometrics.] Cerion is a classic among evolutionary classics for its supposed protean speciation. Clench (1957) lists 302 recorded species from the Bahama islands alone. Whether the plethora of specific names be justified or not, we may safely state that phenetic diversity within the genus can only be described as fantastic (Fig. 1). Moreover, variation within any local population is strictly average in extent; thus, local popu1 This is the third in a series of papers on The Natural History of Cerion. Previous items are Gould (1969a and 1971). 2 Department of Zoology, Duke University, Durham, North Carolina 27708. lations are strongly marked, and demes in close geographic proximity can be very different in mean morphologies. How can we deal with such a genus in a modern way? The old practice of erecting a name for each local variant does not, in all likelihood, reflect the biology of reproductive isolation. Yet this procedure has colored all previous approaches to Cerion. For example, the standard hypothesis (Clench, 1957; Mayr and Rosen, 1956) that species of Cerion are haphazardly distributed by vagaries of hurricane transport depends upon the general validity of current specific nomenclature. If the "species"