Evolution and systematics of Cerion (Mollusca, Pulmonata) on New Providence Island : a radical revision. Bulletin of the AMNH ; v. 182, article 4

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[New York] : American Museum of Natural History
"Cerion has been described by its leading student W.J. Clench as 'the most difficult genus of pulmonate mollusks to classify.' No other pulmonate genus shows greater diversity of form; moreover, almost all these divergent morphologies hybridize at their areas of geographic contact. The result is a taxonomic morass that has effectively debarred fruitful biological work on these fascinating snails. The taxonomy of New Providence Island is the most elaborate for the entire genus; more than 90 species of Cerion have been designated, and the distribution of existing names makes no geographic or ecological sense. We use morphometric and genetic techniques to conclude that the living Cerion of New Providence reduce to two semispecies, C. glans (the ribby morphotype) and C gubernatorium (the mottled morphotype). Traces of an extinct C. agassizi line (a prominent taxon of fossil dunes) survive as introgressions into C. gubernatorium populations of southeastern New Providence. These two groups form the basis of Cerion faunas throughout the northern Bahamas; their separation and recognition provide a key to resolving Cerion's taxonomy both here and elsewhere. Ribby and mottled morphotypes are recurrently evolved 'developmental packages,' not homologous taxa from place to place -- thereby illustrating the importance of developmental channeling in parallel evolution. We base our taxonomic decision upon a conciliance among many independent criteria. Geographic distribution: parental morphotypes occupy their expected positions, with narrow hybrid zones at predicted points of transition. Morphology of parental populations: differences between ribby and mottled are not simple consequences of one minor alteration in growth, but summed results of several independent covariance sets. Morphotypes differ consistently in amounts of variation and patterns of covariance. Hybrid populations: we find both enhanced variation and patterns of covariance based on developmental disturbance never before detected in parental populations of Cerion. Genetics: samples grouped by frequencies of electromorphs yield the same clusters of ribby and mottled populations specified by morphology. Morphotypes can be distinguished by allele frequencies, while hybrid populations contain rare alleles found in neither parental taxon. The genetic hybrid zone is wider than the morphological zone and asymmetric about it. An appendix allocates all previously named taxa, and resolves the status of 19 designated fossil taxa, plus a new species (Cerion clenchi), into three successive faunas, marking waves of migration correlated with rise and fall of Pleistocene seas"--P. 391.
p. 390-490 : ill., maps ; 26 cm.
Includes bibliographical references (p. 488-490).