Character Variation in Two Land Snails from the Dutch Leeward Islands: Geography, Environment and Evolution

Cerion tva (L.) and Tudora megacheilos (Potiez and Michaud) are the dominant land snails of Aruba, Bonaire and Curacao; both display the high degree of intraspecific character variation so often noted in terrestrial gastropods. This study correlates patterns of morphologic diversity with geographic and environmental distribution. Projected upon factor axes, the samples of each species fall into four groups corresponding to degrees of geographic isolation: Aruba, Bonaire, eastern and western Curacao. Factor axes represent the major trends of variation in shell size and shape. Subgrouping within geographic clusters is influenced by environmental parameters. In Cerion, for example, shells collected from volcanic rocks tend to be larger than those from limestone. In all geographic regions, these volcanic samples are distinguished by their higher projection on an axis representing increased size and concomitant changes of shape. Pulmonates usually favor limestone, but the anomaly of Cerion's smaller size is related to the unfavorable climatic conditions associated with limestone areas in the Dutch West Indies (dry, windy, and poorly vegetated). If the morphologic differences among geographic regions are similar to ecologicallyinduced variation within regions, then interregional differences are probably adaptive, and not a fortuitous result of geographic isolation, as many have claimed. Cerion is small ancd Tudora large in eastern Curacao; Tudora is small and Cerion large in western Curacao. This interregional relationship is parallelled by significant negative correlations between sizes of the two species for samples within regions. Diversity patterns did not change significantly during 16 years separating the studies of Baker and Hummelinek. Baker's data show the same distinction of four geographic areas and similar patterns of distribution within areas; significant positive correlation exists between measures made by Baker and Hummelinck on samples collected from the same localities. Fossil samples from cave phosphates do not conform to modem distributions. The basic character diversity pattem-good (presumably genetic) distinction among regions and ecologically induced variation within regions-requires a revision of Baker's classification. Separate names for the four regions are warranted, but the cumbersome nomenclature for intraregional diversity should be dropped.