The Origin of Geographic Diversity in the Bahamian Land Snail Cerion: The Fossil History of Modern Patterns

George Washington University, Washington DC

Investigators

Abstract

ABSTRACT The origin of geographic diversity in the Bahamian land snail Cerion: the fossil history of modern patterns Glenn A.Goodfriend, Stephen J. Gould, and M. G. Harasewych EAR-0106936 The land snail Cerion is noteable for its extreme geographic diversity in shell morphology: very different forms occur adjacent to each other, with steep character gradients between them. This study is aimed at revealing the history and possible causes of the geographic diversity along a stretch of ca. 12 km of the east coast of Long Island, Bahamas, where 5 distinctive forms occur. Starting with the geographic patterns of the modern (living) forms, we will follow these patterns back in time through excavation and analysis of the Holocene fossil record of the snails present in the eolian sand deposits along the coast. Ca. 20-35 shells from each of ca. 25-30 sites will be dated by amino acid racemization (calibrated against a series of radiocarbon ages) and analyzed morphmetrically to obtain temporal and geographic series. PIs will evaluate several alternative scenarios concerning the origin of geographic diversity in these snails: 1. The patterns are relatively stable and old, resulting from in situ differentiation of geographically continuous populations of snails. Selection would be the primary cause of geographic diversity. 2. Geographic patterns are relatively dynamic and result from population dynamic processes such as dispersal and extirpation. These patterns could have various origins: a. Destriuction of Cerion populations across a broad region by, e.g., a hurricane, opens up a broad region for recolonization by snails originating from nearly sources or long-distance dispersal; the populations expand and meet. Broad scale environmental events would be the primary control of geographic diversity. b. Cerion populations are typically in a state of flux, with local extirpations and re-establishment of populations, sometimes from long-distance dispersal. Stochastic population processes and smaller scale environmental events (e.g., washover fans from storms) would be the primary control. Based on preliminary results, we can expect to have a detailed record of Cerion back to ca. 5000-6000 yr BP. A parallel study will look at the relationships of living forms using mDNA sequences. These studies will contribute to understanding the origin and maintenance of geographic diversity of organisms.