The emergence of the Central American isthmus (approximately 3.5 Ma) presents an ideal natural experiment for testing the causal basis of the rates and mechanisms of evolutionary change of marine organisms. This event isolated previously contiguous populations in the eastern Pacific and Caribbean, and initiated the development of different selective regimes on opposing sides of the barrier. Molluscan stable isotopic profiles can provide an excellent record of ambient environmental conditions. In particular, molluscan isotopic profiles record the contrasting patterns of seasonal change in temperature, salinity, and upwelling that characterize the eastern Pacific and southwestern Caribbean today. Stable isotopic analyses will be conducted for selected mollusc shells from extensive and well-dated Plio-Pleistocene sections on either side of the Central American isthmus. The purpose of these analyses is to: 1) track the development of the differences between eastern Pacific and Caribbean environments, and 2) provide a paleoenvironmental context for future evolutionary studies of marine invertebrate taxa.
