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The Geography of Speciation in Marine Organisms
 Understanding the geographic scale at which new species arise in the sea has challenged marine biogeographers for well over a century, and our lab has been investigating the geographic circumstances of speciation in two different systems: a) nearshore faunas of cool-temperate rocky-shores and b) sister-species pairs currently separated by the Isthmus of Panama.
Research on rocky-shore Nucella suggests that even though areas of sympatry among living species can be extensive, sympatric distributions on rocky shores likely represent secondary contact between species that evolved in geographic isolation. These species often also show significant morphological differentiation where they co-occur, suggesting that secondary contact and competitive interactions between recently diverged sister-species plays an important role driving morphological evolution.
In tropical America, so-called “geminate” species pairs found on either side of the Central American Isthmus provide a unique model system to understand the geography speciation. Given geological and paleo-oceanographic information that dates separation of the tropical eastern Pacific and western Atlantic oceans at 3.1-3.5 million years ago, geminates living today on either side of the isthmus are believed to represent the descendants of single ancestral species that were broadly distributed throughout the region prior to the formation of the land bridge between North and South America.
But is this all there is to this story? Molecular sequence data gathered in my lab indicates that geminate species in the bivalve family Arcidae split significantly earlier than 3 MY ago with three of six pairs possessing divergence times significantly greater than 10 MY. Discriminant analyses of Recent and fossil shell material corroborates this result, showing that arcid sister-species were distinct long before the earliest impacts of the the rising isthmus. The data suggest that the faunas of these two oceans were likely on independent evolutionary trajectories long before the time of final seaway closure.
 This work is supported by a grant from the National Science Foundation.
Related publications:
Marko, P. B. 2005. An intraspecific comparative analysis of character divergence between sibling species. Evolution 59: 554-564.[PDF]
Marko, P. B. 2002. Fossil calibration of molecular clocks and the divergence times of geminate species pairs separated by the Isthmus of Panama. Molecular Biology & Evolution 19: 2005-2021. [PDF]
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