Meeting Abstract

28.5  Jan. 5  Larval Mode and Species Accumulation in a Transisthmian Marine Bivalve MARKO, P.B.*; MORAN, A.L.; Clemson University; Clemson University pmarko@clemson.edu

Evolutionary biologists have employed the formation of the Isthmus of Panama and the closure of the Central American Seaway approximately 3.1 million years ago as a model system for the study of allopatric speciation. The fossil record indicates that the closure of the Seaway was not an abrupt occurrence, but a complex macroevolutionary event involving massive species turnover (speciation and extinction) in response to environmental upheaval associated with the gradual separation of the Atlantic and Pacific oceans. Here, we use molecular phylogenies of the bivalve subgenus Acar to investigate the rate of accumulation of clades in tropical America over the course of Isthmus formation in the context of larval developmental mode. Molecular analyses suggest the existence of nearly an order of magnitude more species than currently recognized in this single subgenus. Lineages-Through-Time-Analysis (LTTA) shows that the accumulation of lineages has been constant throughout most of the history of the subgenus, including the period of Isthmus formation, suggesting no detectable macroevolutionary response to seaway closure in this group. Most lineage splitting events in the last seven million years, however, are concentrated in a single clade distinguished by brooded development. If brooded development tends to increase rates of both speciation and extinction, this clade may have experienced significant species turnover that cannot be detected with LTTA and which can probably only be characterized through analysis of morphologically distinct forms in the fossil record.