Visiting Assistant Professor of Biology Simon Coppard recently presented a talk titled “Phylogeography of mellitid sand dollars: implications regarding the Central American Isthmus and rates of molecular evolution” at the Eighth North American Echinoderm Conference in Worcester, Mass.
His research focused on sand dollars of the genera Encope, Leodia, Mellita, Mellitella and Lanthonia, which are members of the sandy shallow-water fauna in tropical and subtropical regions on the two coasts of the Americas.
Using fossil calibrated phylogenies, Coppard showed that the most recent separation between eastern Pacific and Caribbean extant clades in Mellita occurred 3.2 million years ago (Ma) and in Encope at 4.9 Ma, indicating that the Isthmus of Panama allowed genetic exchange until the Pliocene. He said this calls into question recent publications that have suggested that the Central American Isthmus was closed by the middle Miocene, 15-13 Ma.
Coppard’s research also shows that the rate of evolution of the mitochondrial cytochrome oxidase I in Encope has been 10 times slower than in the closely related genera Mellita and Lanthonia. He suggests that this probably relates to differences in generation time between genera, and more importantly shows that splits between eastern Pacific and Caribbean biota, previously dated on the assumption of a “universal” mitochondrial DNA clock, are unreliable.