PHYLOGENETIC DIVERSITY OF THE LUCINIDAE IN THE WESTERN ATLANTIC USING FOSSILS AND MOLECULES

While molecular phylogenies are a powerful tool for inferring the evolutionary relationships of extant taxa, they cannot include the 99% of taxa that are now extinct, except under exceptional circumstances. Recently, estimates of tree topology and divergence time has been improved using ‘Total Evidence’ approaches, combining molecular sequences with morphological data to fit molecules and fossils onto the same tree. We created two time calibrated phylogenies of the Lucinidae, a group of marine, chemosymbiotic bivalves, to assess how adding morphological data to molecular phylogenies affected tree topology, branch lengths, and estimates of phylogenetic diversity from the Pliocene through modern in the Western Atlantic.

We coded 47 morphological characters for 25 species of lucinids and added this data to a global dataset of 18s rRNA sequences and fossil temporal occurrences. We created two Bayesian phylogenies in MrBayes. One analysis excluded morphology and only used molecular sequences, with fossils added as topological constraints, and the other included morphological data to place fossil taxa in the tree. We extracted 1000 trees at random from each of these two datasets to assess topology and calculate the distribution of branch lengths and phylogenetic diversity.

Overall topologies were grossly similar between the two analyses, but branch lengths of taxa in the morphological analysis were proportionally greater than the average and less skewed than the molecular-only analysis. This suggests that the morphological data allows the analysis to constrain the fossil taxa into the total molecular phylogeny, improving branch length estimates. Phylogentic diversity is the sum of the branch lengths of extant taxa on the tree, and can be used to determine how much evolutionary history was lost in a clade. For lucinids in the Western Atlantic, more phylogenetic diversity was lost than would be expected by chance north of Florida using the molecular+morphological analysis, suggesting extinction/extirpation was clumped in this area. This occurs because taxa that lived in northern areas during the Pliocene restricted to Florida today. By integrating fossils and molecules we can test how evolutionary history changed through time and gain a better understanding of a clade’s past than using either type of data alone.