HOW SIMILAR ARE SISTERS? THE RELATIONSHIP BETWEEN MORPHOLOGY, ECOLOGY, AND EVOLUTIONARY HISTORY

A fundamental question in paleobiology is whether morphology and ecology are correlated and reflect trends in evolutionary history. With phylogenetic trees, we can better explore this relationship and address questions that lie at the nexus of the evolutionary and ecological sciences.

We examined the relationship between morphologic distance, ecological distance, and phylogenetic distance between species pairs for 61 taxa used in a recent phylogenetic revision of the brachiopod order, Strophomenida. Morphological distance was calculated as the pairwise Euclidean distance in a principal coordinate ordination of character data. Ecological distance was calculated as the pairwise distance along gradients of water depth and carbonate affinity. Phylogenetic distance was calculated as the pairwise branch length between tips of the tree. Our results show a strong positive correlation between morphological distance and phylogenetic distance and a weak positive relationship between ecological affinity and morphology or phylogeny.

Although ecology is only weakly correlated with morphology and phylogeny, this relationship suggests a lower limit to the amount of ecological difference possible between morphologically or phylogenetically similar taxa, possibly suggesting weak phylogenetic conservatism. However, the strong correlation between morphology and phylogeny makes it difficult to interpret if this pattern is caused by morphological adaptation to the environment or shared evolutionary history. The weak correlation between ecology and morphology could also represent an artifact of using phylogenetic character data, since the most ecologically important characters may be continuous variations that were not coded in the phylogeny. While these results could be an artifact of the methods or the scale of the study, they suggest that these morphological characters do not correlate with water depth or substrate preference in this group. This underscores the importance of collaboration between phylogeneticists and paleoecologists, as it is the combination of both tool sets that is best poised to answer big evolutionary and ecological questions.