BARBEITOS, MS; SUNY at Buffalo: Morphological constraints in the evolution of shape in Scleractinian corals

Most hypotheses on morphological evolution in scleractinian corals are based on the assumption that certain combinations of traits occur because they are adaptive. These combinations are thought to optimize energy acquisition under different environmental regimes. By always treating the problem of morphological diversity as a matter of niche partitioning, reef biologists have systematically failed to acknowledge that organic designs are not forged de novo by natural selection and crafted thereafter into an optimal design. The variation that can be produced by natural selection is limited by developmental, phylogenetic and/or architectural constraints, and so is the amount of variation available for natural selection to work upon. In this study, I test a non-adaptive hypothesis on the evolution of colony morphology in Scleractinians. Specifically, I propose that shape in colonial corals is constrained by the polyp size. Small-polyp colonies have the potential for smaller radii and are thus capable of generating features such as verrucae or branches without considerable increase in colonial size. Therefore, such species are able to occupy a broader portion of the potential morphospace defined by colony shape and polyp size. I define a metric to quantify colony shape using information theory and test the hypothesis by mapping 209 individuals belonging to 64 different species onto this morphospace. Data were acquired from dry specimens deposited at the National Museum of Natural History in Washington, D. C. The statistical significance of the test was assessed using a bootstrap approach. Additionally, the existence of phylogenetic constraints was also tested by assessing the correlation between morphological and phylogenetic distances computed respectively from the morphospace and from a molecular phylogeny based on the 12S and 28S ribosomal RNA genes.