UNDERSTANDING THE ROLE OF PHOTOSYMBIOSIS IN CORAL MACROEVOLUTION WITH MOLECULAR PHYLOGENETICS AND THE FOSSIL RECORD

What are the contributions and consequences of the coral-zooxanthellae mutualism to the patterns of scleractinian coral macroevolution? Mutualisms are advantageous for the organisms involved but their macroevolutionary consequences are not generally known. The photosymbiotic zooxanthellate (Z) and non-photosymbiotic azooxanthellate (AZ) corals originated together in the Triassic and today are equally diverse, suggesting that photosymbiosis has no net advantage over long timescales. However, corals have a rich fossil record showing that Z corals diversify early and decline in diversity toward the Recent, while AZ corals steadily increase in diversity. To understand how photosymbiosis influences macroevolution, the temporal patterns of coral diversification, extinction, and habitat range of Z and AZ corals can be described using molecular phylogenetics in combination with paleontological analyses. We use Price’s theorem to quantify the independent contributions of differential diversification, the probability and temporal pattern of gain and loss of photosymbiosis to the change in frequency of Z and AZ corals over time. This allows us to measure the magnitude and direction of all processes that contribute to the change in frequency of photosymbiosis independently. The phylogenetic component contributes a measure of the pattern and timing of shifts in photosymbiosis and also an independent estimate of differential diversification rates that can be compared to those derived from the fossil record.