Nonrandom Diversification with in Taxonomic Assemblages

Hollow curve distributions (HCDs) of taxonomic diversity are common in nature and are a result of assemblages being dominated by one or a few very diverse taxa. Models of random extinction and speciation can produce HCDs; however, the similarity between real and simulated HCDs has been examined previously only qualitatively. We compare quantitatively dominance observed in 85 real taxonomic assemblages to dominance predicted by five null models (1. Poisson; 2. Raup et al.'s 1973 simulation model; 3. Anderson and Anderson's 1975 simulation model; 4. simultaneous broken-stick; and 5. canonical lognormal distributions). Real assemblages were dominated to a significantly greater extent by one unit than predicted by all null models. This dominance is compounded as one proceeds down the taxonomic hierarchy. We demonstrate that overdominance is common whether phylogenetic or traditional classification schemes are used to assemble taxa. We propose that overdominance reflects real differences in the evolutionary success of units within an assemblage.