Natural clades differ from “random” clades: simulations and analyses

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Abstract
Using computer simulations and analytic calculations, we have evaluated whether conspicuous expansions and contractions of natural clades may have represented chance fluctuations that occurred while probabilities of speciation and extinction remained equal and constant. Our results differ from those of previous workers, who have not scaled generating parameters empirically at the species level. We have found that the waxing and waning of many real clades have almost certainly resulted from changes in probabilities of speciation and extinction. For some of these changes, likely explanations are evident. The emplacement of adaptive innovations, for example, has at times elevated probability of speciation. We conclude that chance factors have not played a dominant role in producing dramatic changes in standing diversity within speciose higher taxa.