Nonrandom Diversification within Taxonomic Assemblages

Abstract

-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. [Hollow curve distributions; nonrandom dominance; null models.] An important theme of biology, and particularly evolutionary biology, is the identification and explanation of general patterns found in nature. One pattern, known as the hollow curve (HCD) (Willis, 1922; Willis and Yule, 1922; Williams, 1964; Anderson, 1974; Anderson and Anderson, 1975; Zima and Horacek, 1978; Flessa and Thomas, 1985), is common in -natural assemblages and yet both its pervasiveness and its possible use in studying evolutionary processes are often not appreciated. The HCD receives its name from the concave shape of a frequency distribution of taxonomic subunits per unit (e.g., number of genera/family), when units are ordered from those with the most to the least subunits. In taxonomic assemblages an HCD implies that one clade (or several clades) has had many more speciation events and/ or fewer extinctions than other clades at the same taxonomic level. Stated another way, HCDs suggest that it is common for a single taxon within an assemblage to be 1 Present address: Division of Biological Sciences, University of Montana, Missoula, Montana 59812. 2 Present address: Star Route 2905, Dryden, Maine