One of the most pervasive, nonrandom evolutionary patterns is extreme domination of a taxon by one subtaxon or only a few subtaxa. Domination refers to taxonomic diversity and the fraction of the taxon that is classified in the most diverse subtaxon. We attempt to explain how subtaxa come to dominate their phyletic counterparts by examining correlations between taxonomic diversity and life history traits such as age of first reproduction, longevity, fecundity and partitioning of reproduction, and resource availability in a variety of vertebrate, invertebrate, and plant groups. Regardless of taxonomic group or rank, the number of taxa within an assemblage, or the school of taxonomy employed, diverse taxa were characterized by short generation time (early age of first reproduction and short life—span) and the ability to contact many resources (high mobility and high resource availability). We suggest that the intrinsic character of short generation time increases diversity because it promotes speciation and reduces extinction. Extrinsic factors such as resource availability and environmental complexity and variability may have a secondary influence on diversity by constraining or enhancing speciation for taxa with short generation times.