The Significance of Few versus Many in the Tree of Life

When biodiversity is examined in the context of species richness, a consistent feature emerges: Most taxonomic groups are species-poor, relatively few are species-rich, and the frequency distribution has the shape of a so-called "hollow curve." Since other models, which generate hollow curves, fail to produce the high number of monotypes and the high dominance when compared with real data, Scotland and Sanderson develop a new "simultaneous broken tree" model comprising three distinct elements: it generates trees according to a Yule-type branching process, contains a model of character evolution in which character changes evolve at a constant rate on this tree, and the character changes are regarded as cues for the recognition of mutually exclusive taxonomic groups.