Speciation and rarity: separating cause from consequence

Abstract

Diversity in any geographical area is a first-order function of immigration, emigration, speciation and extinction (Cracraft, 1986; Archibald, 1993), except at the largest scales where it is an exclusive outcome of differential rates of species production and extinction (Cracraft, 1985). Both ecologists and evolutionary biologists have recognized the importance of these processes, although they have tended to focus their work at either local or more regional scales (Brooks and Wiley, 1988; Ricklefs, 1989). Extinction was of great interest to Darwin, but was of less concern to neoDarwinian theory until recently (Raup, 1994). Almost contemporaneously it was realized that past mass extinctions could have been externally driven (Alvarez et al., 1980), and that humans are precipitating a mass extinction event unparalleled in the earth's history (Ehrlich and Ehrlich, 1981). As a result, both ecologists (see Pimm, 1991) and evolutionary biologists (e.g. Raup, 1992, and almost any recent volume of Paleobiology) turned their attention to the processes leading to a decrease in range size and abundance culminating in extinction. Our understanding of the influence of extinction on local, regional and temporal patterns of diversity is consequently progressing at an unprecedented rate.