Biological Diversification and Its Causes

Abstract

Three major patterns of diversity are defined and a general hypothesis proposed to explain them. These patterns include (a) macroevolutionary diversity, encompassing temporal changes in species richness within and among clades, (b) global diversity, in which gradients of diversity among communities or biotas vary spatially and temporally, and (c) Phanerozoic diversity, which describes largescale temporal variation in species richness within the entire biosphere. Current explanatory hypotheses for these patterns are generally formulated for systems that are assumed to be in a state of ecological equilibrium, with speciation and extinction rates being diversity-dependent. This paper describes an alternative model of diversification in which speciation and extinction rates are independent of standing diversity. It is postulated that speciation rate is controlled primarily by large-scale changes in lithospheric (geomorphological) complexity. This hypothesis is a deductive consequence of biological data showing that allopatric speciation is the general mode of differentiation, and of geological data showing that tectonic changes within the lithosphere are responsible for the formation of geographic and ecological barriers. This hypothesis makes a number of predictions about patterns of endemism, historical biogeography, and spatial gradients of diversity, and data consistent with these predictions are presented. Other potential regulators of speciation rate (degree of morphogenetic variability within species, behavioral-ecological variability within species, intensity of sexual selection) are discussed and their potential roles in shaping diversity patterns are evaluated. Although they may occasionally be important for explaining some intracladal patterns of diversification, they are insufficient by themselves to account for spatial patterns or long-term changes of diversity within biotas. Extinction rate is postulated to be controlled primarily by spatial and temporal changes in environmental harshness, particularly as the latter is manifested by gradients of temperature and moisture. Considerable neontological and paleontological data suggest that change in harshness is a major factor shaping temporal and spatial patterns of diversity through its effects on extinction rate. Other causal mechanisms of extinction, particularly the tectonic elimination of habitats, may be of importance for specific groups of organisms (e.g., marine shelf communities following continent-continent collisions) at specific localities and times (e.g., near a volcanic eruption) but are not likely to play as important a role as does change in harshness. Together, the two main controls on speciation and extinction define a diversity-independent process of diversification. The biosphere can be viewed as an open thermodynamic system that can be expected to grow in complexity (including diversity) through time as a result of the inflow of matter and energy. This increase in complexity is constrained by external factors (physical changes in the biosphere or