MECHANISMS OF LARGE-SCALE EVOLUTIONARY TRENDS

Large-scale evolutionary trends may result from driving forces or from passive diffusion in bounded spaces. Such trends are persistent direc tional changes in higher taxa spanning significant periods of geologic al time; examples include the frequently cited long-term trends in siz e, complexity, and fitness in life as a whole, as well as trends in le sser supraspecific taxa and trends in space. In a driven trend, the di stribution mean increases on account of a force (which may manifest it self as a bias in the direction of change) that acts on lineages throu ghout the space in which diversification occurs. In a passive system, no pervasive force or bias exists, but the mean increases because chan ge in one direction is blocked by a boundary, or other inhomogeneity, in some limited region of the space. Two tests have been used to disti nguish these trend mechanisms: (1) the test based on the behavior of t he minimum; and (2) the ancestor-descendant test, based on comparisons in a random sample of ancestor-descendant pairs that lie far from any possible lower bound. For skewed distributions, a third test is intro duced here: (3) the subclade test, based on the mean skewness of a sam ple of subclades drawn from the tail of a terminal distribution. With certain restrictions, a system is driven if the minimum increases, if increases significantly outnumber decreases among ancestor-descendant pairs, and if the mean skew of subclades is significantly positive. A passive mechanism is more difficult to demonstrate but is the more lik ely mechanism if decreases outnumber increases and if the mean skew of subclades is negative. Unlike the other tests, the subclade test requ ires no detailed phylogeny or paleontological time series, but only te rminal (e.g., modern) distributions. Monte Carlo simulations of the di versification of a clade are used to show how the subclade test works. In the empirical cases examined, the three tests gave concordant resu lts, suggesting first, that they work, and second, that the passive an d driven mechanisms may correspond to natural categories of causes of large-scale trends.