Fig. 8

The test of the minimum. This figure shows the results of computer simulations designed to illustrate passive versus driven trends as distinguished by the behavior of the minimum values in the distribution. In both cases, there is an increase in the mean value of a certain characteristic among species (e.g., body size); however, the mechanism generating this average increase differs greatly between passive and driven trends. In the passive trend (a), there is a lower bound to the parameter (e.g., a smallest possible body size) and the increase in the average results only from an expansion of the largest value—the minimum value in the distribution does not change and remains at or near the lower bound where it began. In a driven trend (b), the increase in the average results from a directional shift in the entire distribution in which both the maximum and the minimum values increase over time. Reprinted from McShea (1994) by permission of Blackwell