Fig. 9

The subclade test. Both passive and driven trends can result in a skewed distribution in which most species exhibit a trait near the low end and a smaller number of species display much higher values for the trait, resulting in a long “tail” of the distribution at the high end. (These distributions are right-skewed, but left-skewed distributions are also possible and the same principles apply). The subclade test involves examining groups of related species within the distribution and away from both the lower and upper ends. If the trend is passive and results from an increase in variance that is bounded at the low end (a), then groups falling within the distribution and away from this lower limit should not show a skewed distribution for the trait. On the other hand, if the trend is driven and there is some factor that drives increases in the trait (b), then groups not near the low or high end should nonetheless display a skewed distribution for the trait as well. Reprinted from McShea (1994) by permission of Blackwell