Fig. 10

The test of ancestor–descendant pairings. Comparisons of ancestors with their descendants can be used to distinguish passive versus driven trends (in this hypothetical example, of changes in average complexity), so long as an appropriate starting point is selected. For example, choosing species B, which is a distant ancestor starting at very low complexity, would not allow one to distinguish between passive or active trends, as in either case its descendants will very likely be more complex than it is. However, choosing species A, which is a more recent ancestor and is of intermediate complexity, allows an informative assessment of the direction of changes among its descendants. In a passive trend, the number of A’s descendants (asterisks) that are more complex than A is roughly equal to the number of descendants that are less complex than A, but the average will still increase because there is a lower limit on complexity. In a driven trend, most of A’s descendants (crosses) will be more complex than A. Figure from McShea (1993), reprinted by permission of Blackwell