Fig. 1.

When is a shift in the mean a trend? (A) A large clade displays an overall increase in the complexity of its constituent subclades. Over time, the distribution of complexity values is shifted to the right, and an apparent trend for increasing complexity results. However, this pattern can derive from two very different although not exclusive processes. (B) Increasing mean complexity of the clade can result from the fortuitous extinction and origination of a small number of constituent major subclades, which coincidentally, have very different mean complexity values. (C) A driven trend results from a parallel increase in complexity within all or most of the constituent subclades. This mechanism is the dominant one in our crustacean data. Determining the relative contributions of this process and the process in the Center requires a phylogenetic approach, which has hitherto been lacking.