Fig. 5

The basis for classifying trends as passive or driven. According to McShea (1994), a trend is passive if the forces influencing change in lineages over time are heterogeneous (a), that is, if they occur in more than one direction. If the forces are homogeneous (i.e., they operate mostly in the same direction), then the trend is driven (b). Figure from McShea (1994), reproduced by permission of Blackwell