 Paleontologists have long been interested in documenting and explaining larger-scale evolutionary trends in the fossil record (McShea 1998). Perhaps the classic example of a macroevolutionary trend, and one that paleontologists have studied extensively, is Cope’s rule of size increase. Paleontologists routinely distinguish within-lineage trends (e.g., average size increase within a single lineage) from among-lineage trends (e.g., increase in the average size of mammals over the Cenozoic). The latter are distinctively macro-level patterns. While it might be tempting to think that trends such as evolutionary size increase are generally attributable to natural selection, Stanley (1973) pointed out that these trends could be merely statistical phenomena. Suppose that a clade, such as mammals, starts out at or near a fixed lower boundary on body size. And suppose that evolutionary size increases and decreases are equally probable. As the clade evolves, the mean body size might do a “random walk” away from the fixed boundary—an idea that Gould (1996) later popularlized. Many scientists also think of this as a passive diffusion model of evolutionary change.