 Hierarchical thinking about evolution is perhaps best illustrated with an example. It has already been noted that one way of explaining morphological stasis is stabilizing selection. Stabilizing selection is a well-understood Darwinian mechanism that operates on organisms in a population. Here, organisms are the units of selection while species are the units of evolution. But Sheldon (1996) introduced another potential explanation of stasis, which he called the “plus ça change” model. Sheldon observed that in times of environmental turmoil, ecological generalists usually have lower extinction risk, while ecological specialists are more prone to extinction. Perhaps surprisingly, then, a pattern of stasis in the fossil record might be due to the persistence of ecological generalists during times of environmental upheaval. This model appeals to species selection, or to the differential extinction and persistence of whole species, rather than to stabilizing selection processes occurring within those species. Hierarchical theorists hold that evolutionary patterns (like stasis) can be generated and sustained by processes operating at different biological levels.