Variation and versatility in macroevolution
Studies of macroevolutionary change are uniquely focused on events and processes that require time, including events that occur infrequently (or just once), or processes that are long in duration. With respect to phenotypic variation, macroevolution is typically the domain of large differences (whose study in aggregate becomes the study of disparity), of the origin of novelties (features that do not correspond to characters or structures present in an ancestor), or of differences (of whatever magnitude or character) that are associated with taxonomic diversification. For variation to be observed, it must not only have been produced but also permitted to persist. The expression and persistence of macroevolutionary variation in a trait is a manifestation of that trait's versatility. This chapter draws attention to the "third component" of evolvability, which has to do with macroevolutionary production of variation that is ecologically and functionally relevant, and considers how two organismal systems dealt with in this study might illustrate such versatility. The examples treated include in one case a structure-teeth in elephantids-whose morphology is extremely divergent from that in most other mammals and very different from their primitive state. Yet by virtue of their reiterated modular structure and dynamic mode of development, the enormous teeth of elephants have readily adjusted to developing and functioning within jaws of animals whose body sizes have undergone radical change. © 2005 Elsevier Inc. All rights reserved.
