FROM PAKICETUS TO PORPOISES: PATTERNS OF CHANGE IN COMPLEXITY IN THE EVOLUTION OF WHALE TEETH

Over billions of years, evolution has given rise to organisms that have increased dramatically in complexity, from microbes alone to communities that include modern humans and the great whales. But change in individual lineages, occurring by chance and in adaptation to unpredictable circumstances, does not necessarily involve increasing complexity. Complexity may increase or decrease depending on the immediate situation. Metrics of change based on a single unit, such as numbers of genes or cells, are inadequate to quantify such shifts in complexity, because the structures of organisms and communities are hierarchical. Teeth and jaws of whales provide an opportunity to assess changes in complexity simultaneously at different levels of organization. A set of 17 standardized variables has been established to characterize the form of each tooth in the whale’s jaw. These include aspects of shape, curvature, carinae, serrations, cusps, and other attributes that vary according to the degree of morphological differentiation within the jaw and among taxa. Measures of complexity for each tooth in the jaw are derived from these variables. These indices of the complexity of individual teeth are integrated to derive simple information functions that quantify the complexity of the dentition as a whole. Our preliminary data show that the earliest whales, derived from terrestrial, hoofed mammals, had fairly simple teeth, with a modest degree of differentiation in the jaw. By Oligocene time, taxa with more complex teeth had emerged, but dentitions did not become significantly more differentiated. Subsequently, the trend changed. Tooth forms in most lineages became simpler and much less differentiated, most dramatically among the mysticetes, where teeth were lost and replaced by baleen. These trajectories of change at two levels of organization represent a common pattern of evolution. A novel structure emerges, it is replicated to constitute a series of elements, and these evolve more or less independently to take on varied functions. Finally, the entire structure is refined as a single, highly integrated functional system, or it is lost as further novelty emerges, typically at a higher level of organization.