By modifying habitats and creating bridges and barriers between landmasses, climate change and tectonic events are believed to have important consequences for diversification of terrestrial organisms. Such consequences should be most evident in phylogenetic histories of groups that are ancient, widespread, and diverse. The squirrel family (Sciuridae) is one of very few mammalian families endemic to Eurasia, Africa, and North and South America and is ideal for examining these issues. Through phylogenetic and molecular-clock analyses, we infer that arrival and diversification of squirrels in Africa, on Sunda Shelf islands, across Beringea, and across the Panamanian isthmus coincide in timing and location with multiple well-documented sea-level, tectonic, and paleontological events. These precise correspondences point to an important role for global change in the diversification of a major group of mammals.

Mammals are characterized by the complex adaptations of their dentition, which are an indication that diet has played a critical role in their evolutionary history. Although much attention has focused on diet and the adaptations of specific taxa, the role of diet in large-scale diversification patterns remains unresolved. Contradictory hypotheses have been proposed, making prediction of the expected relationship difficult. We show that net diversification rate (the cumulative effect of speciation and extinction), differs significantly among living mammals, depending upon trophic strategy. Herbivores diversify fastest, carnivores are intermediate, and omnivores are slowest. The tempo of transitions between the trophic strategies is also highly biased: the fastest rates occur into omnivory from herbivory and carnivory and the lowest transition rates are between herbivory and carnivory. Extant herbivore and carnivore diversity arose primarily through diversification within lineages, whereas omnivore diversity evolved by transitions into the strategy. The ability to specialize and subdivide the trophic niche allowed herbivores and carnivores to evolve greater diversity than omnivores. macroevolution | ecological specialization | character evolution L iving mammals are remarkably diverse: they span eight orders of magnitude in mass, occupy a variety of habitats across the globe, and exploit subterranean, aquatic, terrestrial, arboreal, and aerial niches. Living mammals also show striking differences in diversity between lineages of similar age, from the more than 2,200 species of rodent to the single species of aardvark (1, 2). Early mammals were small, homoeothermic endotherms with tribosphenic molars. Homoeothermic endothermy enabled mammals to survive in a wider range of ambient temperatures and achieve higher sustained activity levels, but it also increased energy demands (3). These increased energetic demands necessitated adaptations or behaviors that either allowed more efficient extraction of energy from the food consumed, entailed consumption of more energy rich foods, or required an increase in the time spent foraging and eating. The tribosphenic molar, which combines shearing and crushing functions in the precisely occluding teeth, is considered to be a key innovation that promoted more effective carnivory and omnivory in early mammalian lineages (4). This type of tooth is also frequently cited as facilitating the diversification of therian mammals (4-6). The tribosphenic molar is an evolutionarily and functionally highly versatile structure (4, 7) that, in combination with heterodonty (different tooth types within the jaw), enabled mammals to evolve a disparate array of specialized dentitions and thus adapt to a broad variety of niches. Indeed, the extraordinary dental diversity of mammals-to the extent that many species can be identified by the morphology of their molars alone (8)-is a testament to the importance of diet to mammalian evolution.Although the adaptations of individual mammalian lineages to d...

Topographically complex regions on land and in the oceans feature hotspots of biodiversity that reflect geological influences on ecological and evolutionary processes. Over geologic time, topographic diversity gradients wax and wane over millions of years, tracking tectonic or climatic history. Topographic diversity gradients from the present day and the past can result from the generation of species by vicariance or from the accumulation of species from dispersal into a region with strong environmental gradients. Biological and geological approaches must be integrated to test alternative models of diversification along topographic gradients. Reciprocal illumination among phylogenetic, phylogeographic, ecological, paleontological, tectonic, and climatic perspectives is an emerging frontier of biogeographic research.

The currently most widely used definitions of homology, which concentrate exclusively on what I call phylogenetic homology, involve comparisons between taxa. Although they share important conceptual relationships with phylogenetic homology and their role in evolutionary biology is significant, serial and other forms of iterative homology have been, by comparison, overlooked.There is need for a more inclusive definition of homology. I propose that the basis of homology in the broad sense is the sharing of pathways of development, which are controlled by genealogicallyrelated genes. Using this definition, one can construct hierarchies of homology, and recognize different degrees or strengths of homology. Because different aspects of structures are controlled by distinct developmental programs, it is sometimes necessary to speak of homologies of different attributes of specific structures, rather than to homologize the structures per se. For good biological reasons, parallelism may be difficult to distinguish from homology, and one must in practice be willing to tolerate some ambiguity between them. The formulation I present leads to some unorthodox conclusions about homology in mammalian dentitions and homology between the foreand hindlimbs of tetrapods.KEY WORDS:--Homology -serial homology -development and evolution -parallelismtetrapod limbs -mammalian dentition.

The jaw, suprahyoid, and extrinsic tongue muscles are described for eight species of New World squirrels, spanning more than an order of magnitude in body mass. Anatomical differences are discussed in the light of body size, natural history, and phylogeny. The relative sizes of different muscles, their orientations, and the shapes and positions of their areas of attachment vary but show few trends in relation to body size. The anatomical differences are likewise not readily explained by the mechanical requirements of the animals' diets, which are similar. The most marked anatomical differences occur in Sciurillus (the pygmy tree squirrel), as well as those genera--Glaucomys (the flying squirrel) and Tamias (the chipmunk)--that are taxonomically most distinct from the tree squirrels. Sciurillus is noteworthy for its unusually small temporalis and an anterior deep masseter that is oriented to assist in retraction of the jaw. Tamias has a more vertically oriented temporalis and greater inclination in the anterior masseter muscles than the other squirrels, features that may be associated with its large diastema and relatively posteriorly situated cheek teeth, which in turn may relate to its having cheek pouches. Our results form a valuable database of information to be used in further studies of functional morphology and phylogeny.