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5 Classification of the Mammals Science is organized knowledge. Wisdom is organized life. —attributed to immanuel kant TEETH ARE ULTIMATELY ABOUT food processing, but they cannot be understood outside of their phylogenetic context. Gentle lemurs and giant pandas both feed on bamboo, but their teeth look entirely different . Ancestral lemurs and pandas had very different dental morphologies, and while their descendants may have converged on similar diets, natural selection had different “raw materials” to work with, leading to alternate solutions to the same problem. The concepts of morphological starting point and “phylogenetic baggage” are important to the study of the adaptive radiation of mammalian tooth form. Moreover, because dental form reflects both function and phylogenetic history, the ability to unravel and separate these two signals has significant implications for understanding both. This chapter focuses on the classification of mammals and their evolutionary relationships. These must be considered together because we arrange and classify mammals on the basis of their relatedness. This helps us organize the thousands of species of mammals and allows us to explore the radiation in a more meaningful way. Do two organisms have similar morphologies because they are closely related, or did their ancestors converge on the same morphological solution to an adaptive problem? Only through a consideration of relationships among organisms can we separate the effects of function and phylogeny on anatomy. This chapter begins with a consideration of some basic terms and concepts important to studying phylogenetic relationships among living organisms. A brief history of biological classification is then sketched, and the scheme used in this book is presented to introduce our remarkable biological class. These are exciting times for researchers who classify mammals. We are in the midst of a major revolution in our approach to inferring relatedness, but as the dust begins to settle, a new and more robust way of classifying mammals is emerging. SOME BASIC TERMS AND CONCEPTS Taxonomy and Systematics Researchers who classify mammals and study their relationships frequently use the terms taxonomy, systematics, classification, and nomenclature. While each has a distinct meaning, they often seem to be used interchangeably. This has led to confusion among students, the extent of which is made clear by a quick search for definitions of each on the Internet. This confusion stems from the fact that classification and nomenclature are part of taxonomy, and modern taxonomy is based on systematics. c l a s s i f i c a t i o n o f t h e m a m m a l s 59 two of three species are more closely related. While the common ancestor of a human, a monkey, and a dog walked on four limbs and had a tail, the symplesiomorphic traits linking the latter two species are not evidence of a close phylogenetic relationship. Resolving relationships among the three species requires synapomorphic traits, those present in the common ancestor of only two of the three. Some traits shared between the monkey and the human, such as fingernails and an enclosed eye orbit, were present in the common ancestor of the two but probably not in their common ancestor with the dog. It is such derived traits that morphological systematists look to in assessing relatedness. Determining relatedness is rarely this simple a task, however , and it is often difficult to distinguish synapomorphic traits from symplesiomorphies, and even homologous traits from homoplasies. Systematists use outgroups, taxa known to have split before those under analysis, to determine the polarity of a given trait, whether it is primitive or derived. Using a lizard as the outgroup in our human-monkey-dog example confirms that quadrupedalism and the presence of a tail are plesiomorphic traits. As more species and more traits are added to an analysis, it becomes increasingly difficult to determine which species are most closely related. In such cases, morphological systematists usually invoke parsimony, the notion that the least complex explanation is usually the right one. The scheme requiring the fewest evolutionary reversals and independently derived traits is often considered the most likely. While evolution does not always work this way, there are at present no other consistent, objective criteria for choosing between competing schemes. The Species Problem We cannot discuss taxonomy or systematics without some appreciation for species as the basic unit of classification. Aristotle wrote in his Metaphysica of species as distinct entities , each defined by an inherent fundamental essence. Indeed , most of us would recognize a dog or cat as such even if we had never seen...

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