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The Comparative Approach in Evolutionary Anthropology and Biology
By Charles L. Nunn
THE UNIVERSITY OF CHICAGO PRESSCopyright © 2011 The University of Chicago
All right reserved.
Chapter OneThe Importance of Comparison
Comparison is fundamental to evolutionary anthropology. When anthropologists attempt to understand the new fossils from Flores, for example, they compare these remains to other fossils and extant humans (Brown et al. 2004; Lieberman 2005; Tocheri et al. 2007), and when psychologists study chimpanzee cognition, they often compare chimpanzee performance on cognitive tasks to the performance of human children on the same tasks (Whiten et al. 1996). Linguists expend tremendous effort describing the grammar and vocabulary of different languages; interesting results also emerge when efforts of multiple linguists are synthesized in a comparative study, for example, to construct language trees (Campbell 2004; Atkinson and Gray 2005; Dunn et al. 2005; McMahon and McMahon 2005). Similarly, the comparative method has a long history in primatology (Crook and Gartlan 1966; Milton and May 1976; Clutton-Brock and Harvey 1977). Primatologists and evolutionary morphologists continue to produce pioneering comparative studies in nonhuman primates, including research on sexual dimorphism, brain size, and social systems. When geneticists create evolutionary trees of primates, they also use data in a comparative context by identifying shared derived characters under specific models of evolution. Paleontologists use similar approaches to reconstruct the phylogeny of extinct hominins using morphological characteristics (e.g., Strait et al. 1997).
These examples demonstrate the central role that comparison plays in evolutionary anthropology. Comparison provides a way to draw general inferences about the evolution of traits and therefore has long been the cornerstone of efforts to understand biological and cultural diversity (Darwin 1859; Murdock and White 1969; Ridley 1983; Brooks and McLennan 1991; Harvey and Pagel 1991; Mace and Pagel 1994). Individual studies of fossilized remains, living species, or human populations are the essential units of analysis in a comparative study; bringing these elements into a broader comparative framework allows the puzzle pieces to fall into place, thus creating a way to test adaptive hypotheses and generate new ones.
Here, comparative study refers to studies of multiple populations or species, including studies of geographic variation at a continental or regional scale. In many cases, a comparative study stands on the shoulders of numerous, painstaking studies of individual populations or species; a single scientist would find it difficult—even impossible—to collect data at such a broad scale. Evolutionary anthropology lies at the interface of biology and anthropology; it aims to understand human behavior, ecology, and evolution. This includes studies of hominin fossils, language diversification, and the genetics of human populations. Evolutionary anthropologists also investigate cognitive abilities in different species of primates (or other animals), and they study primate morphology and behavioral ecology. Thus, evolutionary anthropology broadly encompasses studies of human evolution, comparative psychology, primatology, functional morphology, linguistics, human behavioral ecology, and comparative anatomy.
The overarching aims of this book are twofold: first, to explore the comparative foundations of evolutionary anthropology and biology in past and present research, and second, to point the way toward new uses of the comparative approach in the future, especially in terms of applying new phylogeny-based methods to address fundamental evolutionary questions. In thinking about the past, present, and future of comparative approaches, the past gives us a sense of how comparative approaches have played a central role in evolutionary anthropology and biology, even before rigorous phylogenetic approaches were possible. In the present, phylogeny-based methods are being applied with greater frequency, and with these methods researchers are addressing both new and old questions. As for the future, new methods are on the horizon, as are new comparative questions that have arisen through previous discoveries.
Thus, this is partly a "how- to" book to increase the use of new phylogeny-based comparative approaches in future anthropological and biological research (chapters 2–8), and partly an exploration of how comparative approaches have been applied to investigate biological and cultural variation (chapters 1 and 9–13). For the methodological sections in the early chapters, I provide an introduction to the methods and how they have been applied rather than giving a detailed statistical examination of the methods, and I focus on the methods that are likely to be of greatest use to evolutionary anthropologists and biologists rather than providing a comprehensive overview of all available methods. My target audiences are anthropologists and biologists who wish to become users of phylogenetic comparative methods and those who want to understand more about the burgeoning literatures that use these methods to test specific hypotheses. I generally (but not exclusively) focus on organismal or cultural traits rather than genetics. Links to computer programs and worked examples are provided on AnthroTree, a website designed around the book (http://www.anthrotree.info). Even though many of the analyses in the book were done with older programs, the website focuses heavily on using R (R Development Core Team 2009), which is emerging as the most flexible and powerful way to run comparative analyses. This site will be updated as new computer programs become available.
While this book is focused primarily on evolutionary anthropology, I tried to write in a way that will also appeal to organismal biologists interested in applying the comparative method. I have done this for several reasons. First, there is a need for an entry-level book on phylogenetic methods in comparative biology, and I hope this book helps fill that opening. In addition, I wish to share with biologists the incredible diversity of comparative questions that are being addressed in evolutionary anthropology. Last, by identifying key questions of general importance to human evolution, I aim to facilitate greater collaboration between evolutionary biologists and anthropologists.
The Need for This Book
Given that comparative approaches are so widely used, and given that there is little controversy over the value of comparison, why do we need this book? Indeed, one could argue that a book focused on comparative methods downplays the importance of other valuable approaches, such as experimental approaches that can uncover causality more directly. Certainly we should make use of experiments when possible. For many of the questions that evolutionary anthropologists and biologists address, however, experiments are neither feasible nor ethical. Even when experiments are possible, comparative approaches are essential for understanding evolutionary phenomena and thus should be used in tandem with experiments. One field in particular—animal cognition—provides opportunities to synthesize experiments on different primate species into a broader evolutionary context (E. MacLean, B. Hare, L. Matthews, and C. Nunn, unpublished data). By giving equivalent cognitive tasks to different species, it becomes possible to assess the roles of ecological and social factors in favoring particular types of cognition and to reconstruct the evolutionary history of primate cognitive abilities (see also Haun et al. 2006a, b).
More generally, a book on this topic is valuable and overdue for at least four reasons. First, innovative phylogenetic approaches have fueled new discoveries in evolutionary biology, and these methods have yet to be applied to their full potential in evolutionary anthropology. Many "nonphylogenetic" comparative studies in the past have yielded fruitful insights. However, phylogeny- based methods enable evolutionary anthropologists and biologists to investigate trait evolution more directly, to identify the drivers of speciation and extinction, and to study coevolutionary dynamics of genes, languages, and cultural traits. Thus, phylogenetic comparative methods represent an increasingly important toolkit for evolutionary anthropologists. The first part of this book reviews and synthesizes many advances in phylogenetic methods from biology and provides selected examples from evolutionary anthropology. The second part of the book reviews in greater depth how these approaches have been (or could be) applied to questions in different fields of evolutionary anthropology; thus, the latter chapters reinforce many concepts and approaches that are covered in the earlier, methodologically focused chapters.
Second, evolutionary anthropology is increasingly interdisciplinary and informatics-driven, and many of these research projects have a strong comparative component. This follows trends in other sciences, including biology, which are propelled by an appreciation that greater integration among fields can produce fundamentally new insights, and by greater availability of funding for large-scale collaborative research. In an interdisciplinary environment, it becomes essential to understand research in other fields, for example as a way to form new collaborative networks aimed at uncovering links between genes and languages (Diamond and Bellwood 2003) or associations between primate behavior and morphology (Plavcan et al. 1995, 1997b). This book links diverse fields of evolutionary anthropology through the common lens of comparative methods.
Third, evolutionary anthropology is largely a nonexperimental, historical science; phylogeny-based methods provide a way to examine independent evolutionary origins and thus overcome problems of statistical non-independence in such data. Many of the most enduring questions in evolutionary anthropology concern empirical patterns of evolution that are unsuited for experimental approaches. Examples include the spatial distribution of nonhuman primates, the spread of modern humans to all corners of the earth, and the evolution of human behavioral, morphological and life history traits in different populations. With their long life spans, most nonhuman primates are poorly suited for experimental manipulations of environmental factors thought to influence genetically determined traits, and the same is true of most questions involving human evolution. Relative to other fields, this means that evolutionary anthropologists rely to a greater extent on observational data to test hypotheses. Importantly in this context, observational data suffer from spatial and historical dependencies—the bane of statistical methods, and the source of many uncertainties in evolutionary anthropology. Given the high costs of acquiring biological and anthropological data from remote locations, it is imperative that we make the best use of these data by correctly applying methods that control for statistical nonindependence while also maximizing statistical power.
Finally, from an intellectual standpoint, it is important to understand the epistemological basis to our science. Given the importance of comparison to all subfields of evolutionary anthropology, the time is right to critically assess these methods and how they have increased our knowledge of primate and human evolution. What are the strengths and weaknesses of different comparative approaches? Are there phylogenetic methods that could be useful for evolutionary anthropology but have yet to be applied to study human and primate evolution? Conversely, can methods in evolutionary anthropology be useful for other fields in both the biological and social sciences? Can theoretical models, including simulations, provide new insights to the drivers of broad-scale patterns and by doing so lead to methodological advances to study these processes empirically?
Five Examples from Across Evolutionary Anthropology
In what follows, I briefly review five examples of how the comparative method is used in evolutionary anthropology. I focus on the past and present uses of the comparative method, and I hope to whet your appetite for learning about how phylogeny-based comparative approaches can be used in the future. This is also a good time to make an important caveat relevant to the entire book: given the large number of examples of comparative research in evolutionary anthropology and space limitations in writing this book, it would be impossible to provide a comprehensive summary of all the different comparative studies that have been undertaken (and it is unclear if such a vast litany of examples would be useful in any case). Instead, I focused on particular examples to illustrate different methods and approaches, often drawing on my own research interests in the evolution of behavioral, ecological, and cultural traits.
1. Sperm competition. A suitable example to start with involves the links between primate behavior and morphology. Most studies of mammalian sexual selection prior to about 1980 focused on female choice or direct competition among males, such as males competing for females. For example, Clutton-Brock et al. (1977) investigated the links between body mass dimorphism and group composition in primates. Consistent with the idea that increasing competition for mates selects for larger male body mass, they found that dimorphism increases in lineages characterized by a greater number of females per male and larger body mass (see also Clutton-Brock and Harvey 1977; Mitani et al. 1996a; Plavcan and Van Schaik 1997b; Lindenfors and Tullberg 1998). These findings suggest that primate males experience intense competition to monopolize access to females. In many primate species, however, females mate with multiple males during their time of fertility (Hrdy and Whitten 1987; Van Schaik et al. 1999). For example, Barbary macaque (Macaca sylvanus) females mate with up to ten males per day (!) during estrus (Taub 1980). This might reflect that males are unable to monopolize access to females when they become fertile, resulting in a failure of effective competition. In addition to overt physical competition, however, competition among males might also occur after mating, that is, within the female reproductive tract, with the sperm from different males competing to fertilize the egg of a female (Parker 1970). This situation is known as sperm competition, and it should result in selection for males to produce larger numbers of sperm and sperm that can more quickly reach a fertilizable egg.
Harcourt et al. (1981) used comparative data on primates to investigate a critical prediction of sperm competition. They predicted that in species characterized by greater female mating promiscuity, males should have larger testes mass (see also Short 1979). As testes mass shows a positive association with body mass, it is also necessary to control for body mass in such a comparison. Using data on thirty-three species of primates, they found compelling support for their prediction. Species in which females mate with multiple males have larger relative testes mass, measured, for example, as positive residuals from a regression of testes mass on body mass (figure 1.1). Harcourt et al. found that humans fall very close to the regression line (see arrow in figure 1.1), which is inconsistent with lifelong monogamy (under pure monogamy, they should fall further below the line). In a follow-up study, Harcourt et al. (1995) showed that mating system effects were significant after controlling for phylogenetic relationships among the primate species in their sample, and that results were independent of effects of seasonality, which might also favor larger testes if males copulate at a higher rate in a shorter breeding season. By building on this pioneering comparative research on primates, sperm competition is now known to be pervasive across many groups of animals (Smith 1984b; Birkhead and Moller 1992, 1998).
2. Comparative cognition. Given that humans are characterized by sophisticated tool use, language, and social learning, evolutionary anthropologists are also interested in uncovering the factors that influence primate cognitive abilities. One way to probe cognition comparatively is to design an experimental task and then challenge individuals of different species to solve this task in a way that reveals how they learn new skills. Is it a case of imitation—"monkey see, monkey do"—or does individual learning play a larger role?
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Table of Contents
1. The Importance of Comparison
2. Basic Phylogenetic Concepts and “Tree Thinking”
3. Reconstructing Ancestral States for Discrete Traits
4. Reconstructing Ancestral States for Quantitative Traits
5. Modeling Evolutionary Change
6. Correlated Evolution and Testing Adaptive Hypotheses
7. Comparative Methods to Detect Correlated Evolutionary Change
8. Using Trees to Study Biological and Cultural Diversification
9. Size, Allometry, and Phylogeny
10. Human Cultural Traits and Linguistic Evolution
11. Behavior, Ecology, and Conservation of Biological and Cultural Diversity
12. Investigating Evolutionary Singularities
13. Developing a Comparative Database and Targeting Future Data Collection
14. Conclusions and Future Directions