Identifying primate species: Themes and Perspectives

Abstract

In guidebooks and museum cabinets or exhibits, living and fossil primates come with clear labels describing the species to which they belong. This is not the case in the forests, savannahs, and paleontological outcrops where living and fossil primates are found. The identifications and names are, and always have been, created and assigned by people. Although all humans all over the world are sure that the plants and animals on earth can be divided into distinct kinds, which were called species well before Darwin addressed the problem of their origin, there is far less agreement about how to identify a species in the face of the individual variation that characterizes most organisms. Accordingly, much to the distress of students and professional anthropologists alike, the numbers of recognized primate species change according to the criteria used by the researchers who study, identify, and name them .1-4 The dozen essays in this special issue of Evolutionary Anthropology, written by experts on various aspects of primate evolution and systematics, offer a broad range of perspectives on how primate species are defined and identified, and the important issues facing the study of primate systematics in 2014. It would be impossible and inappropriate to try to summarize the many issues and views put forth by the authors. However, there are a variety of common themes, as well as divergent views and alternate perspectives, that can be identified in this collection of thoughtful and insightful essays. First, each author was asked “What is a species and are species important?” Not surprisingly, since they agreed to contribute to this exercise, virtually all of the authors felt that species are important as the basic unit of animal life and the one used for measuring diversity in both the past and the present, despite the difficulties in reaching any consensus about how to define and identify them 1, 5-8 Nevertheless, some authors 9, 10 suggest that understanding the processes of genetic diversification between and among populations of extant primates is perhaps a more important goal than the ability to assign taxonomic levels or names. Surprisingly, in light of much criticism of taxonomic inflation, the authors most involved in primate conservation 2 argue that, from a conservation perspective, the unit recognized as “species” should not have greater significance than any other taxonomic unit, such a subspecies, in considerations of taxonomic diversity that needs to be preserved. Many of the authors follow Simpson and Darwin in equating a species with a lineage 1, 5, 11 or a piece of a lineage .4 For extant taxa, which sample one slice of geological time, the significance of this is that different species have, to some extent, distinct heritages. However, in the paleontological record, which samples primates through time, the relationship between species and lineages becomes more complicated. Thus, several authors 4, 5, 7, 8 address the question of how to deal with morphological change through time within what seems like a single lineage. On one hand, breaking a continuous lineage into a series of species with different names may suggest more overall lineage diversity within a geological period, such as an epoch, than actually existed at any one time. Assigning a single species name to a lineage that changes through time obscures morphological diversity within the same time frame. Moreover, if the fossil record is sufficiently dense and well-sampled, any break in a continuous lineage is likely to be arbitrary, and often is largely a consequence of the sequence of discoveries 5, 8. However, there are some guidelines based on the variation found among extant taxa 7. Almost all of the authors advocate and use the Phylogenetic Species Concept in their research, usually for practical reasons .1-5, 9-11 Only one author advocates the use of Mayr's Biological Species Concept .6 Several authors suggest a philosophical or practical relationship, for better or worse, between increased use of the Phylogenetic Species Concept and the widespread use of Phylogenetic Systematics or cladistic approaches to phylogeny .4-6, 8 Most authors agree with de Queiroz' 13 suggestion that many of the different species concepts address what are usually separate and successive events or steps in a continuous process of lineage diversification. Equally relevant, perhaps, is Ruse's observation that the more theoretically satisfying species concepts are the least applicable, while most practical approaches are the least theoretically satisfying 9. Confronted with the practical issue of actually identifying the number of species present in a collection of teeth, bones, skins, vocalizations, and bits of DNA. the authors vary in the specificity with which they described their approaches, both because of their diverse perspectives and the limited space they had available. Some describe relatively specific analytical criteria, 4, 7, 12 while others emphasize the difficulty of the process and the many potential pitfalls involved in it .1, 8, 10 Most authors stress the importance of finding concordance among many different types of evidence, such as genetic differences, vocalizations, behavioral differences, and geographic isolation 4, 6, 9-12. Likewise, virtually all of the contributors emphasize that the features distinguishing individual species vary from taxa to taxa, so that the features that delineate one group of primates are likely to be different from those that distinguish other taxa. Thus, species of diurnal primates are often distinguished by aspects of pelage that are readily visible to their conspecifics (as well as other diurnal taxa, including humans), while for nocturnal species, data on vocalizations and genetics have been critical for distinguishing many “cryptic species” in which pelage differences are much less obvious or important .6, 9, 11, 12 Several authors also address the question of whether the criteria and methods used for identifying fossil species should be the same as those used among extant taxa. In addition to the obvious differences in the available information (teeth, bones and, rarely, bits of DNA), small samples, and morphological change through time, there is also evidence that the ranges of variation found among living species (specifically sexual dimorphism) might not encompass the extreme limits that have characterized all of primate evolution 1, 5, 6. Much of the increase in the number of newly recognized primate species has come from the elevation of taxa previously considered to be subspecies .1-4 Thus, the ‘taxonomic inflation” of recent decades among extant primates, rather than resulting from a tremendous number of new creatures, is, in large part, a consequence of change in the criteria used to identify these two taxonomic levels and the value accorded to them. It is also a result of the increased information that is available about the genetics, morphology, and biogeography of previously known organisms. New paleontological discoveries have been more important in the increased numbers of extinct species, but species concepts and new morphological information certainly have played a part 1, 4-6, 8 The way biologists identify species has never been constant. Many factors contribute to these diverse and changing perspectives, including changes in theory, knowledge of the organic and geographical world, and increasing technology. Over the past centuries, all of these have contributed to the ongoing flux in efforts to organize and catalog the primates, as well as the plants and other animals with which we share this planet. Half a century ago, primate systematics was driven almost exclusively by the Biological Species Concept; now most active systematists follow the Phylogenetic Species concept. Until recently, primate systematics was based mostly on details of pelage and cranial morphology in the context of geography. During the past decade, technological advances in genetics have greatly increased our understanding of hereditary material underlying the similarities and differences among extant primates, so that researchers can determine which aspects of the genome are unique to individual populations and which are shared. As the technology of genomic sequencing advances at a rapid pace, it seems likely that we will see increasing complexities that will have profound effects on our understanding of both the processes and the patterns involved in primate speciation, with corresponding changes in the way primate species are identified and classified .4, 9-12 John Fleagle is Distinguished Professor of Anatomical Sciences at Stony Brook University and the editor of Evolutionary Anthropology. His research is focused on many aspects of primate evolution. E-mail: john.fleagle@stonybrook.edu