Species, Species Concepts and Primate Evolution
A world of categones devmd of spirit waits for life to return. Saul Bellow, Humboldt's Gift The stock-in-trade of communicating hypotheses about the historical path of evolution is a graphical representation called a phylogenetic tree. In most such graphi
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ADVANCES IN PRIMATOWGY Series Editors:
JOHN G. FLEAGLE State Umverstty of New York at Stony Brook Stony Brook, New York R. D. E. MacPHEE American Museum of Natural Htstory New York, New York Editorial Board:
JOHN M. ALLMAN, Cahfornta Institute of Technology, Pasadena, California RUSSELL L. CIOCHON, Umverstty of Iowa, Iowa Ctty, Iowa FRAN, and Pnmale Evulutwn, edited by William II. Kimbel and Lawrence B. Martin. Plenum Press, New York, 1993.
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PRIMATES AND PARADIGMS
(I 985), where each level of understanding informs and modifies the other. believe, with Szalay (this volume), that this relationship is basic to the hypothetico-deductive method, and I trust this chapter will illustrate its heuristic value. To begin from a relatively uncontroversial starting point, species may be viewed as clusters in phenotypic and genotypic space. Taxonomic species concepts emphasize the phenotypic aspect, while biological or genetic species concepts emphasize the genotypic; specifically, genetic concepts identify species in terms of gene exchange (or, in the case of allopatric populations, the potential for gene exchange). This chapter concentrates on genetic species, although phenotypic concepts will also be considered. I shall not discuss ecological or evolutionary species concepts, since they have been adequately dealt with elsewhere (e.g., Eldredge, 1985; Kimbel, 1991).
Why Genetic Species? The best answer to this question lies in the ontological nature of species. Evolutionary biologists are crucially interested in the origin of diversity, and (apparently) irreversible change; genetic species are the measurable units of this change. In the words of Mayr ( 1963, p. 11), "The origin of new species, signifying the origin of essentially irreversible discontinuities with entirely new potentialities, is the most important single event in evolution." Futuyma (I 987, p. 467) has provided a cogent and succinct argument for why this should be so: ... the evtdence from geographiC variation tells us that character evolution does not require speciation. But m the absence of speciation, much of the geographical variation we observe is ephemeral, leaving httle imprmt on evolution m the long term . ... over even moderately short spans of evolutiOnary time (tens or hundreds of thousands of years), the habitats to which populations are adapted shtft, often over large distances, m consequence of climatic change. . . the consequence 1s occasional (perhaps at mtervals
of thousands of years) but massive gene flow (i.e., hybridization). on a scale far greater than the tnckle that often characterizes populations at equilibnum . ..
More eloquently still, Futuyma (1987, p. 470) stated, "If we envision an adaptive landscape ... of character values, successive speciation events are the pitons affixed to the slopes of an adaptive peak." Genetic species thus derive their significance from their role in the process of evolutionary diversification. To proponents of pattern (or transformed) cladistics, however, their "process dependence" renders
