Defining Species: A Multi-Level Approach
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Defining Species: A Multi-Level Approach Tudor M. Baetu
Received: 7 May 2011 / Accepted: 12 December 2011 / Published online: 27 December 2011 Springer Science+Business Media B.V. 2011
Abstract Different concepts define species at the pattern-level grouping of organisms into discrete clusters, the level of the processes operating within and between populations leading to the formation and maintenance of these clusters, or the level of the inner-organismic genetic and molecular mechanisms that contribute to species cohesion or promote speciation. I argue that, unlike single-level approaches, a multi-level framework takes into account the complex sequences of cause-effect reinforcements leading to the formation and maintenance of various patterns, and allows for revisions and refinements of pattern-based characterizations in light of the gradual elucidation of the causes and mechanisms contributing to pattern formation and maintenance. Keywords
Species concepts Speciation Levels Mechanisms Pattern Process
1 Introduction Most biologists agree that discrete clusters exist among organisms. They disagree however, about which clusters constitute species, and what are their defining characteristics. Many of these disagreements, especially those touching on the defining characteristics of groups of organisms we may want to call species, stem from divergences in research interests (Harrison 1998; Lide´n and Oxelman 1989; Reydon 2005). Evolutionary biologists are interested in the causes leading to the formation and maintenance of distinct clusters, and tend to define species in terms of processes like gene flow, as well as the factors that promote or hinder these processes, such as geographic and ecological barriers that promote speciation T. M. Baetu (&) Department of Philosophy, University of Maryland, 1125D Skinner Building, College Park, MD 20742, USA e-mail: [email protected]
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[e.g., (Mayr 1963)]. In contrast, systematists are more interested in the final effects than in the causes leading to the formation and maintenance of species, namely the formation of diagnosable clusters or exclusive groups of common ancestry [e.g., (Cracraft 1983; Shaw 1998)]. More recently, geneticists and molecular biologists became interested in elucidating the molecular basis of speciation. With this novel research interest came yet another family of species concepts, placing an emphasis on the biological mechanisms present in each organism (as opposed to extrinsic barriers of a geographical or ecological sort) that make possible or hinder processes like gene flow [e.g., (Coyne and Orr 1998)]. In light of the above considerations, I identify three levels at which groups of organisms we may want to call species are defined: the pattern-level grouping of organisms into discrete clusters; the underlying process-level of gene flow and other processes operating within/between populations, and ultimately contributing to the formation and maintenance of discrete clusters; or the inner-organismic mechanismlevel
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