19 A hierarchy of species concepts: the denouement in the saga of the species problem R. 1. Mayden Contacting address: Department of Biological Sciences, P.O. Box 0344, University of Alabama, Tuscaloosa, AL 35487, USA ABSTRACT At least 22 concepts of species are in use today and many of these are notably incompatible in their accounts of biological diversity. Much of the traditional turmoil embodied in the species problem ultimate- ly derives from the packaging of inappropriate criteria for species into a single concept. This results from a traditional conflation of function of concepts with their applications, definitions with con- cepts, taxonomic categories with groups, and the ontological status of real species with teleological approaches to recover them. Analogous to classifications of supraspecific taxa, our forging inappropriate and ambiguous information relating to theoretical and operational dis- cussions of species ultimately results in a trade-off between conve- nience, accuracy, precision, and the successful recovery of natural biological diversity. Hence, none of these expectations or intentions of species or classifications is attainable through composite, and pos- sibly discordant, concepts of biological diversity or its descent. Reviewing and evaluating the concepts of species for their theo- retical and operational qualities illustrates that a monistic, primary concept of species, applicable to the various entities believed to be species, is essential. This evaluation reveals only one theoretical con- cept as appropriate for species, the Evolutionary Species Concept. This conceptualization functions as a primary concept and is essential in structuring our ideas and perceptions of real species in the natural world. The remaining concepts are secondary, forming a hierarchy of definitional guidelines subordinate to the primary concept, and are essential to the study of species in practice. Secondary concepts Species: The Units of Biodiversity. Edited by M.F. Oaridge, H.A. Dawah and MR. Wilson. Published in 1997 by Chapman & Hall. ISBN 0 412 63120 2

382 A hierarchy of species concepts should be used as operational tools, where appropriate, across the variance in natural diversity to discover entities in accord with the primary concept. Without this theoretical and empirical structuring of concepts of species our mission to achieve reconciliation and understanding of pattern and process of the natural world will fail. 19.1 INTRODUCTION 'I believe that the analysis of the species problem would be consid- erably advanced, if we could penetrate through such empirical terms as phenotypic, morphological, genetic, phylogenetic, or biological, to the underlying philosophical concepts. A deep, and perhaps widen- ing gulf has existed in recent decades between philosophy and empirical biology. It seems that the species problem is a topic where productive collaboration between the two fields is possible'. (Mayr, 1957) Little has changed with regard to the species problem since Mayr com- posed this piece. Some researchers argue for a particular concept of diver- sity known as species, while others prefer a pluralistic approach (Mishler and Donoghue, 1982). Today, the controversy continues over the concep- tualization of species. This volume reflects some of this diversity of thought across multiple taxonomic groups. This seemingly timeless debate has generated a heterogeneous proliferation of concepts, most hoping to capture the operational and/or theoretical qualities of a good concept. The search has been for a concept-definition that is biologically relevant and meaningful, one that is easily applied, and one that encom- passes natural biodiversity. That is, a concept of real species assisting in and ensuring their recognition and our understanding of them in nature. This goal has not been achieved for several reasons. The 20th century history of biological classification illustrates why this so-called silver bullet species concept, one that will attend to all our per- ceived needs, has not yet been achieved. In phylogenetic systematics (or cladistics) the Linnaean classification scheme represents a hierarchical sys- tem of categories coordinate with a phylogenetic tree of named taxa. Represented in the classification is the idea of monophyly of taxa, or sister group (genealogical) relationship. Classifications are information retrieval systems about genealogical relationships. In evolutionary systematics the classification is purported to represent sister group relationship and evo- lutionary distinctiveness. Paradoxically, while this may be viewed as an expedient method to group information in a retrieval system, under this method one can never be sure which criteria are optimized at any part of a classification. Thus, confusion is inherent in an ambiguous information retrieval system. The ultimate trade-off of combining too many desired

Methodology 383 functions into a convenient method is that it is not always possible to iso- late anyone function (e.g. genealogy versus distinctiveness). Much of the turmoil embodied in the species problem ultimately derives from our packaging inappropriate criteria for species into a single concept. This results from a traditional conflation of function of concepts with their applications, definitions with concepts, taxonomic categories with groups, and ontological status of real species with teleological approaches to recover them. Analogous to classifications of supraspecific taxa, our forging inappropriate and ambiguous information relating to theoretical and operational discussions of species ultimately results in a trade-off between convenience, accuracy, precision, and the successful recovery of natural biological diversity. None of these expectations or intentions of species or classifications is attainable through composite, and possibly discordant, concepts of biological diversity or its descent. With this in mind can one tease apart the theoretical concepts and operational definitions of species and develop a primary concept applica- ble to the various entities believed to be species? I think this is possible through a hierarchical view of species concepts and their definitions. Below, I review the various species concepts and propose a hierarchical classification for them. Each of these concepts is briefly evaluated relative to their consequential qualities thought to be important in a concept (Hull, 1997: Chapter 18). This evaluation reveals only one appropriate primary and theoretical concept of species. The remaining definitions are sec- ondary concepts, forming a hierarchy of definitional guidelines subordi- nate to this primary concept. The secondary concepts are engaged only as operational tools, where appropriate, across the variance in natural diver- sity to discover entities in accord with the primary concept. 19.2 METHODOLOGY Probably more is written about species than any other topic in evolution- ary biology. There are many opinions and studies addressing this ques- tion. Hence, an exhaustive survey of these is impossible. Concepts are ideas or intuitions uniquely developed in the minds of every person. Definitions of these concepts are the only form with which one can com- pare them. Sometimes, these definitions may be poorly developed or mis- interpreted, ultimately leading to miscommunication of ideas. Regardless, I have endeavoured to understand the arguments on the various species concepts (Table 19.1), and compare and evaluate them. In section 19.7 I have also made an effort to identify synonyms of concepts these are list- ed by assigned standard abbreviations or full titles. Where concepts were formerly identified as synonymous, credit is provided in part refers to the observation that portions of concepts are equivalent.

384 A hierarchy of species concepts Table 19.1 Species concepts and standardized abbreviations 14. Morphological (MSC) 15. Non-dimensional (NDSC) 16. Phenetic (PhSC) 17. Phylogenetic (PSC) 1. Diagnosable Version (PSC1) 2. Monophyly Version (PSCz) 3. Diagnosable and Monophyly Version (PSC3) Polythetic (PtSC) Recognition (RSC) Reproductive Competition (RCC) Successional (SSC) Taxonomic (TSC) 1. Agamospecies (ASC) 2. Biological (BSC) 3. Cohesion (CSC) 4. Cladistic (CISC) 5. Composite (CpSC) 6. Ecological (EcSC) 7. Evolutionary Significant Unit (ESU) 8. Evolutionary (ESC) 9. Genealogical Concordance (GCC) 18. 10. Genetic (GSC) 19. 11. Genotypic Cluster Definition (GCD) 20. 12. Hennigian (HSC) 21. 13. Internodal (ISC) 22. 19.3 IMPORTANT QUALITIES TO CONSIDER Few concepts can be viewed as more fundamental to the natural sciences than that of the species. Species as individuals (Hull, 1976) represent a unique level of organization of the natural world they are self-organizing entities or particulars. This level of universality is the upper-most limit to involve tokogenetic relationships and the lower-most level participating in phylogenetic relationships. They are purported to be the highest level of integration to participate in natural processes while being spatiotempo- rally constrained. Thus, they are essentially fundamental units of evolu- tion. In plain English, this means that for all disciplines using any species in pure or applied research, education, management, conservation, etc., success at accomplishing identified missions or deriving informative answers to particular questions is inextricably tied to a basic assumption that the species involved are real by-products of natural processes and not misguided fabrications of our own invention. One method to compare and contrast the various concepts of species is through three criteria that have traditionally been employed to evaluate sci- entific concepts. Hull (1997: Chapter 18) provides such a review for seven of the most frequently used concepts for their theoretical significance, gener- ality (or universality), and applicability (or operationality). Following a review of definitions, metaphysical topics, and species concepts, I extend a similar evaluation to all 22 of the various concepts of species. The relation- ships of the concepts within and among the three criteria, together with the question of monism versus pluralism, reveals a hierarchy of species con- cepts that should finally put the species problem to rest. 19.4 CONCEPTS, DEFINITIONS, GROUPS, CATEGORIES AND NAMES: THE UNFORTUNATE CONFLATION OF TERMINOLOGY Important in the discussion and resolution of the species problem is the correct usage and understanding of ideas and terms related to the species

Concepts. definitions, groups, categories and names 385 issue. Historically, discussions of species have involved the use of four critical terms: concept, definition, group and category. These terms have been central to the fundamental melange that both scientists and philoso- phers have encountered with the species problem. Communication of ideas or concepts in science is of utmost importance and hinges upon statements or definitions developed by persons formu- lating or discussing the concepts. Critically important to exact communi- cation of ideas embodied in concepts demands that we do not obfuscate the terms or words used in definitions or statements of the concepts. Here, we confront difficulties both in the logical treatment and the evaluation of definitions. Concepts of biological systems serve as fundamental links between pat- tern and process in nature, are employed in every discipline, and help guide our perception of natural systems. They are formulated by individ- ual persons through observation, study and synthesis (impressions and imagination) of both theory and empirical data. Concepts may be real or abstract. Real concepts are those representing easily agreed discrete objects. Abstract concepts are those representing hypothetical and tran- sient phenomena. A concept may be relayed from one person to one or more other persons by adapting it into a statement or definition, either verbally, in writing, or graphically. Such a definition mayor may not induce the same concept in the mind of the other persons, depending upon the appropriateness, precision, and accuracy of the words used in the definition and the level of understanding of the other person. With real concepts (e.g. round versus square) one may compare statements devel- oped by different observers with discrete objects to see if they agree. With abstract concepts (evolution, natural selection, species as taxa) it is difficult to know for sure if statements represent the same transient or hypothetical things, and the respective definitions can only be compared using previ- ously agreed definitions of words used in the statement. One may also observe the effectiveness of such a concept through direct examination. The term'group' refers to a collection of objects or things. In the inter- section of natural sciences, taxonomy, and systematics the term taxon is often used synonymously with the term group. A group can be real and have objective reality if it corresponds to qualities that are real and exclu- sive to it, and if it consists of things that have material existence. They may be arranged hierarchically, either as non-reticulate or reticulate groups. They may be represented at various levels of universality from groups of things to more inclusive groups of things, etc. They may be of any size and arise on the basis of intrinsic attributes and/or extrinsic decisions. Organisms can be members of any number of groups so long as they pos- sess the attributes of the said groups. Groups, however, are not like con- cepts. Groups develop from sense impressions of concepts and can be agreed upon and definite if the statements about them are unambiguous and decisive.