A number of authors argue that while species are evolutionary units, individuals and real entities, higher taxa are not. I argue that drawing the divide between species and higher taxa along such lines has not been successful. Common conceptions of evolutionary units either include or exclude both types of taxa. Most species, like all higher taxa, are not individuals, but historical entities. Furthermore, higher taxa are neither more nor less real than species. None of this implies that there is no (...) distinction between species and higher taxa; the point is that such a distinction is more subtle than many authors have claimed. (shrink)

I defend a view of the species category, pluralistic realism, which is designed to do justice to the insights of many different groups of systematists. After arguing that species are sets and not individuals, I proceed to outline briefly some defects of the biological species concept. I draw the general moral that similar shortcomings arise for other popular views of the nature of species. These shortcomings arise because the legitimate interests of biology are diverse, and these diverse interests are reflected (...) in different legitimate approaches to the classification of organisms. In the final section, I show briefly how the pluralistic approach can help to illuminate some areas of biological and philosophical dispute. (shrink)

This paper shows that species are individuals with respect to evolutionary theory in the sense that the laws of the theory deal with species as irreducible wholes rather than as sets of organisms. 'Species X' is an instantiation of a primitive term of the theory. I present a sketch of a proof that it cannot be defined within the theory as a set of organisms; the proof relies not on details of my axiomatization but rather on a generally accepted property (...) of speciation; hence the same argument should work for any axiomatization which captures this generally accepted property of speciation. (shrink)

I construe the question Are species sets? as a question about whether species can be conceived of as sets, as the term set is understood by contemporary logicians. The question is distinct from the question Are species classes?: The conception of classes invoked by Hull and others differs from the logician's conception of a set. I argue that species can be conceived of as sets, insofar as one could identify a set with any given species and that identification would satisfy (...) three desiderata: the set would be a set of organisms, the identification would be apposite, and the identification would permit the formulation of statements about species in set-theoretic terms. One cannot, however, identify a species with any given set. Understanding the claim that species are sets in this way enables one to understand better the dispute between some who accept the claim (e.g., Kitcher 1984, 1987) and some who apparently reject it (e.g., Mayr 1987). (shrink)

The cladistic species concept proposed by Ridley (1989) rests on an undefined notion of speciation and its meaning is thus indeterminate. If the cladistic concept is made determinate through the definition of speciation, then it reduces to a form of whatever species concept is implicit in the definition of speciation and fails to be a truly alternative species concept. The cladistic formalism advocated by Ridley is designed to ensure that species are monophyletic, that they are objectively real entities, and that (...) they are individuals. It is argued that species need not be monophyletic in order to be real entities, and that ancestor-descendant relations are not the only relations that confer individuality on entities. The species problem is recast in terms of a futile quest for a definition of that single kind of entity to which the term species should uniquely apply. (shrink)

The concept of individuality as applied to species, an important advance in the philosophy of evolutionary biology, is nevertheless in need of refinement. Four important subparts of this concept must be recognized: spatial boundaries, temporal boundaries, integration, and cohesion. Not all species necessarily meet all of these. Two very different types of pluralism have been advocated with respect to species, only one of which is satisfactory. An often unrecognized distinction between grouping and ranking components of any species concept is necessary. (...) A phylogenetic species concept is advocated that uses a grouping criterion of monophyly in a cladistic sense, and a ranking criterion based on those causal processes that are most important in producing and maintaining lineages in a particular case. Such causal processes can include actual interbreeding, selective constraints, and developmental canalization. The widespread use of the biological species concept is flawed for two reasons: because of a failure to distinguish grouping from ranking criteria and because of an unwarranted emphasis on the importance of interbreeding as a universal causal factor controlling evolutionary diversification. The potential to interbreed is not in itself a process; it is instead a result of a diversity of processes which result in shared selective environments and common developmental programs. These types of processes act in both sexual and asexual organisms, thus the phylogenetic species concept can reflect an underlying unity that the biological species concept can not. (shrink)

I defend a view of the species category, pluralistic realism, which is designed to do justice to the insights of many different groups of systematists. After arguing that species are sets and not individuals, I proceed to outline briefly some defects of the biological species concept. I draw the general moral that similar shortcomings arise for other popular views of the nature of species. These shortcomings arise because the legitimate interests of biology are diverse, and these diverse interests are reflected (...) in different legitimate approaches to the classification of organisms. In the final section, I show briefly how the pluralistic approach can help to illuminate some areas of biological and philosophical dispute. (shrink)

Biological species have been treated traditionally as spatiotemporally unrestricted classes. If they are to perform the function which they do in the evolutionary process, they must be spatiotemporally localized individuals, historical entities. Reinterpreting biological species as historical entities solves several important anomalies in biology, in philosophy of biology, and within philosophy itself. It also has important implications for any attempt to present an "evolutionary" analysis of science and for sciences such as anthropology which are devoted to the study of single (...) species. (shrink)

The central thesis of this book is that the semantic conception is a logical methodologically and heuristically richer and more accurate account of scientific theorizing, and in particular of theorizing in evolutionary biology, than the ...

Traditionally, species have been treated as classes. In fact they may be considered individuals. The logical term “individual” has been confused with a biological synonym for “organism.” If species are individuals, then: 1) their names are proper, 2) there cannot be instances of them, 3) they do not have defining properties, 4) their constituent organisms are parts, not members. “ Species " may be defined as the most extensive units in the natural economy such that reproductive competition occurs among their (...) parts. Species are to evolutionary theory as firms are to economic theory: this analogy resolves many issues, such as the problems of “reality” and the ontological status of nomenclatorial types. (shrink)

Oaks exemplify problems with the reproductive species concept which motivate a reconsideration of the use and nature of species. Ecology is important in the reconsideration. The species level is usually overemphasized in evolutionary thought; selection acts on phenotypes and any mutualistic units. Standard definitions tend to inhibit free conceptual progress. Multispecies, sets of broadly sympatric species that exchange genes, may occur among animals as well as plants and may conceivably bridge kingdoms. This phenomenon can be adaptively important. There may be (...) taxa without species. The degree of modality of adaptive zones can be investigated empirically. (shrink)