Opinion
On the failure of modern species concepts

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The modern age of species concepts began in 1942, when Ernst Mayr gave concept names to several different approaches to species identification. A long list of species concepts then followed, as well as a complex literature on their merits, motivations and uses. Some of these complexities arose as a consequence of the semantic shift that Mayr introduced, in which procedures for identifying species were elevated to concepts. Much of the debate in recent decades over concepts, and over pluralism versus monism, can be seen as an unnecessary consequence of treating species identification criteria as if they were more fundamental concepts. Recently, biologists have begun to recognize both the shortcomings of a lexicon of multiple species concepts and a common evolutionary idea that underlies them.

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Ernst Mayr and the origins of the species-concept debate

Concepts are ideas that are typically neither very broad nor overly specific. In the rough hierarchy of referential scope, concepts tend to fall somewhere below theories and somewhere above definitions. Thus, we have an all-embracing theory of evolution, within which occur more modest concepts, such as the concept of fitness. A general concept can, in turn, be defined more specifically, and in different ways at different times, depending on the context. This is particularly true of concepts

The history of ‘concept’ in regard to species

Debates over the meaning of the word ‘species’ were already common before Darwin, although they became even more so afterward. For example, Darwin wrote about the difficulties associated with species definitions [3]: ‘Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.’

The discovery that species evolve and give rise to new

The diversity of modern species concepts

Mayr's purpose for delineating multiple species concepts was to emphasize the different kinds of methods and criteria used for species identification. Mayr's equating of species concepts with criteria for identification is clear in 1942, and especially in his writings after 1969 15, 16. Interestingly, for several years in between, Mayr identified two levels of concepts: a primary theoretical level and a secondary level of definitions (that included the biological species concept) 17, 18 (Box 1).

Species pluralism: real or apparent?

An effect of the vigorous debate on species is the often-repeated claim that there can be no single species concept, and that some kind of pluralistic view of species should be adopted. Some proposals for pluralism are philosophical in nature and attempt to accommodate different metaphysical aspects of species 25, 26. More typically, proposals for pluralism are motivated by the fact that particular criteria for identifying species are not applicable in all situations and the observation that

Returning to a common concept of species

One piece of evidence suggesting that we do share a common concept of species is that biologists discuss species regularly without explicit reference to species concepts, and that disagreement over ‘species’ does not necessarily arise unless a discussant explicitly brings up the subject of species concepts. In this literal linguistic sense, we appear to share a species concept 34, 35. Perhaps more to the point: are there any participants in species debates who actually think that species do not

Lessons on the method of multiple concepts

Perhaps the long-running debate over the best species concept is dying. If so, it comes without the oft-hoped for widespread recognition of a common protocol for species identifications. In this light, it is useful to appreciate that some of our most persistent questions on how best to identify species were present long before Mayr's book, and that the fundamental form of those questions has not changed greatly. There were different species definitions in discussion and use before the

Acknowledgements

I thank J. Wilkins, M. P. Winsor, R. Harrison, J. Mallet and K. de Queiroz, as well as A. Templeton and two anonymous reviewers for helpful comments and critique.

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