What Is a Species, and What Is Not? Ernst Mayr Philosophy of Science, Vol. 63, No. 2. (Jun., 1996), pp. 262-277. Stable URL: http://links.jstor.org/sici?sici=0031-8248%28199606%2963%3A2%3C262%3AWIASAW%3E2.0.CO%3B2-H Philosophy of Science is currently published by The University of Chicago Press. Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/ucpress.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. The JSTOR Archive is a trusted digital repository providing for long-term preservation and access to leading academic journals and scholarly literature from around the world. The Archive is supported by libraries, scholarly societies, publishers, and foundations. It is an initiative of JSTOR, a not-for-profit organization with a mission to help the scholarly community take advantage of advances in technology. For more information regarding JSTOR, please contact [email protected]. http://www.jstor.org Tue Aug 21 14:59:32 2007 WHAT IS A SPECIES, AND WHAT IS NOT?" ERNST MAYRT I analyze a number of widespread misconceptions concerning species. The spe- cies category, defined by a concept, denotes the rank of a species taxon in the Linnaean hierarchy. Biological species are reproducing isolated from each other, which protects the integrity of their genotypes. Degree of morphological difference is not an appropriate species definition. Unequal rates of evolution of different characters and lack of information on the mating potential of isolated populations are the major difficulties in the demarcation of species taxa. 1. What is a Species, and What is Not? As someone who has published books and papers on the biological species for more than 50 years, and who has revised and studied in detail more than 500 species of birds and many species of other groups of organisms, the reading of some recent papers on species has been a rather troubling experience. There is only one term that fits some of these authors: armchair taxonomists. Since many authors have never personally analyzed any species populations or studied species in nature, they lack any feeling for what species actually are. Darwin already knew this when, in September 1845, he wrote to Jo- seph Hooker: "How painfully true is your remark that no one has hardly the right to examine the question of species who has not minutely de- scribed many" (Darwin 1987, 253). These authors make a number of mis- takes that have been pointed out again and again in the recent literature. Admittedly, the relevant literature is quite scattered, and some of it is perhaps rather inaccessible to a non-taxonomist. Yet, because the species concept is an important concept in the philosophy of science, every effort should be made to clarify it. It occurred to me that instead of criticizing certain recently published papers individually, it would be more construc- tive and helpful if I would here attempt to present, from the perspective of a practicing systematist, a concise overview of the philosophically im- portant aspects of the problem of the 'species'. There is nothing of the sort in the literature. The species is the principal unit of evolution and it is impossible to write about evolution, and indeed about almost any aspect of the philosophy of biology, without having a sound understanding of the meaning of bi- ological species. A study of the history of the species problem helps to dispel some of the misconceptions (Mayr 1957, Grant 1994). *Received December 1994; revised October 1995. ?Send reprint requests to the author, Museum of Comparative Zoology, Harvard Uni- versity, 26 Oxford Street, Cambridge, MA 02138. Philosophy of'Science, 63 (June 1996) pp. 262-277. 0031-824819616302-0007S2.00 Copyright 1996 by the Philosophy of Science Association. All rights reserved. WHAT IS A SPECIES, AND WHAT IS NOT? 263 2. Species of Organisms Are Concrete Phenomena of Nature. Some recent authors have dealt with the concept of species as if it were merely an arbitrary, man-made concept, like the concepts of reduction, demarcation, cause, derivation, prediction, progress, each of which may have almost as many definitions as there are authors who have written about them. How- ever, the concept biological species is not like such concepts. The term 'species' refers to a concrete phenomenon of nature and this fact severely constrains the number and kinds of possible definitions. The word 'species' is, like the words 'planet' or 'moon', a technical term for a concrete phe- nomenon. One cannot propose a new definition of a planet as "a satellite of a sun that has its own satellite," because this would exclude Venus, and some other planets without moons. A definition of any class of objects must be applicable to any member of this class and exclude reference to attributes not characteristic of this class. This is why any definition of the term 'species' must be based on a careful study of the phenomenon of nature to which this term is applied. Alas, this necessity is not appreciated by all too many of those who have recently discussed the species problem after a mere analysis of the literature. The conclusion that there are concrete describable objects in nature which deserve to be called "species" is not unanimously accepted. There has been a widespread view that species are only arbitrary artifacts of the human mind, as some nominalists, in particular, have claimed. Their ar- guments were criticized by Mayr (1949a, 371). 3. Why Are There Species of Organisms? Why is the total genetic varia- bility of nature organized in the form of discrete packages, called species? Why are there species in nature? What is their significance? The Darwinian always asks why questions because he knows that everything in living nature is the product of evolution and must have had some selective sig- nificance in order to have evolved.' He therefore asks: What selection forces in nature favor the origin and maintenance of species? The answer to this question becomes evident when one makes a certain thought ex- periment. "It is quite possible to think of a world in which species do not exist but are replaced by a single reproductive community of individuals, each one different from every other one, and each one capable of reproducing with those other individuals that are most similar to it. Each individual would then be the center of a concentric series of circles of genetically more and more unlike individuals. What would be the consequence of the continuous uninterrupted gene flow through such a large system? In each generation certain individuals would have a selective advantage because '1 am not aware of a single major feature of living nature of which this claim could be refuted. 264 ERNST MAYR they have a gene complex that is specially adapted to a particular ecolog- ical situation. However, most of these favorable combinations would be broken up by pairing with individuals with a gene complex adapted to a slightly different environment. In such a system there is no defense against the destruction of superior gene combinations except the abandonment of sexual reproduction. It is obvious that any system that prevents such un- restricted outcrossing is s~perior"~(Mayr 1949b, 282). The biological spe- cies is such a system. The biological meaning of species is thus quite apparent: "The segre- gation of the total genetic variability of nature into discrete packages, so called species, which are separated from each other by reproductive bar- riers, prevents the production of too great a number of disharmonious incompatible gene combinations. This is the basic biological meaning of species and this is the reason why there are discontinuities between sym- patric species. We do know that genotypes are extremely complex epige- netic systems. There are severe limits to the amount of genetic variability that can be accommodated in a single gene pool without producing too many incompatible gene combinations" (Mayr 1969, 3 16). The validity of this argument is substantiated by the fact that hybrids between species, particularly in animals, are almost always of inferior viability and more extreme hybrids are usually even sterile. "Almost always" means that there are species interpreted to be the result of hybridization, particularly among plants, but except for the special case of allopolyploidy, such cases are rare. Among the attributes members of a species share, the only ones that are of crucial significance for the species definition are those which serve the biological purpose of the species, that is, the protection of a harmo- nious gene pool. These attributes were named by Dobzhansky (1935) iso- lating mechanisms. It is immaterial whether or not the term isolating mech- anism was well chosen, nor is it important whether one places the stress on the prevention of interbreeding with non-conspecific individuals or the facilitation ("recognition") of breeding with conspecific individuals. The concept I have just developed is articulated in the so-called biological species definition: "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." The isolating mechanism by which reproductive isolation is effected are properties of individuals.