What is a species, and What is not? / Ernst Mayr

As an “outsider” I have often complained (something I do well, and often, LOL) about the chaotic “idea” of species, which has gotten more confusing with the advent of DNA research. I think this problem may be more easily observed by an outsider, who is trying to “make sense” of the variety of uses of “species” one encounters in related, but somehow disconnected fields. This article by Ernst Meyer presents a Darwinian approach; he is quite emphatic as to the correct definition, as a fact of nature. This article is quite long: please use the link below. His writing is such a relief from contemporary sloppy “hand-waiving” that passes for discussion.



What is a Species, and What is Not?

Originally Published in Philosophy of Science, Vol. 63 (June 1996) pp. 262-277.

by Ernst Mayr (Brief bio at bottom: impossible to “sum up” his stature and contributions)

I analyze a number of widespread misconceptions concerning species. The species 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 Joseph Hooker: “How painfully true is your remark that no one has hardly the right to examine the question of species who has not minutely described many.” (Darwin 1987, 253).

These authors make a number of mistakes 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 constructive and helpful if I would here attempt to present, from the perspective of a practicing systematist, a concise overview of the philosophically important 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 biological species. A study of the history of the species problem helps to dispel some of the misconceptions (Mayr 1957, Grant 1994).

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. However, 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 phenomenon. 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 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 arguments were criticized by Mayr (1949a, 371).

3. Why are there species of organisms? Why is the total genetic variability 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 significance in order to have evolved. (1) 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 experiment. “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 they have a gene complex that is specially adapted to a particular ecological 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 unrestricted outcrossing is superior” (2) (Mayr 1949b, 282). The biological species is such a system. The biological meaning of species is thus quite apparent: “The segregation of the total genetic variability of nature into discrete packages, so called species, which are separated from each other by reproductive barriers, 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 sympatric species. We do know that genotypes are extremely complex epigenetic 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, 316). 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.


Born on July 5, 1904, in Kempten in southern Germany, Ernst Mayr passed away, age 100, on February 3, 2005, in Bedford near Cambridge, Massachusetts. Mayr was, by the accounts of his Harvard colleagues the late Stephen Jay Gould and Edward O. Wilson, not only the greatest evolutionary biologist of the 20th century, but even its greatest biologist overall. Thomas Henry Huxley was dubbed “Darwin’s bulldog” for fighting for the acceptance of Darwinian ideas soon after their inception in the last decades of the 19th century. Similarly, Ernst Mayr has been called “Darwin’s apostle” or the “Darwin of the 20th century” for promoting and dispersing Darwin’s hypotheses throughout the past century.



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