Going to school in the late 1960’s, we were always taught that a species consisted of a group of individuals that shared the same physical characteristics and could naturally interbreed with each other and produce viable offspring.  In addition, they could not reproduce with any other species and produce viable offspring.  However, in reviewing the literature, I find that this definition is far from the accepted norm today, and if we are involved in a discussion about the process of speciation, then we must determine what the prevailing definition of a species is.

Webster’s New Collegiate Dictionary of 1976 defines species as:

a category of biological classification ranking immediately below the genus or subgenus, comprising related organisms or populations potentially capable of interbreeding, and being designated by a Latin or latinized uncapitalized noun or adjective agreeing grammatically with the genus.[i] 

Twenty years later, Webster’s New Universal Unabridged Dictionary, defines a species as:

1. A class of individuals having some common characteristics or qualities; distinct sort or kind. 2. Biol. The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.[ii]

Now the most current dictionary definition of a species is:

b : a class of individuals having common attributes and designated by a common name; specifically : a logical division of a genus or more comprehensive class <confessing sins in species and in number> c : the human race : human beings —often used with the <survival of the species in the nuclear age> d (1) : a category of biological classification ranking immediately below the genus or subgenus, comprising related organisms or populations potentially capable of interbreeding, and being designated by a binomial that consists of the name of a genus followed by a Latin or latinized uncapitalized noun or adjective agreeing grammatically with the genus name (2) : an individual or kind belonging to a biological species.[iii]

In his treatise on evolution in 1931, Wright discusses a species as:

. . . an intricate network of living matter, physically continuous in space-time, and with modes of response to external conditions which it appears can be related to the genetics of individuals only as statistical consequences of the latter.  From a still broader viewpoint (compare Lotka, 1925) [reference by Wright] the species itself is merely an element in a much more extensive evolving pattern….[iv]

Part of the uncertainty and problems of what constitutes a species were expressed in a 1957 paper by Prosser:

Most physiologists seek homogeneous material, whereas in population studies, controlled heterogeneity is desired.  When differences are observed in similar animals, the differences may be among individuals within a genetically similar population, they may be in mutants carried at some level in the population, they may be racial between ecotypes of the same species, or they may be between the total genetic systems of true species.  Our operative assumption has been that speciation progresses from individual variation through strains and races to species.  Since the complete sequence cannot be observed in any single species but is inferred, it is necessary to piece together as much indirect evidence as possible to test the basic assumption.[v]

Prosser then discussed the physiological criteria that he saw for variation, including survival, reproduction, measurement of an internal state as a function of external stress, mechanisms and limits of regulation, recovery from a deviated state, rate function, and behavior.  He then carefully laid out a list of inadequacies of a simplified speciation scheme:

1.  Every serious systematist is impressed more by the diversity than by the uniformity of his species…

2.  The notion that new characteristics are retained only if they have adaptive advantage for survival is unrealistic, and there are numerous examples of establishment of neutral morphological and ethological patterns…

3.  While the range of a species may be very great, actual breeding populations are often very small…

4.  Many of the so-called physiological races turn out to be true species…

5.  Physiological characteristics tend to be more sensitive to the environment in their expression than do structural ones, and the potential lability of individual animals is very great…

6.  Most physiological characters which have been analyzed in animals are polygenic…

7.  It may be argued that the physiological criteria which have been applied are too gross, that measurement of oxygen consumption by an animal is no more precise than stating the number of segments on an appendage, that what is needed are more sensitive tests, measures of single enzymes…

In summary, physiological adaptedness to the environment may exist in the absence of clear morphological speciation (a) in balanced polymorphic populations (these include advantageous heterozygotes and genic expression differing with genetic environment), (b) in clines, (c) in sibling species, and (d) in the extensive phenotypic lability in response of individuals to environmental stress.  Physiological characters may not conform to ‘good’ specific characters because (a) taxonomic characters are often nonadaptive, (b) physiological characters are complex genetically (multifactor), (c) adaptive characters usually have large safety factors and parallel or alternate paths, and functional characters are either (d) quantitatively so sensitive to the environment that phenotypic variation exceeds genotypic, or (e) qualitatively so complex that reproductive isolation has already been established.  If evolution is considered as the development of adapted organisms and if adaptation is basically functional, it is of prime importance to analyze populations in terms of physiological characters.  However, the origin of species per se may not coincide with the development of adapted organisms.[vi]

In 1959, the Darwin Centennial Celebration was held at the University of Chicago in commemoration of the one hundredth anniversary of the first publication of Darwin’s infamous work, Origin of Species.  At that celebration, a number of papers were presented and in one of those papers by Prosser, “Comparative Physiology in Relation to Evolutionary Theory”, he discussed the criteria for a species (Prosser, 1960). Prosser stated that there were three ways of understanding biological species; morphology, reproductive isolation, and physiological adaptation.  Morphology, is listed as the simplest approach and is accomplished by the arbitrary cataloging of morphological traits of similar organisms and is the basis of most taxonomic keys.  However as Prosser points out, the morphological classification is not always based upon phylogeny.  Reproductive isolation may be due to a number of causes such as genetic, behavioral, anatomical, hormonal, spatial, and others.  Prosser mentions that reproductive isolation is difficult to test under natural conditions while the occurrence of hybridization under laboratory conditions is not sufficient reason to extrapolate to natural isolation.   Physiological adaptation, at least at the time of Prosser’s presentation in 1960, was an approach to defining a species that had scarcely been examined and dealt with the ecological separation.  He divided the ecological separation into two general aspects:

(a) No two related species can successfully occupy the same ecological niche (possibly excepting a few overlaps which are isolated by breeding behavior).  (b) No two species have identical distribution ranges.  Hence, if all of the microclimatic and biotic features of an ecological niche and distribution range are known, a description of the distinctive physiological adaptations to the niche and range should describe a species.[vii]

The emphasis of Prosser’s paper was that although the morphological features were important, more emphasis should be placed on the determination of the reproductive isolation mechanisms and even more emphasis on physiological or ecological adaptation as a determining factor in defining a species.

In 1957(a), Mayr presented a brief history of the species concept with a statement that denotes the current problem with the term: 

Linnaeus will be cited as the champion of two characteristics of the species, their constancy and their sharp delimitation (their ‘objectivity’).  One of the minor tragedies in the history of biology has been the assumption during the hundred and fifty years after Linnaeus that constancy and clear definition of species are strictly correlated and that one must make a choice of either believing in evolution (the ‘inconsistency’ of species) and then having to deny the existence of species except as purely subjective, arbitrary figments of the imagination, or, as most early naturalists have done, believing in the sharp delimitation of species but thinking that this necessitated denying evolution . . .

 The insistence of Linnaeus on the reality, objectivity, and constancy of species is of great importance in the history of biology for three reason.  First it meant the end of the belief in spontaneous generation as far as higher organisms are concerned, a belief which at the time was still widespread . . .

 A second reason why his emphasis was important is that it took the species out of the speculations of the philosophers who approached the species problem in the spirit of metaphysics and stated, for instance, that ‘only individuals exist.  The species of a naturalist is nothing but an illusion’ (Robinet, 1768) [referenced by Mayr]…

 A third reason why the insistence on the sharp delimitation of species in the writings of Linnaeus is of historical importance is that it strengthened the viewpoint of the local naturalist and established the basis for an observational and experimental study of species in local faunas and floras, of which Darwin took full advantage.[viii]

Darwin took a completely opposite view of species as did Linnaeus.  As Mayr stated:

In Darwin, as the idea of evolution became firmly fixed in his mind, so grew his conviction that this should make it impossible to delimit species.  He finally regarded species as something purely arbitrary and subjective.  ‘I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety which is given to less distinct and more fluctuating forms…The amount of difference is one very important criterion in settling whether two forms should be ranked as species or variety.’  And finally he came to the conclusion that ‘in determining whether a form should be ranked as a species or a variety, the opinion of the naturalists having sound judgment and wide experience seems the only guide to follow’ (Darwin, 1859) [referenced by Mayr].[ix]

Those that followed Darwin only helped to increase the disagreement on what the criterion for a species was.  Mayr points out those disagreements as:

Subjective versus objective;

Scientific versus purely practical;

Degree of difference versus degree of distinctness;

Consisting of individuals verses consisting of populations;

Only one kind of species versus many kinds of species;

To be defined morphologically versus to be defined biologically.[x]

Mayr, then moves to clarify the main three aspects that are derived from the current discussion on species:

(1) they are based on distinctness rather than on difference and are therefore to be defined biologically rather than morphologically, (2) they consist of populations, rather than of unconnected individuals, a point particularly important for the solution of the problem of speciation, (3) they are more succinctly defined by isolation from non-conspecific populations than by the relation of conspecific individuals to each other.  The crucial species criterion is thus not the fertility of individuals, but rather the reproductive isolation of populations.[xi]

Therefore the question arises: “What is a species?”  Again turning to Mayr for the reply:

Depending on the choice of criteria, it leads to a variety of ‘species concepts’ or ‘species definitions.’  At one time I listed five species concepts, which I called the practical, morphological, genetic, sterility, and biological (Mayr, 1942) [referenced by Mayr].  Meglitsch (1954) [referenced by Mayr] distinguishes three concepts, the phenotypic, genetic, and the phylogenetic, a somewhat more natural arrangement.  Two facets emerge from these and other classifications.  One is that there is more than one species concept and that it is futile to search for the species concept.  The second is that there are at least two levels of concepts.  Such terms as ‘practical,’ ‘sterility,’ ‘genetic’ signify concrete aspects of species which lead to what one might call ‘applied’ species concepts.  They specify criteria which can be applied readily to determine the status of discontinuities found in nature.  Yet they are secondary, derived concepts, based on underlying philosophical concepts, which might also be called primary or theoretical concepts.  I believe that the analysis of the species problem would be considerably advanced, if we could penetrate through such empirical terms as phenotypic, morphological, genetic, phylogenetic, or biological, to the underlying philosophical concepts.[xii]

In 1970, Mayr did not seem as unsure of what constituted a species.  Theoretically, they constituted a reproductive community, plus an ecological unit and finally a genetic unit.  This took the concept of species away from the typological classification of objects.  He ends with the following definition: “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”.[xiii]

With all of the difficulties in defining exactly what a species is, Mayr also points the importance of maintaining the use of the term: 

There are many difficulties in applying the species concept to the vast variety of discontinuities found in organic nature.  Yet many biological phenomena would make no sense if individuals were not organized into populations, and these populations into species.  Species are an important biological phenomenon, important to every biologist because ever biologist works with species.[xiv]

By 1982, Mayr recognized that the controversy over what constitutes a species still existed and probably will for some time to come.  He states:

The entities which the taxonomist assembles into genera and still higher taxa are the species.  They are the basic kinds of living beings that make up the diversity of nature.  They represent the lowest level of genuine discontinuity above the level of the individual.  The song sparrow and the fox sparrow are different species and so are the red oak and pin oak.  The entity designated by the term ‘species’ would, at first sight, seem obvious, simple, and easily defined.  But this is not the case.  There is probably no other concept in biology that has remained so consistently controversial as the species concept.  One should have thought that the animated debate of the post-Darwinian period would have produced clarity and unanimity or, at least, that the new systematics of the 1930s and 40s would have brought final clarity, but this was not the case.  Even today several papers on the species problem are published each year and they reveal almost as much difference of opinion as existed one hundred years ago.  The advance that has been made is that the nature of the disagreement is much more clearly formulated than in the earlier periods.  What is particularly interesting for the student of ideas is that the history of the species problem is, to a large degree, quite independent of the history of the problem of classification.  The branch of systematics dealing with the species problem can be designated as microtaxonomy . . . .[xv]

Regardless of the inability to define what a species is, Mayr believes that it is still important to use the term as if we did know what it was.

Therefore the question is raised one last time, “What is a species?”  For if the concept of what defines a species is so variable, then how can one honestly discuss the process of speciation?  Provine adds to this concept by stating:

The corresponding book on observed and documented cases of speciation in action would be very thin indeed, and some evolutionary biologists claim even those pages would be mostly blank.  That is one good reason why, in the face of great mounds of evidence of the results of speciation processes, there is so much controversy about mechanisms of speciation.[xvi]

Demonstrating that there is no clear and concise definition of what a species is, one must question use of the term in the scientific literature, especially since the process of speciation is discussed as one of the main evidences for evolution.  Therefore, the point in question is: “How can a process such as speciation be used as one of the main bodies of evidence for evolution when the very item at its core, the species, is such a vague and controversial term?”

From a biblical perspective, we would prefer to use the term ‘kind’ or ‘created kind’.  We know that God created certain animal and plant kinds to reproduce according to their kinds (Genesis 1:11-12, 20-25).  How do kinds compare to species?  In some cases, they may be the same.  In other cases, a created kind may equate to a genus or family. 


[i] Webster’s New Collegiate Dictionary.  Springfield: G. & C. Merriam Company, 1976: 1116.

[ii] Webster’s New Universal Unabridged Dictionary.  New York: Barnes & Noble, Inc., 1996: 1832.

[iii] Merriam Webster Dictionary, 2010, http://www.merriam-webster.com/dictionary/species.

[iv] Wright, S.  “Evolution in Mendelian Populations,”  Genetics 16, (1931): 97-159.

[v] Prosser, C. L.  “The Species Problem From the Viewpoint of a Physiologist,”  In The Species Problem.  ed. Mayr, E., Reprinted 1974,  New York: Arno Press, 1957: 339-369. 

[vi] Ibid.

 [vii]Prosser, C. L.   Comparative Physiology in Relation to Evolutionary Theory,” Evolution After Darwin: The Evolution of Life: It’s Origin, History and Future. Vol. 1,  ed. Tax, S.  Chicago: The University of Chicago Press, 1960: 569-594.

[viii] Mayr, E.  “Species Concept and Definition,” The Species Problem.  ed. Mayr, E.,  Reprinted 1974, New York: Arno Press, 1957a: 1-23.

[ix] Ibid.

[x] Ibid.

[xi] Ibid.

[xii] Ibid.

[xiii] Mayr, E.  Populations, Species, and Evolution.  Cambridge: Belknap Press of Harvard University Press, 1970: 10-20; 106-128; 296-330.

[xiv] Mayr, E.  “Difficulties and Importance of the Biological Species Concept,” The Species Problem.  ed. Mayr, E., Reprinted 1974, New York: Arno Press, 1957b: 371-388.

[xv] Mayr, E.  The Growth of Biological Thought.  Cambridge: The Belknap Press, 1982. 

[xvi] Provine, W. B.  “Founder Effects and Genetic Revolutions in Microevolution and Speciation: A Historical Perspective,” Genetics, Speciation and the Founder Principle.  ed. Giddings, L.V., Kaneshiro, K.Y., and Anderson, W.W.,  New York: Oxford University Press, 1989: 43-77.

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