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Species Concepts
Michael T Ghiselin, California Academy of Sciences, San Francisco, California, USA
The species is one of the most fundamental units in biology. However, there are different
concepts of what is meant by the term ‘species’ that reflect diverse goals and priorities
on the part of scientists.
Introduction
The species, like the cell and the organism, is one of the
most fundamental units in biology. It is fundamental in
two very important ways. On the one hand it is a basic
populational and evolutionary unit. Species become
transformed through time. They undergo speciation,
giving rise to separate and distinct lineages that diversify
through time. On the other hand, it is also a basic unit in
classification. Systematics is often defined as the study of
biodiversity, but this is a bit too broad. Systematics
emphasizes those aspects of biodiversity that are the result
of evolution. Its results are expressed as taxonomy, or
formal biological classification. Each species is given a
name that serves as a unique identifier and has a place in a
hierarchical arrangement that at once reflects its evolu-
tionary relationships and provides a basis for biologists’
language.
Essentialism or typology
This dual role of species creates serious problems because
ordinary ways of classifying the objects around us are not
always appropriate to dealing with an evolving universe.
Systematic biology had its origins in the language of
everyday life, and the modern science of taxonomy that
goes back to Linnaeus was pre-evolutionary. It was
supposed that ‘kinds’ of plants and animals are as
immutable as are kinds of minerals. They might vary
within certain limits, but for one species to change into
another was contrary to the kind of philosophy that was
accepted at the time. A groupwas supposed to havewhat is
called an ‘essence’ or ‘nature’ that made it what it is.
Classification meant discovering this essence, and defining
groups on the basis of supposedly ‘essential’ properties.
Much of the history of evolutionary thinking reflects a
struggle to get rid of what is called ‘essentialism’ or
‘typology’. Essentialism presupposes the reality of es-
sences, leading people to think in terms of stereotypes and
to screen out that which is unique or variable.
Nominalism and realism
Even if we reject essentialism, the species presents
difficulties for an evolutionist. If a species gradually splits
into two descendant species, for example, there is no
moment in time at which one can say that we now have two
species instead of one. One way of coping with such
situations has been to adopt a philosophy called ‘nomin-
alism’, which views classification as something arbitrary or
conventional. It often takes the form of claims that species
are not ‘real’, and has been opposed to ‘realism’. The
traditional claim of nominalists was that individuals are
real, but classes are not, and that species are classes and
therefore not real. A third possibility, which is that species
are not classes, but individuals in the sense of supraindi-
vidualwholes (populations), has graduallywonacceptance
during the latter half of the twentieth century.According to
this view, species are like baseball teams, groups of
organisms, but not kinds of organisms. Here ‘individual’
is used in amore general sense than ‘organism’ andmeans a
concrete particular thing. It is a technical term in the
branch of philosophy called ‘ontology’, which deals with
such issues.
Species Concepts and Definitions
Scientists and philosophers have thus taken quite a variety
of positions about what it means when we say that
something is, or is not, a species. The philosophical
positions just mentioned have sometimes been called
essentialist, nominalist and individualistic species con-
cepts. When, however, we speak of species concepts we
generallymean somethingmore particular, for example the
biological species concept, which is just one example of an
individualistic concept. A conceptual analysis of the
biological species concept and its possible alternatives
allows us to understand the ongoing debates among
biologists and philosophers. What we mean by a concept
is obscure, and here wewill focus onwhat to all intents and
purposes is the same thing, the definition of a term.
In any such discussion it is crucial to make it clear both
what a species concept, or definition, is andwhat it is not.A
species definition is a definition of a general term, ‘species’.
Article Contents
Introductory article
. Introduction
. Species Concepts and Definitions
. The Biological Species Concept: Species as Populations
. Recognition, Cohesion and Evolutionary Species
Concepts
. Species as Kinds, or Classes
. Kinds of Species: Polymorphic, Polytypic and Cryptic
(Sibling)
. Monism versus Pluralism with Respect to Species
Concepts
1ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net
It definitely is not a definition ofHomo sapiens or anything
else that might be given as an example of the general term.
It is a definition of it, not of one of them. Some
misunderstanding has resulted from the fact that ‘species’
can be read as either singular or plural. The practice of
saying ‘the species’ when one means species in general
sometimes helps. However, modern taxonomy offers a
more satisfactory solution. It treats the general term as
referring to a level in a hierarchical system of classification.
These levels are called ‘categories’ and the groups that
occupy the levels are called ‘taxa’ (singular ‘taxon’). Thus
the genus is a category, and each and every group that has
that rank, such asHomo, is a taxon. Likewise, the species is
a category, and each and every species, such as Homo
sapiens, is a taxon. So when we want to make sure that our
meaning is clear, we say that a species definition is a
definition of the species category.
Because the species category is a class, we should be able
to define it in a straightforward manner, by listing the
properties that are necessary and sufficient for something
to be a species (the defining properties). Aswe shall see, this
is indeed possible, but by nomeans easy.Howone defines a
species taxon, however, is an entirely differentmatter. It all
depends on one’s definition of the species category, and
that in turn may depend upon one’s underlying philoso-
phical assumptions. If one believes that species taxa are,
like the species category, classes, then it should be possible
to define them in the sense of listing defining properties. If
one rejects that, however, and says that particular species
are individuals, then, just like individual organisms, they
have no defining properties. They can be described, but not
defined.Much confusion results because the term ‘define’ is
sometimes used in a loose sense, where to be precise we
really ought to say ‘describe’ or ‘diagnose’.
The Biological Species Concept:
Species as Populations
The biological species concept that is generally, if some-
times reluctantly, accepted by evolutionary biologists
treats species as reproductively isolated populations. Such
populations may consist of lesser groups that are also
populations, including local ones. A (biological) species is
reproductively isolated from all other species, and there-
fore it cannot interbreed with them. The component
populations of a species, however, have the capacity,
either directly or indirectly, to interbreedwith one another.
Such interbreeding is not a property of the component
organisms. It is the exchange of genetic material, or gene
flow, throughout the population as a whole. As a
consequence of interbreeding a species is a reproductive
community that is held together by sex.
The following formal definition of the species category is
that ofMayr:Groups of interbreeding natural populations
that are reproductively isolated from other such groups.
Such formal definitions are apt to be misleading,
especially since their meaning depends upon the definition
of other terms, and upon the theoretical context inwhich
they are used. For this reason some background was
provided before giving the quotation, but that is only a
beginning. The biological species concept is linked to the
theory of species formation, or speciation. When a species
speciates, it gives rise to populations that are considered
distinct species only when they have come to differ in away
that would keep them separate, even if they occupied the
same geographical area. It is generally accepted that under
ordinary circumstances, populations must be geographi-
cally isolated (allopatric) before such divergence can take
place to the point that they will remain distinct even when
they are in the same place (sympatric). The evolution of
such intrinsic isolatingmechanisms is thereby equatedwith
speciation, and serves as the basis for defining the species
category in terms of reproductive isolation. The biological
species concept does not, however, exclude the possibility
of sympatric speciation, i.e. without geographical isola-
tion.
However, the biological species concept has many
difficulties, some imagined, others real. Reproductive
isolation between entire species is often confused with the
inability of any two organisms within a species to produce
offspring. The definition covers populations under natural
conditions: artificial crosses produced in the laboratory are
irrelevant. Likewise a certain amountof hybridizationdoes
not contradict the definition, so long as the species remain
distinct and can evolve separately. A problem not so easily
dismissed arises because allopatric populations are con-
sidered parts of the same species so long as they are
potentially, not just actually, capable of interbreeding.
Again, isolation between populations refers only to a given
moment in time, or to the so-called ‘nondimensional’
situation. Changes that would isolate contemporary
populations can evolve within a single population over a
long series of generations, and some persons would like to
use such changes as a criterion for delimiting ‘chronos-
pecies’ in the fossil record. The biological species definition
provides no criterion for dividing up a single lineage into
so-called chronospecies.Nor does it tell uswhat happens to
the ancestral species when it splits into daughter species,
perhaps by budding off a small peripheral one. Does it, or
does it not, cease to exist? Furthermore, many organisms
are not parts of (sexually reproducing) populations. They
are asexual, and not just on a temporary basis, but
permanently asexual. This circumstance burdens taxono-
mists with a serious problemof what to dowith the asexual
lineages or clones. Nomenclature proceeds as if each and
every organism belongs to one and only one species;
evolutionary theory tells us that they belong to at most one
species. There are also many practical problems. It is very
Species Concepts
2
difficult to study populations when all one has is a single
museum specimen. Palaeontologists likewise often must
deal with quite fragmentary fossil remains.
Recognition, Cohesion and
Evolutionary Species Concepts
Some alternatives to the biological species concept are
merely variations on the same populational theme. Let us
consider someof these before proceeding tomorphological
and other concepts that treat species as classes of similar
organisms rather than as populations.
Recognition concept
The recognition concept emphasizes what holds species
together, in place of what keeps themapart. The organisms
are supposed to share a ‘specific mate recognition system’,
or SMRS, which enables them to recognize conspecifics of
the appropriate sex. According to this concept, species are
reproductive populations, and on the whole they are
equivalent to biological species. However, there are two
important differences. First, a population, in principle at
least, might vary geographically in how mates are
recognized. Second, a single lineage might evolve a
different SMRS. Would it then be a different species?
Cohesion concept
The cohesion concept emphasizes the point that something
does in fact hold species together. This provision of
cohesion is precisely the role of sex in the biological species
concept. It has been suggested that something other than
sex might play this role. However, no convincing evidence
has yet been provided that such a thing actually exists.
Evolutionary concept
The evolutionary species concept emphasizes the point that
species are not just populations as they exist at any given
instant in time: they are also lineages, and the fact that they
evolve is very important. Thus evolutionary species are
basically the same things as biological species, but
considered from a somewhat different point of view.
Cladistic species concepts
Cladistic species concepts have been advocated by those
who reject the biological species as the appropriate unit for
the study of relationships among lineages. There is no
agreement as to what this unit should be, but in general it is
a part of a biological species. Advocates of this concept
argue that what others call subspecies or local populations
should have their taxonomic rank elevated to that of full-
blown species. This move would havemany serious effects.
The number of species names would be vastly increased.
Also, human beings would not all belong to the same
species.
Species as Kinds, or Classes
Folk taxonomy and common sense suggest that species
taxa are kinds of organisms. However, the biological
species concept does not treat them as kinds of organisms
or for that matter as kinds of anything. Rather they are
wholes, with organisms as parts. In this respect they are
very much like organizations. A university is made up of
departments and professors, but it is not a kind of
department or a kind of professor. Likewise when we say
that somebody is a humanbeing, we donotmean that he or
she is ‘a Homo sapiens’ but rather that he or she is an
organism-level component of Homo sapiens. This may
seem to contradict common usage, but it really does not.
Most people would consider a human sperm cell a part of
our species, even though it is not a human being.
Some species concepts do treat species as kinds. It
deserves emphasis that these concepts treat species taxa,
andnot just the species category, as kinds.According to the
biological species concept the species category is akind, but
the taxa are not kinds. If species taxa are kinds, then the
species category is a kind of kind. We shall now explain
what these concepts are and consider their merits.
Species as natural, or objective kinds
The physical sciences, such as chemistry, generally classify
material objects upon the basis of properties that
supposedly reflect the laws of nature that govern such
objects. The periodic table of the elements, for example,
consists of groups such as the halogens that encompass
lesser groups, such as chlorine and bromine. Chlorine has
the properties that it does because the laws of nature
necessitate that whenever matter assumes a given config-
uration those properties simply must be present. Such
kinds are very important in chemical theory, for the laws of
nature allow us to predict the properties of molecules
whenever andwherever they occur, irrespective of time and
place. And in principle at least it should be possible to
define such kinds in terms of those properties: e.g. by
specifying the atomic number of each element.
The species category may likewise be interpreted as such
a natural kind. The biological species concept picks out a
kind of population about which laws of nature may be
formulated, at least in principle. These might include laws
for speciation, for example. Likewise a good case can be
made out for the existence of natural kinds that are in fact
kinds of organisms, for example autotrophs and hetero-
Species Concepts
3
trophs, for which laws of nature may be enunciated.
Biological species, and likewise the genealogical units
(clades) that arise as a consequence of their speciating, owe
their properties to their history,rather than to laws of
nature. Just as astronomy has laws about a natural kind
called ‘planet’ but none for the planet Saturn, evolutionary
biology has laws about a kind called the species category,
but none for the species taxon calledHomo sapiens. And in
general it is easy to see how a particular planet, such as
Saturn, or a particular species, such asHomo sapiens, could
originate or evolve; butwe are hard pressed to imagine how
the planet or the species, taken in the abstract, could do
anything whatsoever.
Some authors, however, have challenged this view, and
maintained that such laws of nature do, or must, exist. On
the one hand such claims have not yet been supported by a
single example of any such law. On the other hand, in
evolutionary biology as it currently exists, species taxa do
not function as natural kinds. They function as historical
units.
Species as subjective kinds: phenetic and
morphological species concepts
Natural kinds may be said to be objective classes, in the
sense that their definition is based upon laws of nature,
laws that exist apart from anybody’s opinion or personal
judgement.However, not all kinds, or classes, have such an
objective character, and this is the main objection to
various species concepts that treat taxa as classes of similar
objects. The notion of similarity is notoriously vague. Of
course natural kinds are classes of similar things, in the
sense that all chlorine atoms have the same valence and
atomic number. However, what we consider similar or
dissimilar may, like the beauty that lies in the eye of the
beholder, be something that tells us about ourselves rather
than themembers of the class.And this is amajor objection
to species concepts that define the category in terms of
degree of similarity. The subjectivity of such species
concepts is particularly evident in the pragmatic, or
practical, concept of Cronquist: ‘Species are the smallest
groups that are consistently and persistently distinct, and
distinguishable by ordinary means.’ What are ordinary
means for one person are extraordinarymeans for another,
and the use, say, of scanning electronmicroscopy has gone
from extraordinary to ordinary in a very short period of
time.
The similarity may be overall similarity, morphological,
physiological, genetical or other. The phenetic species
concept is based upon the idea of overall similarity thatwas
championed by the numerical pheneticist school of
taxonomy that flourished in the 1960s. The idea was that
a large sample of attributes, carefully assembled and
processed by the appropriate statistical techniques, would
give clusters that could be treated as taxa. To get a species
definition, however, one would have to make an arbitrary
and subjective decision as to howmuch the organismswere
to differ to justify calling them species.
The morphological species concept treats species as
classes of organisms that are similar in terms of their
anatomy and other so-called morphological properties. It
is an older and narrower form of phenetic species concept,
and remains popular, especially where the biological
species concept is hard to apply.
The genetical species concept treats species as classes of
organisms with similar genes.
The physiological species concept treats species as kinds
of organisms that share the physiological capacity to mate
and produce fertile offspring. It is important not to confuse
this physiological species concept with the biological
species concept. Rather it is an effort to treat the sterility
test that does provide some evidence of whether two
organisms belong to the same species as if it gave a defining
property. According to the biological species concept,
some of the organisms within the same species can be
unable to cross with one another, provided that the
population as a whole has at least the potential for gene
flow.What are called ‘ring species’ are a case in point. Two
populations that donot have the capacity for interbreeding
directly with one another in an area of sympatry may
nonetheless be connected by intermediate populations that
do have that capacity, and the species is thereby united into
a single reproductive community and evolutionary unit.
Kinds of Species: Polymorphic,
Polytypic and Cryptic (Sibling)
Even those who attempt to apply a phenetic or morpho-
logical species concept generally put both males and
females that mate and produce fertile offspring in the same
species, no matter how dissimilar they may seem. ‘Poly-
morphism’ means having many forms, and this term is
often used in the sense of genetic polymorphism, but it also
is sometimes used to refer to morphs within a species. It
alsomay refer to phenotypic variants, some of which differ
remarkably. Sexual dimorphism is moderate within our
own species, but in some it can reach an extreme degree, as
in the tinymales of themarine echiuranwormBonellia that
live commensally on the much larger females. There are
also mating types, such as those we find in plants with
dimorphic and trimorphic flowers. There may also be
extreme differences between asexual and sexual genera-
tions of bothplants andanimals, andbetweenhaploids and
diploids. Another example is morphs within a single insect
species that mimic different models.
A polytypic species is one that exists as discrete
populations that are somewhat divergent from one
another. Advocates of the biological species concept treat
these as single species, on the basis of their retained
Species Concepts
4
capacity to interbreed. Not everybody accepts this move,
preferring to treat such parts of biological species as
species. This difference of opinion as to how to define the
species category is sometimes confusedwith the questionof
whether the biological species concept applies to such
cases. Obviously if speciation occurs gradually there will
inevitably be transitional periods during which the
distinction between species and incipient species is
arbitrary at best.
Cryptic or sibling species are biological species that are
hard for us human beings, including the experts, to tell
apart. The organisms themselves do not necessarily
experience any such difficulty. Both ‘cryptic’ and ‘sibling’
are somewhat misleading terms. To call them ‘cryptic
species’ suggests that the organisms, rather than the species
are cryptic (hidden). To call them ‘sibling species’ implies
that they are one another’s closest relatives, something that
may or may not be the case. Cryptic species are often
detected by careful study of the organisms’ reproductive
biology or by the use of molecular techniques. Application
of a morphological species concept would tend to keep
such organisms together. The effort that may be necessary
to separate them is sometimes given as a reason for not
using the biological species concept. Supposedly it is not
practical. Onemight argue on the other side, however, that
it may be well worth the effort in dealing with certain kinds
of practical problems. The example usually given is the
control of mosquitoes that serve as vectors for parasites.
Obviously here, as elsewhere, value judgements enter into
the controversies,making it harder to follow the arguments
or pass sound judgement in such matters.
Monism versus Pluralism with Respect
to Species Concepts
In discussing species concepts we have come to grips with
one of the most controversial, indeed downright conten-
tious, topics in biology. Efforts to reach some kind of
resolution or consensus have not been altogether success-
ful. Recently two distinct ways of dealing with the problem
have been recognized as important enough to deserve
names: ‘monism’ and ‘pluralism’. A monist maintains that
there should be one, and only one, definition of the word
‘species’. A pluralist advocates having that term mean at
least two, but perhaps quite a variety of things, depending
upon the circumstances. Such an effort at solution is
anything but new. Advocates of ‘cluster concepts’ and
‘disjunctive definitions’ suggested that species might be
quite a variety ofthings, so that for example both
biological species and asexual complexes would qualify
as species. One version of pluralism suggests that wemight
choose any of a variety of species concepts depending upon
what happens to interest us in a given problem-solving
situation. Another version that pluralism might take is for
different species concepts to be applied by those who work
on different groups of organisms.
Pluralism is sometimes advocated as part of a liberal or
tolerant point of view. At the risk of seeming dogmatic or
intolerant, let me briefly sketch the basic arguments for
monism. First, we have the problems that result from the
use of ambiguous (equivocal) terms. If ‘species’ sometimes
means an entire genetic population, sometimes a small
subunit of such a population, then what the word means
when it occurs in a sentence may be pure guesswork.
Second, and perhaps more important, is that the laws of
nature refer to natural kinds. Although laws of nature
apply to particular things and historical events, the
theoretical generalizations themselves are about classes
of objects that have something of causal importance in
common. Different kinds of objects play different roles in
biological theory. Only if such classes are rigorously
defined is it possible to make theoretically significant
generalizations about them. Species diversity, for example,
can only be meaningfully compared from group to group
or place to place if ‘species’ means the same thing for each
and every comparison that is made. Species are of
fundamental theoretical importance because it is they that
speciate. Of course other kinds of things also play
important roles in evolution. But asexual lineages and
population-level subunits do not do the same things that
biological species do. One can have a pluralistic theory but
a monistic terminology by using different terms for the
different classes of object. In other words, whatever the
benefits of pluralism may be, we need not pay for them by
calling such a heterogeneous class of things species.
Further Reading
Claridge MF, Dawah HA and Wilson MR (1997) Species: The Units of
Biodiversity. London: Chapman & Hall.
EreshefskyM (ed.) (1992)TheUnits of Evolution: Essays on theNature of
Species. Cambridge, MA: MIT Press.
GhiselinMT (1997)Metaphysics and the Origin of Species. Albany, NY:
State University of New York Press.
Hull DL (1988) Science as a Process. Chicago: University of Chicago
Press.
Mayr E (1942) Systematics and the Origin of Species. New York:
Columbia University Press.
Mayr E (1963)Animal Species and Evolution. Cambridge, MA: Harvard
University Press.
Minelli A (1993) Biological Systematics: the State of the Art. London:
Chapman & Hall.
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