Forthcoming in Routledge Encyclopedia of Philosophy
Natural Kinds in Biology
Marc Ereshefsky
University of Calgary
ereshefs@ucalgary.ca
It is commonly held that objects in the world form natural kinds. Rabbits form a natural kind
and so do all pieces of gold. The traditional account of natural kinds asserts that the members of
a kind share a common essence. The essence of gold, for example, is its unique atomic structure.
That structure occurs in all and only pieces of gold, and it is a property that all pieces of gold
must have.
Species are seen as paradigmatic natural kinds in biology. However, that assumption poses a
potential problem for the traditional account of natural kinds. Many biologists and philosophers
argue that species lack essences. They maintain that there is no biological trait that must occur in
all and only the members of a species. If species are natural kinds and lack essences, then
species are a counterexample to the view that natural kinds have essences.
Philosophers have responded to this problem in several ways. Some defend traditional
essentialism. Others offer accounts of biological kinds that depart from traditional essentialism.
Still others suggest that there are different types of natural kinds in biology: some biological
kinds have essences, while other biological kinds lack them.
The issue of natural kinds in biology is important because such kinds serve as a basis for
prediction and explanation in science. Furthermore, biological kinds are taken to be real
categories in nature. For these reasons, the topic of natural kinds in biology is of special interest
to the philosophy of science and metaphysics.
1 Essentialism
The traditional account of natural kinds in biology is essentialism. Essentialism is not limited to
biological kinds but is a general theory of natural kinds held by many philosophers (see
NATURAL KINDS §3). Essentialism concerning biological kinds goes back to Aristotle and
Locke (see Ayers 1981) and is found in the work of such late 20th Century philosophers as
Kripke (1972) and Putnam (1975).
Essentialism concerning natural kinds has three main tenets. First, all and only the members
of a kind share a common essence. Second, that essence is a property, or a set of properties, that
all the members of a kind must have. And third, a kind’s essence causes the other properties
associated with that kind. The essence of the natural kind gold, for example, is gold’s atomic
structure. That atomic structure occurs in all and only pieces of gold. Furthermore, that
structure is a property that all gold must have, as opposed to, for example, the accidental
property of being valuable to humans. Finally, the atomic structure of gold causes pieces of gold
to have the properties associated with that kind, such as dissolving in certain acids and
conducting electricity. Knowing the essence of a kind has practical benefits: it allows us to
predict and explain the properties associated with the members of a kind. For instance, the
atomic structure of gold provides the basis for explaining why gold conducts electricity, and it
allows us to predict that a particular chunk of gold will conduct electricity.
The above example is from chemistry. What are the comparable essences for natural kinds in
biology? Species are thought to be paradigmatic examples of biological kinds (see SPECIES).
Many philosophers hold that the essence of a species is found in the species-specific
microstructure of its members. Putnam (1975), for example, speculates that the essence of
lemons is the unique genetic code of lemons. Despite such speculation, biologists have been
hard-pressed to find the essence of any particular species. Biological theory provides ample
reason why the essences of species are not forthcoming (Hull 1965). For a biological trait to be
the essence of a species, that trait must occur in all and only the members of that species.
However, a number of biological forces work against the uniqueness and universality of a trait in
a species. Suppose a trait were found in all the members of a species. The forces of mutation,
recombination, and random drift can cause the disappearance of that trait in a future member of
the species. Suppose, nevertheless, that a trait occurs in all the members of a species. That trait
is the essence of a species only if it is unique to that species. Yet organisms in different species
often have common traits because they inherit similar genes and developmental resources from
their common ancestors. Given the stringent requirements of essentialism and the confounding
forces of evolution, many philosophers and biologists reject the view that species have essences.
The philosophical literature contains three responses to the observation that species lack
essences. One response denies that species are natural kinds and looks elsewhere in biology for
kinds with essences (Ghiselin 1974, Hull 1978). A second response argues that species are
kinds, but that kinds do not require essences (Boyd 1999). A third response claims that species
are indeed kinds with essences, but their essences are of a non-traditional variety (Griffiths 1999,
Okasha 2002). Let us look at each of these responses in turn.
2 Species as individuals
Ghiselin (1974) and Hull (1978) suggest that the lack of essences among species does not show
that there are no essentialist kinds in biology. Species, they argue, are not natural kinds in
biology but individuals. The problem, then, is not with essentialism but with the assumption that
species are natural kinds. To understand Ghiselin and Hull’s answer we must first understand
their distinction between natural kinds and individuals. They draw this distinction between kinds
and individuals along the lines of spatiotemporal restrictedness. Kinds are spatiotemporally
unrestricted entities. A chunk of metal belongs to the kind copper so long as it has the essential
atomic structure of copper, regardless of its location in space and time. Individuals, on the other
hand, are spatiotemporally restricted entities. The parts of an individual must be located in a
restricted space-time region to be parts of that individual. The parts of an individual coin, for
example, cannot be scattered across the universe but must be connected to one another.
Membership of a kind has no such spatiotemporal restriction.
Ghiselin and Hull argue that species are not natural kinds but individuals because species are
spatiotemporally restricted entities. Their argument turns on the assumption that species are
genealogical entities. All the organisms in a species are connected by heredity relations – they
are offspring of conspecifics, or parents of conspecifics, or both. Such relations require that
parent and offspring, or their appropriate parts, be connected in space and time. Though not
every member of a species must come in contact with every other member, they all must be
connected to the same genealogical nexus. The members of a species, in other words, must form
a spatiotemporally continuous and hence spatiotemporally restricted entity. Consequently,
species are individuals and not natural kinds.
If species are not natural kinds in biology, then they are the wrong examples for testing the
theory that biological kinds have essences. Nevertheless, according to Hull and Ghiselin, other
classes of biological entities may be natural kinds. For example, the sexes (male and female) and
the class of all predators may be natural kinds. Certain genetic features, such as the Hox gene, or
body parts, such as the eye, may be natural kinds as well. Whether or not such classes of
biological entities are essentialist natural kinds is not settled.
3 Homeostatic property cluster theory
Another approach to natural kinds in biology is Homeostatic Property Cluster Theory (Boyd
1999). Homeostatic Property Cluster Theory (HPC Theory) drops the essentialist requirement
that all and only the members of a natural kind must have a kind-specific essence. By dropping
that requirement, HPC Theory allows species to be natural kinds.
According to HPC Theory, the members of a natural kind share a cluster of similar properties,
but no property is necessary for membership in a natural kind. Such properties must nevertheless
be stable enough to allow for successful induction. That is, they must be stable enough to allow
us to predict with better than chance probability that a member of a natural kind will have certain
properties. For example, the members of the kind Canis familiaris share many similar
properties, so that if we know that an organism is a dog we can predict with better than chance
probability that it will have four legs. According to HPC theory, the co-occurrence of properties
among the members of a natural kind is caused by the homeostatic mechanisms found among the
members of a kind. For example, the members of Canis familiaris are exposed to such
homeostatic mechanisms as interbreeding, shared ancestry, and common developmental
constraints. Those mechanisms cause members of Canis familiaris to share a number of similar
traits.
HPC Theory is a more inclusive account of natural kinds than essentialism. It allows any class
of biological entities to be a natural kind so long as the members of such classes share a number
of co-occurring properties caused by a cluster of homeostatic mechanisms. For HPC Theory, not
only are species natural kinds, so are many other classes of biological entities. For instance, the
class of all males is a natural kind so long as males have significant similarities caused by a
common set of homeostatic mechanisms. Though HPC Theory drops the essentialist search for
essences, it retains the essentialist goal of highlighting classes of entities that reflect the causal
structure of the world.
4 Historical essences
Some philosophers have argued that we should not reject essentialism when it comes to
biological kinds (Griffiths 1999, Okasha 2002). They suggest that species do have essences, but
not the type of essences described by traditional essentialism. Traditional essentialists believe
that the essence of a kind is a property, or set of properties, found within the members of a
natural kind. Such essences are intrinsic essences, such as the microstructure of lemons.
Griffiths and Okasha observe that in contemporary biology membership of a species turns on
being genealogically related to a particular ancestor. Accordingly, they suggest that the essence
of the kind Canis familiaris is being a descendant of that species’ ancestral population. This sort
of essence is an extrinsic or historical essence: the property of being a descendant of a particular
ancestor is a relational property between an organism and its ancestral population. Arguably,
knowing a species’ ancestry is useful for predicting and explaining the traits typically associated
with its members.
5 Summary
In summary, there are three major approaches to natural kinds in biology. The first is traditional
essentialism. A worry about traditional essentialism is that species lack essences, and thus
species are not kinds according to traditional essentialism. Many find this conclusion
unacceptable because species are seen as paradigmatic examples of kinds in biology.
Alternatively, one can adopt Homeostatic Property Cluster Theory as the appropriate account of
biological kinds. HPC theory does not require that kinds have essences and thus it allows that
species are kinds. Or, one might adopt the view that species have historical essences and
recognize that species are essentialist kinds, though historical essences are different from the
intrinsic essences of traditional essentialism.
A worry about the alternatives to traditional essentialism, namely HPC Theory and historical
essentialism, is that they conflate two different types of approaches to biological classification.
The members of a species are classified by their historical relations to one another, whereas the
members of a traditional kind like gold are classified by their intrinsic similarities regardless of
their historical relations to one another. One might worry that treating both classes of entities,
those with intrinsic essences and those with historical essences, as natural kinds conflates two
different ways scientists construct classifications (Ereshefsky and Matthen 2005). Others argue
that such a worry is unfounded, because there are different types of natural kinds in biology
(Hacking 1991, Dupré 2002).
References and Further Reading
Ayers, M. (1981) ‘Locke versus Aristotle on Natural Kinds’, The Journal of Philosophy
78 (5): 247-272. (An introduction to Aristotle’s and Locke’s theories of
natural kinds.)
Boyd, R. (1999) ‘Homeostasis, species and higher taxa’, in R. Wilson (ed.) Species: new
interdisciplinary essays, Cambridge, MA: MIT Press, 141-185. (A discussion of the Homeostatic
Property Cluster Theory of natural kinds and its application to biological kinds.)
Dupré, J. (2002) Humans and other Animals, Oxford: Oxford University Press. (A collection of
essays on natural kinds, biological kinds, and human kinds. Dupré argues that there are multiple
correct accounts of biological kinds.)
Ereshefsky, M. (2001) The Poverty of the Linnaean Hierarchy, Cambridge: Cambridge
University Press. (An introduction to philosophical problems in biological taxonomy. This book
also contains an introduction to theories of biological kinds.)
Ereshefsky, M., Matthen, M. (2005) ‘Taxonomy, polymorphism, and history: an introduction to
population structure theory’, Philosophy of Science 72: 1-21. (A discussion of biological kinds
from the perspective of evolutionary biology. This article also criticizes the Homeostatic
Property Cluster Theory of biological kinds.)
Ghiselin, M. (1974) ‘A radical solution to the species problem’, Systematic Zoology 23: 536-544.
(An influential article suggesting that species are not natural kinds but individuals.)
Griffiths, P. (1999) ‘Squaring the Circle: Natural Kinds with Historical Essences’ in R. Wilson
(ed.) Species: new interdisciplinary essays, Cambridge, MA: MIT Press, 209-228. (Griffiths
develops the thesis that species are biological kinds with historical essences.)
Hacking, I. (1991) ‘A Tradition of Natural Kinds’, Philosophical Studies 61: 109-126. (A survey
of philosophical theories of natural kinds. Hacking argues that there are different types of
biological kinds in the world.)
Hull, D. (1965) ‘The effect of essentialism on taxonomy: two thousand years of stasis’, British
Journal for the Philosophy of Science. 15: 314-326, 16: 1-18. (An influential article suggesting
that species lack essences. Hull also surveys various philosophical approaches to biological
taxonomy.)
Hull, D. (1978.) ‘A matter of individuality’, Philosophy of Science 45: 335-360. (An influential
article suggesting that species are not natural kinds but individuals. Hull also discusses the
implications of species being individuals for both biology and philosophy.)
Kripke, S. (1972) ‘Naming and necessity’, in D. Davidson and G. Harmon (eds.) Semantics of
Natural Language, Dordrecht: Reidel, 253-355. (An influential article outlining a modern theory
of essentialism.)
Okasha, S. (2002) ‘Darwinian Metaphysics: Species and the Question of Essentialism’, Synthese
131: 191-213. (This article responds to the view that species lack essences by suggesting that
they have historical essences.)
Putnam, H. (1975) Mind Language and Reality; Philosophical Papers, volume 2, Cambridge:
Cambridge University Press. (A collection of essays containing Putnam’s essentialist approach to
natural kinds; see chapters 8 and 10-12.)
Sober, E. (1980) ‘Evolution, population thinking and essentialism’, Philosophy of Science 47:
350-383. (An influential discussion of evolutionary theory and philosophical theories of
biological kinds; discusses the role of essentialism in the history of biology.)