Species and Taxonomy
Species play two central roles in biology. Species are units of classification in biology:
organisms are sorted into species, and species are sorted into genera. We belong to the species
Homo sapiens and that species belongs to the genus Homo. Species are also units of evolution.
They are the groups of organisms that evolve as a unit. Species are so important in biology that
Darwin named his book, The Origin of Species, after them. One of the challenges of that book
was to discover how species come into existence. Darwin was successful in arguing for the
existence of natural selection and evolution, but his account of species remains controversial.
Even before Darwin’s time, biologists disagreed on the correct definition of ‘species.’ In
fact, biologists still disagree on that definition. Over a dozen prominent definitions of ‘species’
can be found in contemporary biology. Controversy over the nature of species is philosophical
as well. One philosophical question is whether biologists should try to discover the correct
definition of ‘species.’ Given that biologists have spent centuries seeking the correct definition
of ‘species’ and have not found it, perhaps there is no single correct definition. Maybe there are
multiple correct definitions of ‘species.’ This raises a fundamental question about taxonomy:
should scientists aim for the right definition of a theoretical term such as ‘species’ or should they
allow multiple correct definitions? Another philosophical controversy concerning species is
their ontological status. Are they natural kinds with essences, akin to elements on the periodic
table? If so, then there is an intrinsic property, perhaps some DNA, which distinguishes the
organisms of a species from all other organisms. Alternatively, maybe species are individuals
akin to particular objects. If that is the case, then membership in a species turns on being a part
of a species’ genealogical lineage rather than having a particular essence.
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Clarifying the biological and philosophical nature of species is important. Species play a
central role in environmental laws and biological preservation. If we want to preserve species,
then we need to know what they are. Our understanding of species also affects our theories of
human nature. Humans are, among other things, the species Homo sapiens. If species have
essences, then humans have an essential biological nature. If species do not have essences, then
there is no biological essence to being a human. This chapter discusses these and other issues,
starting with the ontological status of species and then turning to the definition of ‘species.’
Species and Essentialism
Philosophers generally believe that species are natural kinds with essences. Such philosophers as
Saul Kripke and Hilary Putnam hold this view, and it has its roots in the works of Aristotle and
John Locke. The idea that species are kinds with essences is part of a more general view called
‘kind essentialism.’ Kind essentialism has two major tenets: first, all and only the members of a
kind share a kind-specific essence; second, a kind’s essence causes the properties typically found
among the members of that kind. The essence of the natural kind gold, for example, is gold’s
atom structure. That structure occurs in all and only chunks of gold. Furthermore, that structure
causes the properties associated with gold, such as melting at certain temperatures and
conducting electricity.
Are species kinds with essences? Philosophers tend to think they are, but work in the
philosophy of biology suggests otherwise. Consider the first tenet of kind essentialism, that all
and only the members of a kind share a common essence. Biologists have been hard-pressed to
find a biological trait that occurs in all and only the members of a particular species.
Evolutionary biology explains why (Hull 1965). Suppose a trait occurs in all the members of a
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species. Mutation can eliminate that trait in an organism in the next generation of a species. All
it takes is the elimination of a trait in one member of a species to show that it is not essential for
that species. Recombination can have the same effect. Recombination reshuffles DNA such that
a trait universal in one generation is not universal in the next.
Evolutionary forces also undermine the uniqueness of a trait within a species. Often
different species live in similar habitats, and that causes the parallel evolution of similar traits.
Birds and bats, for example, both have wings even though those types of wings have different
evolutionary origins. Organisms in different species also share common ancestors.
Consequently, they draw on common genetic material and developmental resources that cause
the members of different species to have similar traits. Consider the multitude of organisms with
four limbs. Just as evolutionary forces work against the universality of a trait in a species, they
also work against the uniqueness of a trait in a species. Evolutionary forces, in other words,
work against the existence of species essences.
Sober (1980) offers a different objection to species essentialism. Sober argues that
evolutionary explanations have replaced essentialist ones in biology. Suppose we want to
explain variation in a population, say variation in height. The essentialist explanation cites the
essence of the organisms in a species, and then cites ontogenetic interference that prevents the
occurrence of that essence in all the organisms of a species. For the essentialist, height variation
is due to a species’ essence and interference with that essence. In contrast, the evolutionist
explains variation in a population without positing essences. Evolutionists cite the gene
frequencies within a population and the evolutionary forces that affect those frequencies.
Variation in height is due to the gene frequencies within a species plus the evolutionary forces
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that affect those frequencies, such as selection and random drift. According to Sober, the
positing of species essences has become theoretically superfluous in biology.
Species as Individuals
If species are not kinds with essences, what are they? Hull (1978) and Ghiselin (1974) suggest
that species are individuals. Hull draws the contrast between kinds and individuals as follows.
Membership in a kind requires that the members of a kind share a common essential property.
For example, a drop of liquid is water so long as it has the molecular structure H2O. It does not
matter where that drop of liquid is located, whether it is on Earth now or in a distant galaxy in a
million years. So long as that drop has the molecular structure H2O it is water. Individuals,
unlike kinds, consist of parts that must exist in a spatiotemporally restricted area. Consider a
paradigmatic individual, an elephant. The parts of an elephant cannot be scattered across the
universe at different times if they are parts of a single living organism. Various biological
processes, such as respiration and digestion, require those parts to be spatiotemporally
connected. Generalizing from these examples, the parts of an individual are spatiotemporally
restricted, whereas the members of a kind are spatiotemporally unrestricted.
Given this distinction between kinds and individuals, why does Hull think that species are
individuals? His argument starts with the assumption that ‘species’ is theoretical term in
evolutionary biology. Hull argues that species are units of evolution in evolutionary biology,
meaning that species are groups of organisms that evolve as a unit. Natural selection is the
primary force that causes species to evolve. One way that selection causes a species to evolve is
by causing a rare trait to become prominent within a species. For such evolution to occur, a trait
must be passed down through the generations of a species. That requires the organisms of a
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species to be connected by reproductive relations: namely, sexual relations between parents, and
parent-offspring relations between parents and offspring. Such relations require organisms, or
their parts (gametes and DNA), to come into contact. In sum, evolution by selection requires the
generations of a species to be spatiotemporally connected. The organisms of a species cannot be
scattered throughout the universe, but must occupy a particular space-time region. Given that
species are units of evolution, they are individuals and not kinds.
The thesis that species are individuals has a number of implications (Hull 1978). One
implication is that the relation between an organism and its species is not a member-class
relation but a part-whole relation. An organism belongs to a particular species only if it is
connected to other organisms in that species’ lineage. If belonging to a species turns on an
organism's being part of a particular lineage, then similarity can be misleading. Two organisms
may be very similar morphologically, genetically, and behaviorally, but unless they belong to the
same lineage they cannot belong to the same species. Consider an analogy. Being part of my
immediate family turns on my wife, my children, and I having certain relations to one another,
not our having similar features. My son's friend might look just like him. Nevertheless, that does
not make him part of our family. By analogy, organisms belong to a particular species because
they are appropriately connected, not because they look similar.
Another implication of the species are individuals thesis concerns our conception of
human nature (Hull 1978). Humans may be a number of things, but one of them is being the
species Homo sapiens. If species are individuals, then there is no biological essence to being a
human. Humans are foremost parts of the genealogical lineage Homo sapiens. Being part of that
lineage does not require that all and only humans have a particular property. Having a certain
cognitive ability, social ability, even being able to communicate with language is not required for
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being a human. Being part of a particular genealogical lineage is all that matters. Traditional
accounts of human nature require that all humans have a distinctive quality. If species are
individuals, then such accounts of human nature lack a biological basis.
The New Biological Essentialism
The debate over the ontological status of species does not end with the claim that species are
individuals. Some philosophers have argued that species are kinds with essences, but not kinds
according to traditional essentialism (Boyd 1999, Okasha 2002, Devitt 2008). Boyd’s
Homeostatic Property Cluster Theory (HPC Theory) is the most prominent form of the new
biological essentialism, so we will focus on it. HPC Theory has two main tenets. First, members
of a kind share a cluster of similar properties. None of those properties is necessary for
membership in a kind, but those properties must 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 kind will have certain properties. The members of the kind Canis
familaris share many similar properties such that we can predict that if an organism is a dog it
will probably have certain properties. Second, HPC Theory requires that the co-occurrence of
properties among the members of a kind be due to that kind’s homeostatic mechanisms. For
species, such processes as interbreeding, shared ancestry, and common developmental
constraints are homeostatic mechanisms that cause the typical properties of a species. Dogs
typically have four legs, two ears, and one head, and they tend to have those properties because
of their species’ homeostatic mechanisms.
HPC Theory provides a better account of species as kinds than traditional essentialism.
Unlike traditional essentialism, it does not require that every member of species have a particular
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biological property. HPC Theory merely requires that the members of a species share a cluster
of co-occurring properties sustained by underlying causal mechanisms. Though HPC Theory is
an improvement over traditional essentialism, it is controversial whether HPC Theory offers an
adequate account of species. Some argue that it does not. Ereshefsky and Matthen (2005)
contend that while HPC Theory focuses on the similarities among the members of a species, it
neglects the existence of persistent differences within a species. Polymorphism, variation within
a species, is an important feature of nearly every species. The males and females of a species
can vary dramatically. In addition, the members of a species can vary in their life stages, as
exemplified by the caterpillar and butterfly stages of a single organism. Stable polymorphism is
a feature of nearly every species. HPC theory focuses on explaining similarities among the
members of a species and finding homeostatic mechanisms that cause those similarities. Yet a
proper account of species also needs to explain persistent variation within a species and find the
heterostatic mechanisms that cause that variation. Consequently, HPC theory’s account of
species as natural kinds leaves out a significant feature of species, polymorphism.
Stepping back from these details, the debate over the ontological status of species is far
from settled. Some reject the idea that species are individuals (Kitcher 1984, Devitt 2008).
Others reject Boyd’s HPC Theory and other forms of the new biological essentialism
(Ereshefsky 2010). Regardless of how this debate is resolved, all parties in this debate agree that
species are not kinds with traditional essences.
Taxonomic Pluralism
A common assumption in biology and philosophy is that one true classification of the organic
world exists. That is, if we had a god’s eye perspective, we would see that each organism
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belongs to a particular species. This view, called ‘monism,’ also assumes that there is one
correct definition of ‘species.’ In contrast, pluralism is the view that there are multiple correct
definitions of ‘species’ (Kitcher 1984, Dupré 1993, Ereshefsky 2001). According to pluralists,
there are different kinds of species and different but equally legitimate classifications of the
organic world.
What is the argument for taxonomic pluralism? It begins with the observation that
biologists provide different definitions of the term ‘species.’ The most prominent definitions fall
into three categories: interbreeding, ecological, and phylogenetic. According to the interbreeding
approach, species are groups of organisms that can interbreed and produce fertile offspring.
Interbreeding species are distinct gene pools, maintained by sexual reproduction. According to
the ecological approach, a species is a lineage of organisms that live in a particular ecological
niche. The selection forces in a species’ niche cause a lineage to be a distinct species. The
interbreeding and ecological approaches come from evolutionary biology; the phylogenetic
approach is from the school of taxonomy called ‘Cladism.’ According to Cladism, species are
lineages of organisms that share a common and unique ancestry.
These approaches to species (interbreeding, ecological, and phylogenetic) assume that
species are genealogical lineages. Nevertheless, these approaches highlight different types of
lineages. Some species are lineages of organisms that interbreed. Other species are lineages of
organisms that share a common ecological niche. Still other species are lineages of organisms
with unique ancestors. These approaches to species pick out different types of lineages on the
Tree of Life. Consequently, these approaches provide different classifications of the organic
world. Some monists respond that this situation is due to our lack of knowledge and is merely
temporary (Sober 1984). They suggest that one of the definitions of ‘species’ discussed, or one
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to be discovered, is the correct approach to species. Once biologists have settled on that correct
definition, we will have a single classification of the world’s organisms. However, species
pluralists maintain that the case for pluralism is not our lack of information about the organic
world. Pluralists maintain that we have substantial information from biology that the Tree of
Life contains different types of species.
Monists offer various objections to taxonomic pluralism. Hull (1987) and Ghiselin
(1987) worry that pluralism is an overly liberal approach to science. They ask how pluralists
determine which definitions of ‘species,’ among suggested definitions, are legitimate. If
pluralism offers no criteria for discerning among species definitions, then pluralism boils down
to a position of ‘anything goes.’ Dupré (1993) and Ereshefsky (2001) respond to this objection
by offering criteria for accepting proposed definitions of ‘species.’ Those criteria include
standard criteria that scientists and philosophers use for judging theories, such as empirical
testability, internal consistency, and inter-theoretic consistency. Pluralists argue that they do not
subscribe to a position of ‘anything goes’ because they offer standards for evaluating definitions
of ‘species.’
Advances in molecular sequencing inspire another monist response to pluralism. Perhaps
the correct definition of ‘species’ should be based on genetic similarity. With molecular
sequencing, we may discover the distinctive genome of each species. We can then use that
information to construct a single classification of the organic world. However, the use of
molecular data may not result in a single correct classification. Molecular data may merely
provide additional classifications of the organic world. Ferguson (2002) offers examples where
overall genetic similarity and the ability to interbreed do not coincide. The result is two different
classifications: one that sorts organisms according to interbreeding, and another classification
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according to overall genetic similarity. Add to these classifications a third classification based
on ecological adaptedness. Wu and Ting (2004) cite cases where a classification based on genes
for ecological adaptedness fails to coincide with a classification based on overall genetic
similarity. Moreover, neither of these classifications coincides with a classification based on
interbreeding. Bringing molecular data to the table may not reduce the number of classifications
but instead increase their number.
Darwin and the Species Category
Even though the debate between monists and pluralists is not resolved, the possibility of
taxonomic pluralism raises pressing questions concerning our use of ‘species.’ If pluralism is
correct, how should the word ‘species’ be defined? Before answering that question, consider a
useful distinction: the difference between species taxa and the species category. Species taxa are
groups of organisms. Homo sapiens and Canis familiaris are examples of species taxa. The
species category is a more inclusive entity. It contains all species taxa. The species category is
one of the categories in the Linnaean hierarchy. Other Linnaean categories include genus,
family, and order. With the species taxa-category distinction in hand, we can more clearly see
what pluralists are affirming. Pluralists do not deny that species taxa exist. In other words,
pluralists do not deny the existence of Homo sapiens and Canis familiaris. Whether or not a
pluralist denies the existence of the species category depends on the pluralist. Some pluralists
(Kitcher 1984, Dupré 1993) believe that the term ‘species’ refers to different types of groups of
organisms (for example, interbreeding lineages and phylogenetic lineages), but they do not deny
the existence of the species category. That category is just a heterogeneous category of different
types of groups of organisms. Other pluralists (Ereshefsky 2001) believe that the heterogeneity
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of the species category implies that there is no species category in nature. They argue that if
there is no distinctive set of features that those taxa called ‘species’ tend to have, we should
doubt the existence of the species category.
The debate between pluralists and monists is not new. It was present in Darwin’s time
and Darwin was well aware of it. Consider what Darwin wrote to his friend, the botanist Joseph
Hooker. “It is really laughable to see what different ideas are prominent in various naturalists'
minds, when they speak of 'species'; in some, resemblance is everything and descent of little
weight –in some, resemblance seems to go for nothing, and Creation the reigning idea– in some,
sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying
to define the indefinable” (F. Darwin 1887, vol. 2, 88). In this quote, Darwin mentions four
definitions of ‘species.’ Just as there are competing definitions of ‘species’ today, there were
competing definitions in Darwin’s time. What was Darwin’s prognosis of the situation? At the
end of the quote, he suggests that ‘species’ is an indefinable term. Does that mean he believed
that the species category exists and that the term ‘species’ is merely indefinable? Or, did Darwin
also doubt the existence of the species category? Scholars disagree on the answer to that
question. Some believe that Darwin was a species category sceptic (Beatty 1992). Others
believe he was not (Mallet 2010).
Concluding Remarks
Philosophical questions concerning species are difficult to answer. Yet answers to those
questions have important implications. Whether one adopts taxonomic monism or pluralism
affects how biologists should conduct biodiversity studies. Such studies often compare like-tolike types of entities. If species are the targets of preservation and there are different types of
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species, then we need to rethink how to conduct biodiversity studies. Turning to the ontological
status of species, the question of whether species are kinds with essences affects our conception
of human nature. If there is no essence to Homo sapiens, then there is no biological essence to
humans. Stepping back from these issues, we see that philosophical questions concerning
species are both vexing and important.
References
Beatty, J. (1992) “Speaking of species: Darwin's strategy,” in M. Ereshefsky (ed.) The Units of
Evolution, Cambridge, MA: MIT Press, pp. 227-246.
Boyd, R. (1999) “Homeostasis, species, and higher taxa,” in R. Wilson (ed.) Species: New
Interdisciplinary Essays, Cambridge: MIT Press, pp. 141-185.
Darwin, F. ed. (1877) The life and letters of Charles Darwin, including an autobiographical
chapter. (London: John Murray)
Devitt, M. (2008) “Resurrecting Biological Essentialism,” Philosophy of Science 75: 344-382.
Dupré, J. (1993) The Disorder of Things: Metaphysical Foundations of the Disunity of Science,
Cambridge, MA: Harvard University Press.
Ereshefsky, M (2001) The Poverty of the Linnaean Hierarchy: A Philosophical Study of
Biological Taxonomy, Cambridge, UK: Cambridge University Press.
Ereshefsky, M. (2010) “What’s Wrong with the New Biological Essentialism,” Philosophy of
Science 77: 674-685
Ereshefsky, M. and Matthen, M. (2005) “Taxonomy, polymorphism and history: an introduction
to population structure theory,” Philosophy of Science 72: 1-21.
Ferguson, J. (2002) ‘On the use of genetic divergence for identifying species,” Biological
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Journal of the Linnean Society 75:509-519.
Ghiselin, M. (1974) “A radical solution to the species problem,” Systematic Zoology 23: 536544.
Ghiselin, M. (1987) “Species concepts, individuality, and objectivity,” Biology and
Philosophy 2: 127-143.
Hull, D. (1965) “The effect of essentialism on taxonomy: two thousand years of stasis,” British
Journal for the Philosophy of Science 15: 314-326.
Hull, D. (1978) “A matter of individuality,” Philosophy of Science 45: 335-360.
Hull, D. (1987) “Genealogical Actors in Ecological Roles,” Biology and Philosophy 2: 168-183.
Kitcher, P. (1984) “Species,” Philosophy of Science 51: 308-333.
Mallet, J. (2010) “Why was Darwin's view of species rejected by 20th Century biologists?”
Biology and Philosophy 25: 497-527.
Okasha, S. (2002) “Darwinian Metaphysics: Species and the Question of Essentialism,” Synthese
131: 191-213.
Sober, E. (1980) “Evolution, population thinking and essentialism,” Philosophy of Science
47: 350-383.
Sober, E. (1984) “Sets, Species, and Natural Kinds,” Philosophy of Science 51: 334-41.
Wu, C. and Ting, C. (2004) “Genes and speciation,” Nature Genetics 5: 114-122.
Further Reading
Marc Ereshefsky’s (ed.) The Units of Evolution: Essays on the Nature of Species (Cambridge,
MA: MIT Press, 1992) contains the principle philosophical essays on species. It also contains
introductions to prominent biological definitions of ‘species.’ Robert Wilson’s (ed.) Species
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(Cambridge, MA: MIT Press, 1999) contains more recent philosophical essays on species. For
books that focus on the biological debate over species see Quentin Wheeler and Rudolf Meier’s
(eds.) Species Concepts and Phylogenetic Theory (New York: Columbia University Press 2000)
and Jerry Coyne and Allen Orr’s Speciation (Sunderland, MA: Sinauer 2004). For an up-to-date
bibliography of philosophical works on species, consult Ereshefsky’s “Species” in Edward N.
Zalta (ed.) The Stanford Encyclopedia of Philosophy, available online:
http://plato.stanford.edu/entries/species/.
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