Species problem
Encyclopedia
The species problem or species concept is a mixture of difficult, related questions that often come up when biologist
s identify species
and when they define the word "species".
One common but sometimes difficult question is how best to decide just which particular species an organism
belongs to. Another challenge is deciding when to recognize a new species. This is a question for the biologist who discovers organisms that appear to be different from those that belong to already described species. A related question arises when new data indicate that one previously described species actually may include two or more separately evolving
groups, each of which could possibly be recognized as a separate species.
Many of the debates on species touch on philosophical issues, such as nominalism
and realism
, as well as on issues of language
and cognition.
This current meaning of the phrase "species problem" is quite different from what was meant by "species problem" during the 19th and early 20th centuries, as used by Darwin and others. For Darwin the species problem was the question of how new species arose
.
is one of several ranks in the hierarchical system of scientific classification. These are called taxonomic ranks
, and the system of classification includes, in addition to species the ranks of genus
and family
and others all the way up to kingdom
. Usually the rank of species is the basal rank, meaning that in the system of scientific classification species is the bottommost rank that includes no other ranks. However sometimes when one species, that is already named and described, is found to actually include two slightly different kinds of organisms, it is necessary to use the rank of subspecies
.
Even though it is not disputed that species is a taxonomic rank, this does not prevent disagreements when particular species are discussed. Consider the case of the Baltimore Oriole
and Bullock's Oriole
, two similar species of birds that have sometimes in the past been considered to be one single species, the Northern Oriole. Currently biologists agree that these are actually two separate species, but in the past this was not the case.
It is common in debates about species for participants to argue at cross purposes. For example, in a debate over the species status of Baltimore Oriole and Bullock's Oriole one person might think that the critical question is about the two kinds of orioles and how similar they are. A second person might think that the critical question concerns the actual taxonomic rank of species, and on what the correct criteria are for identifying a species. If one person is talking about the birds, and another person is talking about the rank of species, then there can be confusion.
Disagreements and confusion also happen over just what the best criteria are for identifying new species. In 1942 the famous biologist Ernst Mayr
wrote that because biologists have different ways of identifying species, they actually have different species concepts. Mayr proceeded to list five different species concepts, and since then many more have been added. The question of which species concept is best has occupied many printed pages and many hours of discussion.
Some debates are philosophical in nature. One common disagreement is over whether a species is defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species organisms belong to. But for many species, all of the individuals that fit the defining criteria also make up a single evolving unit. These two different ways of thinking about species, as a category and as an evolving population, are quite different from each other.
introduced the biological concept that species were distinguished by always producing the same species, and this was fixed and permanent, though considerable variation was possible within a species. Carolus Linnaeus (1707–1778) formalized the taxon
omic rank of species, and devised the two part naming system of binomial nomenclature
that we use today. However this did not prevent disagreements on the best way to identify species.
The history of definitions of the term "species" reveal that the seeds of the modern species debate were alive and growing long before Darwin.
's famous book On the Origin of Species (1859) offered an explanation as to how species changed over time (evolution
). Although Darwin did not provide details on how one species splits into two, he viewed speciation
as a gradual process
. If Darwin was correct, then when new incipient species are forming there must be a period of time when they are not yet distinct enough to be recognized as species. Darwin's theory suggested that there was often not going to be an objective fact of the matter, on whether there were one or two species.
Darwin's book triggered a crisis of uncertainty for some biologists over the objectivity of species, and some came to wonder whether individual species could be objectively real — i.e. have an existence that is independent of the human observer.
In the 1920s and 1930s, Mendel's
theory of inheritance and Darwin's theory of evolution by natural selection
were joined in what was called the modern evolutionary synthesis
. This conjunction of theories also had a large impact on how biologists think about species. Edward Poulton
anticipated many ideas on species that today are well accepted, and that were later more fully developed by Theodosius Dobzhansky
and Ernst Mayr
, two of the architects of the modern synthesis. Dobzhansky's 1937 book articulated the genetic processes that occur when incipient species are beginning to diverge. In particular Dobzhansky described the critical role, for the formation of new species, of the evolution of reproductive isolation
.
's 1942 book was a turning point for the species problem. In it he wrote about how different investigators approach species identification, and he characterized these different approaches as different species concepts. He also argued strongly for, what came to be called, a Biological Species Concept (BSC), which is that a species consists of populations of organisms that can reproduce with one another and that are reproductively isolated from other such populations.
Mayr was not the first to define "species" on the basis of reproductive compatibility. Many others before Mayr had suggested this idea, as Mayr makes clear in his book on the history of biology. For example Mayr discusses how Buffon
proposed this kind of definition of "species" in 1753.
Theodosius Dobzhansky was a close contemporary of Mayr's and the author of a classic book, that came out a few years before Mayr's, that was about the evolutionary origins of reproductive barriers between species. Many biologists credit Dobzhansky and Mayr jointly for emphasizing the need to consider reproductive isolation when studying species and speciation
.
Mayr was persuasive in many respects and from 1942 until his death in 2005 he and the biological species concept (BSC) played a central role in nearly all debates on the species problem. For many, the Biological Species Concept was a useful theoretical idea because it leads to a focus on the evolutionary origins of barriers to reproduction between species. But the BSC has been criticized for not being very useful for deciding when to identify new species. It is also true that there are many cases where members of different species will hybridize and produce fertile offspring when they are under confined conditions, such as in zoos. One fairly extreme example is that lions and tigers will hybridize in captivity, and at least some of the offspring have been reported to be fertile. Mayr's response to cases like these is that the reproductive barriers that are important for species are the ones that occur in the wild. But even so it is also the case that there are many cases of different species that are known to hybridize and produce fertile offspring in nature.
After Mayr's 1942 book many more species concepts were introduced. Some, such as the Phylogenetic Species Concept (PSC), were designed to be more useful than the BSC for actually deciding when a new species should be described. However not all of the new species concepts were about identifying species, and some concepts were mostly conceptual or philosophical.
About two dozen species concepts have been identified or proposed since Mayr's 1942 book, and many articles and several books have been written on the species problem. At some point it became common for articles to profess to "solve" or "dissolve" the species problem.
Some have argued that the species problem is too multidimensional to be "solved" by one definition of species or one species concept. Since the 1990s articles have appeared that make the case that species concepts, particularly those that specify how species should be identified, have not been very helpful in resolving the species problem.
Although Mayr promoted the Biological Species Concept for use in systematics
, the concept has been criticized as not being useful for those who do research in systematics. Some systematists have criticized the BSC as not being operational. However for many others the BSC is the preferred description of species. For example many geneticists who work on the process of species formation prefer the BSC because it emphasizes the role of barriers to reproduction between species.
and Nominalism
are philosophical subjects that come up in debates over whether or not species literally exist. From one perspective, each species is a kind of organism and each species is based on a set of characteristics that are shared by all the organisms in the species. This usage of "species" refers to the taxonomic sense of the word, and under this kind of meaning a species is a category, or a type, or a natural kind
. For example, the species that we call giraffe
is a category of things that people have recognized have a lot in common with each other and to which we have given the name "giraffe". This is a category in the same sense that the words "mountain
" and "snowflake
" identify categories of things in nature.
This view of a species as a type, or natural kind, raises the question of whether such things are real. The question is not whether the organisms exist, but whether the kinds of organisms exist. There is a school of philosophical thought, called realism
that says that natural kinds and other so called universals do exist. But what kind of existence would this be? It is one thing to say that a particular giraffe exists, but in what way does the giraffe category exist? This question is the opening for Nominalism which is a philosophical view that types and kinds, and universals in general, do not literally exist.
If the nominalist view is correct then kinds of things, that people have given names to, do not literally exist. It would follow then that because species are named types of organisms, that species do not literally exist. This can be a troubling idea, particularly to a biologist who studies species. If species are not real, then it would not be sensible to talk about "the origin of a species" or the "evolution of a species". As recently at least as the 1950s, some authors adopted this view and wrote of species as not being real.
A useful counterpoint to the nominalist view, in regard to species, was raised by Michael Ghiselin
who argued that an individual species is not a type, but rather an actual individual, an actual entity
. This idea comes from thinking of a species as an evolving dynamic population. As an entity a species exists quite regardless of whether or not people have observed it and whether or not it has been given a name based on traits shared by the organisms in the species.
Several authors have noted the similarity between "species", as a word of ambiguous meaning, and points made by Wittgenstein on family resemblance
concepts and the indeterminacy of language.
Jody Hey
described the species problem as a result of two conflicting motivations by biologists:
Under the first view, species appear to us as typical natural kinds, but when biologists turn to understand species evolutionarily they are revealed as changeable and without sharp boundaries. Hey
argued that it is unrealistic to expect that one definition of "species" is going to serve the need for categorization and still reflect the changeable realities of evolving species.
description of species could be developed and agreed upon, then the species problem would be solved.
In contrast some authors have argued for pluralism, claiming that biologists cannot have just one shared concept of species, and that they should accept multiple, seemingly incompatible ideas about species.
David Hull
argued that pluralist proposals were unlikely to actually solve the species problem.
"No term is more difficult to define than "species," and on no point are zoologists more divided than as to what should be understood by this word". Nicholson (1872) p. 20
"Of late, the futility of attempts to find a universally valid criterion for distinguishing species has come to be fairly generally, if reluctantly, recognized" Dobzhansky (1937) p. 310
"The concept of a species is a concession to our linguistic habits and neurological mechanisms" Haldane (1956)
"The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word 'species'." Hey (2001)
"First, the species problem is not primarily an empirical one, but it is rather fraught with philosophical questions that require - but cannot be settled by - empirical evidence." Pigliucci (2003)
"An important aspect of any species definition whether in neontology
or palaeontology is that any statement that particular individuals (or fragmentary specimens) belong to a certain species is an hypothesis (not a fact)"
Biologist
A biologist is a scientist devoted to and producing results in biology through the study of life. Typically biologists study organisms and their relationship to their environment. Biologists involved in basic research attempt to discover underlying mechanisms that govern how organisms work...
s identify species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
and when they define the word "species".
One common but sometimes difficult question is how best to decide just which particular species an organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
belongs to. Another challenge is deciding when to recognize a new species. This is a question for the biologist who discovers organisms that appear to be different from those that belong to already described species. A related question arises when new data indicate that one previously described species actually may include two or more separately evolving
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
groups, each of which could possibly be recognized as a separate species.
Many of the debates on species touch on philosophical issues, such as nominalism
Nominalism
Nominalism is a metaphysical view in philosophy according to which general or abstract terms and predicates exist, while universals or abstract objects, which are sometimes thought to correspond to these terms, do not exist. Thus, there are at least two main versions of nominalism...
and realism
Philosophical realism
Contemporary philosophical realism is the belief that our reality, or some aspect of it, is ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc....
, as well as on issues of language
Language
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...
and cognition.
This current meaning of the phrase "species problem" is quite different from what was meant by "species problem" during the 19th and early 20th centuries, as used by Darwin and others. For Darwin the species problem was the question of how new species arose
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
.
Confusion on the meaning of "Species"
SpeciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
is one of several ranks in the hierarchical system of scientific classification. These are called taxonomic ranks
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, and the system of classification includes, in addition to species the ranks of genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
and family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
and others all the way up to kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
. Usually the rank of species is the basal rank, meaning that in the system of scientific classification species is the bottommost rank that includes no other ranks. However sometimes when one species, that is already named and described, is found to actually include two slightly different kinds of organisms, it is necessary to use the rank of subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
.
Even though it is not disputed that species is a taxonomic rank, this does not prevent disagreements when particular species are discussed. Consider the case of the Baltimore Oriole
Baltimore Oriole
The Baltimore Oriole is a small icterid blackbird that averages 18 cm long and weighs 34 g. This bird received its name from the fact that the male's colors resemble those on the coat-of-arms of Lord Baltimore...
and Bullock's Oriole
Bullock's Oriole
The Bullock's Oriole, , is a small New World blackbird. At one time, this species and the Baltimore Oriole were considered to be a single species, the Northern Oriole...
, two similar species of birds that have sometimes in the past been considered to be one single species, the Northern Oriole. Currently biologists agree that these are actually two separate species, but in the past this was not the case.
It is common in debates about species for participants to argue at cross purposes. For example, in a debate over the species status of Baltimore Oriole and Bullock's Oriole one person might think that the critical question is about the two kinds of orioles and how similar they are. A second person might think that the critical question concerns the actual taxonomic rank of species, and on what the correct criteria are for identifying a species. If one person is talking about the birds, and another person is talking about the rank of species, then there can be confusion.
Disagreements and confusion also happen over just what the best criteria are for identifying new species. In 1942 the famous biologist Ernst Mayr
Ernst Mayr
Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
wrote that because biologists have different ways of identifying species, they actually have different species concepts. Mayr proceeded to list five different species concepts, and since then many more have been added. The question of which species concept is best has occupied many printed pages and many hours of discussion.
Some debates are philosophical in nature. One common disagreement is over whether a species is defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species organisms belong to. But for many species, all of the individuals that fit the defining criteria also make up a single evolving unit. These two different ways of thinking about species, as a category and as an evolving population, are quite different from each other.
Before Darwin
The idea that one organism reproduces by giving birth to a similar organism, or producing seeds that grow to a similar organism, goes back to the earliest days of farming. While people tended to think of this as a relatively stable process, many thought that change was possible. The term species was just used as a term for a sort or kind of organism, until in 1686 John RayJohn Ray
John Ray was an English naturalist, sometimes referred to as the father of English natural history. Until 1670, he wrote his name as John Wray. From then on, he used 'Ray', after "having ascertained that such had been the practice of his family before him".He published important works on botany,...
introduced the biological concept that species were distinguished by always producing the same species, and this was fixed and permanent, though considerable variation was possible within a species. Carolus Linnaeus (1707–1778) formalized the taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
omic rank of species, and devised the two part naming system of binomial nomenclature
Binomial nomenclature
Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...
that we use today. However this did not prevent disagreements on the best way to identify species.
The history of definitions of the term "species" reveal that the seeds of the modern species debate were alive and growing long before Darwin.
From Darwin to Mayr
Charles DarwinCharles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
's famous book On the Origin of Species (1859) offered an explanation as to how species changed over time (evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
). Although Darwin did not provide details on how one species splits into two, he viewed speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
as a gradual process
Gradualism
Gradualism is the belief in or the policy of advancing toward a goal by gradual, often slow stages.-Politics and society:In politics, the concept of gradualism is used to describe the belief that change ought to be brought about in small, discrete increments rather than in abrupt strokes such as...
. If Darwin was correct, then when new incipient species are forming there must be a period of time when they are not yet distinct enough to be recognized as species. Darwin's theory suggested that there was often not going to be an objective fact of the matter, on whether there were one or two species.
Darwin's book triggered a crisis of uncertainty for some biologists over the objectivity of species, and some came to wonder whether individual species could be objectively real — i.e. have an existence that is independent of the human observer.
In the 1920s and 1930s, Mendel's
Gregor Mendel
Gregor Johann Mendel was an Austrian scientist and Augustinian friar who gained posthumous fame as the founder of the new science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance...
theory of inheritance and Darwin's theory of evolution by natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
were joined in what was called the modern evolutionary synthesis
Modern evolutionary synthesis
The modern evolutionary synthesis is a union of ideas from several biological specialties which provides a widely accepted account of evolution...
. This conjunction of theories also had a large impact on how biologists think about species. Edward Poulton
Edward Bagnall Poulton
Sir Edward Bagnall Poulton, FRS was a British evolutionary biologist who was a lifelong advocate of natural selection...
anticipated many ideas on species that today are well accepted, and that were later more fully developed by Theodosius Dobzhansky
Theodosius Dobzhansky
Theodosius Grygorovych Dobzhansky ForMemRS was a prominent geneticist and evolutionary biologist, and a central figure in the field of evolutionary biology for his work in shaping the unifying modern evolutionary synthesis...
and Ernst Mayr
Ernst Mayr
Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
, two of the architects of the modern synthesis. Dobzhansky's 1937 book articulated the genetic processes that occur when incipient species are beginning to diverge. In particular Dobzhansky described the critical role, for the formation of new species, of the evolution of reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
.
Mayr and recent history
Ernst MayrErnst Mayr
Ernst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
's 1942 book was a turning point for the species problem. In it he wrote about how different investigators approach species identification, and he characterized these different approaches as different species concepts. He also argued strongly for, what came to be called, a Biological Species Concept (BSC), which is that a species consists of populations of organisms that can reproduce with one another and that are reproductively isolated from other such populations.
Mayr was not the first to define "species" on the basis of reproductive compatibility. Many others before Mayr had suggested this idea, as Mayr makes clear in his book on the history of biology. For example Mayr discusses how Buffon
Georges-Louis Leclerc, Comte de Buffon
Georges-Louis Leclerc, Comte de Buffon was a French naturalist, mathematician, cosmologist, and encyclopedic author.His works influenced the next two generations of naturalists, including Jean-Baptiste Lamarck and Georges Cuvier...
proposed this kind of definition of "species" in 1753.
Theodosius Dobzhansky was a close contemporary of Mayr's and the author of a classic book, that came out a few years before Mayr's, that was about the evolutionary origins of reproductive barriers between species. Many biologists credit Dobzhansky and Mayr jointly for emphasizing the need to consider reproductive isolation when studying species and speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
.
Mayr was persuasive in many respects and from 1942 until his death in 2005 he and the biological species concept (BSC) played a central role in nearly all debates on the species problem. For many, the Biological Species Concept was a useful theoretical idea because it leads to a focus on the evolutionary origins of barriers to reproduction between species. But the BSC has been criticized for not being very useful for deciding when to identify new species. It is also true that there are many cases where members of different species will hybridize and produce fertile offspring when they are under confined conditions, such as in zoos. One fairly extreme example is that lions and tigers will hybridize in captivity, and at least some of the offspring have been reported to be fertile. Mayr's response to cases like these is that the reproductive barriers that are important for species are the ones that occur in the wild. But even so it is also the case that there are many cases of different species that are known to hybridize and produce fertile offspring in nature.
After Mayr's 1942 book many more species concepts were introduced. Some, such as the Phylogenetic Species Concept (PSC), were designed to be more useful than the BSC for actually deciding when a new species should be described. However not all of the new species concepts were about identifying species, and some concepts were mostly conceptual or philosophical.
About two dozen species concepts have been identified or proposed since Mayr's 1942 book, and many articles and several books have been written on the species problem. At some point it became common for articles to profess to "solve" or "dissolve" the species problem.
Some have argued that the species problem is too multidimensional to be "solved" by one definition of species or one species concept. Since the 1990s articles have appeared that make the case that species concepts, particularly those that specify how species should be identified, have not been very helpful in resolving the species problem.
Although Mayr promoted the Biological Species Concept for use in systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
, the concept has been criticized as not being useful for those who do research in systematics. Some systematists have criticized the BSC as not being operational. However for many others the BSC is the preferred description of species. For example many geneticists who work on the process of species formation prefer the BSC because it emphasizes the role of barriers to reproduction between species.
Realism and nominalism
RealismPlatonic realism
Platonic realism is a philosophical term usually used to refer to the idea of realism regarding the existence of universals or abstract objects after the Greek philosopher Plato , a student of Socrates. As universals were considered by Plato to be ideal forms, this stance is confusingly also called...
and Nominalism
Nominalism
Nominalism is a metaphysical view in philosophy according to which general or abstract terms and predicates exist, while universals or abstract objects, which are sometimes thought to correspond to these terms, do not exist. Thus, there are at least two main versions of nominalism...
are philosophical subjects that come up in debates over whether or not species literally exist. From one perspective, each species is a kind of organism and each species is based on a set of characteristics that are shared by all the organisms in the species. This usage of "species" refers to the taxonomic sense of the word, and under this kind of meaning a species is a category, or a type, or a natural kind
Natural kind
In philosophy, a natural kind is a "natural" grouping, not an artificial one. Or, it is something that a set of things has in common which distinguishes it from other things as a real set rather than as a group of things arbitrarily lumped together by a person or group of people.If any natural...
. For example, the species that we call giraffe
Giraffe
The giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant...
is a category of things that people have recognized have a lot in common with each other and to which we have given the name "giraffe". This is a category in the same sense that the words "mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...
" and "snowflake
Snowflake
Snowflakes are conglomerations of frozen ice crystals which fall through the Earth's atmosphere. They begin as snow crystals which develop when microscopic supercooled cloud droplets freeze. Snowflakes come in a variety of sizes and shapes. Complex shapes emerge as the flake moves through...
" identify categories of things in nature.
This view of a species as a type, or natural kind, raises the question of whether such things are real. The question is not whether the organisms exist, but whether the kinds of organisms exist. There is a school of philosophical thought, called realism
Platonic realism
Platonic realism is a philosophical term usually used to refer to the idea of realism regarding the existence of universals or abstract objects after the Greek philosopher Plato , a student of Socrates. As universals were considered by Plato to be ideal forms, this stance is confusingly also called...
that says that natural kinds and other so called universals do exist. But what kind of existence would this be? It is one thing to say that a particular giraffe exists, but in what way does the giraffe category exist? This question is the opening for Nominalism which is a philosophical view that types and kinds, and universals in general, do not literally exist.
If the nominalist view is correct then kinds of things, that people have given names to, do not literally exist. It would follow then that because species are named types of organisms, that species do not literally exist. This can be a troubling idea, particularly to a biologist who studies species. If species are not real, then it would not be sensible to talk about "the origin of a species" or the "evolution of a species". As recently at least as the 1950s, some authors adopted this view and wrote of species as not being real.
A useful counterpoint to the nominalist view, in regard to species, was raised by Michael Ghiselin
Michael Ghiselin
Michael T. Ghiselin is an American biologist, philosopher/historian of biology currently at the California Academy of Sciences.B.A., University of Utah ; Ph.D., Stanford University ; Postdoctoral Fellow, Harvard University ; Postdoctoral Fellow, Marine Biological Laboratory ; Assistant Professor of...
who argued that an individual species is not a type, but rather an actual individual, an actual entity
Entity
An entity is something that has a distinct, separate existence, although it need not be a material existence. In particular, abstractions and legal fictions are usually regarded as entities. In general, there is also no presumption that an entity is animate.An entity could be viewed as a set...
. This idea comes from thinking of a species as an evolving dynamic population. As an entity a species exists quite regardless of whether or not people have observed it and whether or not it has been given a name based on traits shared by the organisms in the species.
Language and the role of human investigators
The nominalist critique of the view that kinds of things exist, raises for consideration the role that humans play in the species problem. For example, Haldane suggested that species are just mental abstractions.Several authors have noted the similarity between "species", as a word of ambiguous meaning, and points made by Wittgenstein on family resemblance
Family resemblance
Family resemblance is a philosophical idea made popular by Ludwig Wittgenstein, with the best known exposition being given in the posthumously published book Philosophical Investigations It has been suggested that Wittgenstein picked the idea and the term from Nietzsche, who had been using it,...
concepts and the indeterminacy of language.
Jody Hey
Jody Hey
Jody Hey is an evolutionary biologist at Rutgers University. In the 1980s and 1990s he did research on natural selection and species divergence in fruit flies . More recently he has worked on the divergence of cichlid fishes from Lake Malawi, on chimpanzees and on human populations...
described the species problem as a result of two conflicting motivations by biologists:
- to categorize and identify organisms;
- to understand the evolutionary processes that give rise to species.
Under the first view, species appear to us as typical natural kinds, but when biologists turn to understand species evolutionarily they are revealed as changeable and without sharp boundaries. Hey
Jody Hey
Jody Hey is an evolutionary biologist at Rutgers University. In the 1980s and 1990s he did research on natural selection and species divergence in fruit flies . More recently he has worked on the divergence of cichlid fishes from Lake Malawi, on chimpanzees and on human populations...
argued that it is unrealistic to expect that one definition of "species" is going to serve the need for categorization and still reflect the changeable realities of evolving species.
Pluralism and monism
Usually it is assumed that biologists approach the species problem with the idea that it would be useful to develop one common viewpoint of species - one single common conception of what species are and of how they should be identified. It is thought that if such a monisticMonism
Monism is any philosophical view which holds that there is unity in a given field of inquiry. Accordingly, some philosophers may hold that the universe is one rather than dualistic or pluralistic...
description of species could be developed and agreed upon, then the species problem would be solved.
In contrast some authors have argued for pluralism, claiming that biologists cannot have just one shared concept of species, and that they should accept multiple, seemingly incompatible ideas about species.
David Hull
David Hull
David Lee Hull was a philosopher with a particular interest in the philosophy of biology. In addition to his academic prominence, he was well-known as a gay man who fought for the rights of other gay and lesbian philosophers....
argued that pluralist proposals were unlikely to actually solve the species problem.
Quotations on the species problem
"... I was much struck how entirely vague and arbitrary is the distinction between species and varieties" Darwin 1859 (p. 48)"No term is more difficult to define than "species," and on no point are zoologists more divided than as to what should be understood by this word". Nicholson (1872) p. 20
"Of late, the futility of attempts to find a universally valid criterion for distinguishing species has come to be fairly generally, if reluctantly, recognized" Dobzhansky (1937) p. 310
"The concept of a species is a concession to our linguistic habits and neurological mechanisms" Haldane (1956)
"The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word 'species'." Hey (2001)
"First, the species problem is not primarily an empirical one, but it is rather fraught with philosophical questions that require - but cannot be settled by - empirical evidence." Pigliucci (2003)
"An important aspect of any species definition whether in neontology
Neontology
Neontology is the part of biology which – in contrast to paleontology – deals with now living organisms. The term neontologist is usually used only by paleontologists to refer to non-paleontologists...
or palaeontology is that any statement that particular individuals (or fragmentary specimens) belong to a certain species is an hypothesis (not a fact)"