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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.
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.
Confusion on the meaning of "Species"
Species 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. 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, (1) as a category and (2) as an evolving population, are quite different from each other.
Carolus Linnaeus (1707-1778) formalized the taxonomic 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.
From Darwin to Mayr
Charles 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.
Mayr and recent history
Ernst Mayr'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. The idea of shared reproduction within species is even contained in the Biblical story of Noah's ark, in which each species was preserved by saving a reproductive pair.
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 .
Realism and nominalism
Realism 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.
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 concepts and the indeterminacy of language 
Hey described the species problem as a result of two conflicting motivations by biologists: (1) to categorize and identify organisms; and (2) 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 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 monistic 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 
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) 
"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)"
- ↑ Robson, G. C. 1928. The Species Problem: an Introduction to the Study of Evolutionary Divergence in Natural Populations. Oliver and Boyd, Edinburgh.
- ↑ 2.0 2.1 Mayr E. 1942. Systematics and the origin of species. Columbia University Press, New York.
- ↑ Mayden RL. 1997. A hierarchy of species concepts: The denouement in the saga of the species problem. Pp. 381-424 in Claridge, MF, Dawah, HA, Wilson, MR, eds. Species: The units of biodiversity. Chapman and Hall, London.
- ↑ 4.0 4.1 Wheeler QD, Meier R. 2000. Species concepts and phylogenetic theory: A debate. Columbia University Press, New York.
- ↑ Britton NL. 1908. The taxonomic aspect of the species question. American Naturalist 42:225-242.
- ↑ 6.0 6.1 Mayr E. 1982. The growth of biological thought. Harvard University Press, Cambridge, MA.
- ↑ Johnson DS. 1908. Aspects of the species question. American Naturalist 42:217.
- ↑ Bailey LH. 1896. The philosophy of species-making. Botanical Gazette 22:454-462.
- ↑ Mallet J. 2004. Perspectives poulton, wallace and jordan: How discoveries in papilio butterflies initiated a new species concept 100 years ago. Systematics and Biodiversity 1:441-452.
- ↑ 10.0 10.1 10.2 Dobzhansky T. 1937. Genetics and the origin of species. Columbia University Press, New York.
- ↑ Mallet J. 2001. The speciation revolution. Journal of Evolutionary Biology 14:887-888.
- ↑ Coyne JA. 1994. Ernst Mayr and the origin of species. Evolution 48:19-30.
- ↑ 13.0 13.1 13.2 Pigliucci M. 2003. Species as family resemblance concepts: The (dis-)solution of the species problem? Bioessays 25:596-602.
- ↑ 14.0 14.1 Ghiselin MT. 1974. A radical solution to the species problem. Systematic Zoology 23:536-544.
- ↑ de Queiroz K. 2005. Different species problems and their resolution. Bioessays 27:1263-1269.
- ↑ Hey J. 1997. A reduction of "species" and a resolution of the species problem.(A Reduction of "Species" Resolves the Species Problem on Rutgers University Department of Genetics laboratory under professor Jody Hey accessed at December 25 2007)
- ↑ Ridley M. 1989. The cladistic solution to the species problem. Biology and Philosophy V4:1-16.
- ↑ Stamos DN. 2003. The species problem: Biological species, ontology, and the metaphysics of biology. Lexington Books, Lanham.
- ↑ Vrana P, Wheeler W. 1992. Individual organisms as terminal entities: Laying the species problem to rest. Cladistics 8:67-72.
- ↑ 20.0 20.1 20.2 Hey J. 2001. Genes categories and species. Oxford University Press, New York, NY.
- ↑ Endler JA. 1989. Conceptual and other problems in speciation. Pp. 625-648 in Otte, D, Endler, JA, eds. Speciation and its consequences. Sinauer Associates, Sunderland, Mass.
- ↑ de Queiroz K. 1998. The general lineage concept of species: Species criteria and the process of speciation. Pp. 57-75 in Howard, DJ, Berlocher, SH, eds. Endless forms: Species and speciation. Oxford University Press, New York.
- ↑ Miller W. 2001. The structure of species, outcomes of speciation and the `species problem': Ideas for paleobiology. Palaeogeography, Palaeoclimatology, Palaeoecology 176:1-10.
- ↑ Hey J. 2006. On the failure of modern species concepts. Trends Ecol Evol 21:447-450.
- ↑ Hull DL. 1999. On the plurality of species: Questioning the party line. Pp. 23-48 in Wilson, RA, ed. Species. MIT Press, Cambridge, MA.
- ↑ Zink RM, McKitrick MC. 1995. The debate over species concepts and its implications for ornithology. Auk 112:701-719.
- ↑ Levin DA. 1979. The nature of plant species. Science 204:381-384.
- ↑ Sokal RR, Crovello TJ. 1970. The biological species concept: A critical evaluation. American Naturalist 104:127-153.
- ↑ Coyne JA, Orr HA. 2004. Speciation. Sinauer Associates, Sunderland, Mass.
- ↑ Gregg JR. 1950. Taxonomy, language and reality. American Naturalist 84:419-435.
- ↑ Burma BH. 1954. Reality, existence, and classification: A discussion of the species problem. Pp. 193-209 in Slobodchikoff, CN, ed. Concepts of species. Dowden, Hutchinson & Ross, Stroudsburg, PA.
- ↑ Ghiselin MT. 1997. Metaphysics and the origin of species. State University of New York Press, Albany, NY.
- ↑ 33.0 33.1 Haldane JBS. 1956. Can a species concept be justified? Pp. 95-96 in Sylvester-Bradley, PC, ed. The species concept in paleontology. Systematics Association, London.
- ↑ Hull DL. 1978. A matter of individuality. Philosophy of Science 45:335-360.
- ↑ Jardine N. 1969. A logical basis for biological classification. Systematic Zoology 18:37-52.
- ↑ 36.0 36.1 Hey J. 2001. The mind of the species problem. Trends in Ecology and Evolution 16:326-329.
- ↑ Dupré J. 1999. On the impossibility of a monistic account of species. Pp. 3-22 in Wilson, RA, ed. Species. MIT Press, Cambridge, MA.
- ↑ Mishler BD, Donoghue MJ. 1982. Species concepts: A case for pluralism. Systematic Zoology 31:491-503.
- ↑ Ereshefsky M. 1992. Eliminative pluralism. Philosophy of Science 59.
- ↑ Hull DL. 1999. On the plurality of species: Questioning the party line. Pp. 23-48 in Wilson, RA, ed. Species. MIT Press, Cambridge, MA.
- ↑ Darwin C. 1859. On the origin of species by means of natural selection. Murray, London.
- ↑ Nicholson HA. 1872. A manual of zoology. Appleton and Company, New York.
- ↑ Bonde, N (1977), "Cladistic classification as applied to vertebrates", in Hecht, M.K., Goody, P.C., and Hecht, B.M., Major Patterns in Vertebrate Evolution, New York: Plenum Press, pp. 741-804
- A catalogue of species conceptions
- A Reduction of "Species" Resolves the Species Problem
- Curiosities of Biological Nomenclature
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