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The species problem is a mixture of difficult, related questions that often come up when biologists identify species and when they define the word "species".

For example, sometimes it is difficult to tell just which particular species an organism belongs to. It can also be hard to decide when to recognize a new species. This can happen when a biologist finds organisms that appear to be different from those that belong to already described species. It can also happen when new data suggests that one previously described species actually includes two or more separately evolving groups, each of which could possibly be recognized as a separate species.

Another aspect of the species problem is one of communication. Because of the different kinds of uncertainties that arise, biologists often do not agree with each other about when a new species should be recognized,  or on what methods to use for identifying 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.

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)

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 thought to be just one single species. Currently there is agreement that these are actually two separate species. But in the past this was not the case, and there are many situations where biologists cannot agree on just how many species there are in a particular situations.

Sometimes in debates about species, different participants end up talking about different things. If this is not realized, it can lead to confusion. For example, in a debate over the species status of Baltimore Oriole and Bullock's Oriole one participant might think that the critical question is about the two kinds of orioles and how similar they are, while a second participant in the debate 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 in the debate is talking about the birds, and another person is talking about the rank of species,  then there can be confusion.

A related kind of confusion and disagreement can arise 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.

Another source of confusion and debate is whether we think of a particular species as being defined by the characteristics by which the species is identified, or whether we think of a particular species as an evolving entity in nature. Every named species has been formally described by biologists as a type of organism with particular defining characteristics. These defining characteristics can be used to identify whether or not a particular organism belongs to a particular species. But for many species, all of the members that fit the defining criteria also make up a single evolving population. These two different ways of thinking about species, (1) as a category and (2)as an evolving population, are quite different from each other. Also sometimes all of the organisms that would be included in a particular species, based on the defining characteristics of that species, do not actually exist in a single evolving population.

Before Darwin
Carolus Linnaeus (1707-1778) formalized the taxonomic rank of species, and devised the two part naming system of bionomial nomenclature that we use today. However this did not prevent there being disagreements on what exactly was the best way to identify species.

Discussions of the history of definitions of the term "species", such as by Britton (1908) and Mayr (1982) reveal that the seeds of the modern debate were growing long before Darwin,  with some descriptions focusing on the need for organisms within a species to be very similar to one another, and other descriptions focusing on sharing in sexual reproduction.

From Darwin to Mayr
Charles Darwin's famous book "On the Origin of Species ..." explained how species evolve and change over time. Although Darwin did not say much on the details of how one species splits into two, he clearly viewed this was a gradual slow process. If so, then there must be a period of time when there are incipient species that are in the process of forming, and that are not yet distinct enough to be recognized as species. Darwin's theory opened up the possibility that sometimes there was 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 observer.

In the first decades of the 20th century the fusion of Mendel's theory of inheritance and Darwin's theory of evolution by natural selection, in what has been called the Modern Evolutionary Synthesis, also had a large impact on how biologists think about species. Edward Poulton anticipated many of the ideas that were later developed by Theodosius Dobzhansky and Ernst Mayr. 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 organisms that can reproduce with one another and that are reproductively isolated from other such groups.

It is sometimes thought that Mayr originated the idea that "species" be defined on the basis of reproductive compatibility. This is quite wrong. 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 was common before Mayr, and ideed it is even contained, in a limited way, in the Biblical myth of Noah's ark, in which each species was preserved by saving a reproductive pair.

A close contemporary of Mayr's was Theodosius Dobzhansky, and just a few years before Mayr's 1942 book, Dobzhansky's treatise focused strongly on 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.

What Mayr did in his 1942 book was to propose explicitly that the reproductive criterion be a basis for species classification and systematics, and to distinguish this idea from other conceptual views of species.

Mayr was persuasive in many respects and from 1942 until his death in 2005 he and the 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 on the other hand the BSC has been criticized for not being very useful, because it does not help very much in deciding when to identify new species. The issue is complicated because there are many cases where members of unambiguously different species (by most criteria) 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. The response to such questions is that reproductive barriers must be assessed 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.

In the decades after Mayr, many more species concepts were introduced. Some, such as the Phylogenetic Species Concept (PSC), where 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 criteria for identifying species, and some were more conceptual or more philosophical.

As many as 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 biologists have argued that the species problem is too multidimensional to be "solved" by one definition of species or one species concept. Since the 1990's 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 systematists, the concept has been criticized as not being useful for those biologists whose research actually falls under the area of systematics. Some biologists who work in the area of systematics have critiqued the BSC as not being operational. . However for many other biologists 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 Nominalsim
Realism and Nominalsim 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, particular 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 crique 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

David Hull argued that pluralist proposals were unlikely to actually solve the species problem.