Species concept
From The Art and Popular Culture Encyclopedia
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"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."--Sholem Stein |
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The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for sexually reproducing organisms such as birds is useless for species that reproduce asexually, such as bacteria. The scientific study of the species problem has been called microtaxonomy.
One common, but sometimes difficult, question is how best to decide which species an organism belongs to, because reproductively isolated groups may not be readily recognizable, and cryptic species may be present. There is a continuum from reproductive isolation with no interbreeding, to panmixis, unlimited interbreeding. Populations can move forward or backwards along this continuum, at any point meeting the criteria for one or another species concept, and failing others.
Many of the debates on species touch on philosophical issues, such as nominalism and realism, and on issues of language and cognition.
The current meaning of the phrase "species problem" is quite different from what Charles Darwin and others meant by it during the 19th and early 20th centuries. For Darwin, the species problem was the question of how new species arose. Darwin was however one of the first people to question how well-defined species are, given that they constantly change.
- ... I was much struck how entirely vague and arbitrary is the distinction between species and varieties --Charles Darwin|On the Origin of Species
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Philosophical aspects
Realism
Realism is the philosophical position that species are real mind-independent entities, natural kinds. Mayr, a proponent of realism, attempted to demonstrate species exist as natural, extra-mental categories. He showed for example that the New Guinean tribesman classify 136 species of birds, which Western ornithologists came to independently recognize:
- "I have always thought that there is no more devastating refutation of the nominalistic claims than the above mentioned fact that primitive natives in New Guinea, with a Stone Age culture, recognize as species exactly the same entities of nature as western taxonomists. If species were something purely arbitrary, it would be totally improbable for representatives of two drastically different cultures to arrive at the identical species delimitations."
Mayr's argument however has been criticized:
- "The fact that independently observing humans see much the same species in nature does not show that species are real rather than nominal categories. The most it shows is that all human brains are wired up with a similar perceptual cluster statistic (Ridley, 1993). On this view we [humans] might have been ‘wired’ differently and different species might now be wired differently from us, so that no one wiring can be said to be ‘true’ or veridical."
Another position of realism is that natural kinds are demarcated by the world itself by having a unique property that is shared by all the members of a species, and none outside the group. In other words, a natural kind possesses an essential or intrinsic feature (“essence”) that is self-individuating and non-arbitrary. This notion has been heavily criticized as essentialist, but modern realists have argued that while biological natural kinds have essences, these need not be fixed and are prone to change through speciation. According to Mayr (1957) reproductive isolation or interbreeding "supplies an objective yardstick, a completely non-arbitrary criterion” and "describing a presence or absence relationship makes this species concept non-arbitrary". The BSC defines species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups". From this perspective, each species is based on a property (reproductive isolation) that is shared by all the organisms in the species that objectively distinguishes them.
Nominalism
Some philosophical variants of nominalism propose that species are just names that people have assigned to groups of creatures but where the lines between species get drawn does not reflect any fundamental underlying biological cut-off point. In this view, the kinds of things that people have given names to, do not reflect any underlying reality. It then follows that species do not exist outside the mind, because species are just named abstractions. 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 counterpoint to the nominalist views 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. If viewed as an entity, a species would exist regardless of whether or not people have observed it and whether or not it has been given a name.
Pragmatism
A popular alternative view, pragmatism, espoused by philosophers such as Philip Kitcher and John Dupre states while species do not exist in the sense of natural kinds, they are conceptually real and exist for convenience and for practical applications. For example, regardless of which definition of species one uses, one can still quantitatively compare species diversity across regions or decades, as long as the definition is held constant within a study. This has practical importance in advancing biodiversity science and environmental science.
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.
Jody Hey 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 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
Many approaches to the species problem have attempted to develop 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 however argued that pluralist proposals were unlikely to actually solve the species problem.
See also
