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| “ | ... I was much struck how entirely vague and arbitrary is the distinction between species and varieties | ” |
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; cryptic species may be present.
Another common problem is how to define reproductive isolation, because some separately evolving groups may continue to interbreed to some extent, and it can be a difficult matter to discover whether this hybridization affects the long-term genetic make-up of the groups.
Many of the debates on species touch on philosophical issues, such as nominalism and realism, as well as 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.[2] For Darwin, the species problem was the question of how new species arose: speciation.
Confusion on the meaning of "Species"
Species is one of several ranks in the hierarchical system of scientific classification, called taxonomic ranks.Even though it is not disputed that species is a taxonomic rank, this does not prevent disagreements when particular species are discussed. In the case of the Baltimore Oriole (Icterus galbula) and Bullock's Oriole (I. bullockii), two similar species of birds have sometimes in the past been considered to be one single species, the Northern Oriole (I. galbula). Currently, biologists agree that these are actually two separate species,[3] but in the past this was not the case.[4]
Disagreements and confusion happen over just what the best criteria are for identifying new species. In 1942, Ernst Mayr wrote that, because biologists have different ways of identifying species, they actually have different species concepts.[5] Mayr listed five different species concepts, and since then many more have been added.[6][7][8] The question of which species concept is best has occupied many printed pages and many hours of discussion.[9]
The debates are philosophical in nature. One common disagreement is over whether a species should be 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 an organism belongs to. For many species, all of the individuals that fit the defining criteria also make up a single evolving unit, but it might not be known whether that is the case. These two different ways of thinking about species, as a category or as an evolving population, may be quite different from each other.
History
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 Ray 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.[10][11] 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.The history of definitions of the term "species"[12][13] reveal that the seeds of the modern species debate were alive and growing long before Darwin.
"The traditional view, which was developed by Cain, Mayr and Hull in the mid-twentieth century, claims that until the ‘Origin of species’ by Charles Darwin both philosophy and biology considered species as invariable natural kinds with essential features. This ‘essentialism story’ was adopted by many authors, but questioned from the beginning by a minority … when Aristotle and the early naturalists wrote about the essences of species, they meant essential ‘functions’, not essential ‘properties’. Richards pointed out [Richard A. Richards, The Species Problem: A Philosophical Analysis, Cambridge University Press, 2010] that Linnaeus saw species as eternally fixed in his very first publication from 1735, but only a few years later he discovered hybridization as a modus for speciation.[14]
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.[15][16]
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.[17] Dobzhansky's 1937 book[18] 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.
Biological species concept
Ernst Mayr's 1942 book was a turning point for the species problem.[5] 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, as Mayr makes clear in his book on the history of biology.[13] For example Mayr discusses how Buffon proposed this kind of definition of "species" in 1753.
Theodosius Dobzhansky was a close contemporary of Mayr and the author of a classic book about the evolutionary origins of reproductive barriers between species, which was published a few years before Mayr's.[18] Many biologists credit Dobzhansky and Mayr jointly for emphasizing the need to consider reproductive isolation when studying species and speciation.[19][20]
Mayr was persuasive in many respects and from 1942 until his death in 2005, both he and the Biological Species Concept 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.[21][22][23][24][25][26][27]
Some have argued that the species problem is too multidimensional to be "solved" by one definition of species or one species concept.[28][29] 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.[28][30][31][32][33]
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.[9][34][35][36] 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.[37]
It has also been argued that the BSC, based on reproductive isolation, is not only a useful preferred description of species, but is also a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.[38][39][40][41] (Also see speciation.)
Philosophical aspects
Realism and nominalism
Realism and Nominalism are philosophical subjects that come up in debates over whether 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 then follow that species do not literally exist, because species are named types of organisms. 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.[42][43]
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.[22][44] 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.[45]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.[21][46][47]
Jody Hey described the species problem as a result of two conflicting motivations by biologists:[28][48]
- to categorize and identify organisms;
- to understand the evolutionary processes that give rise to 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.[49][50][51]
David Hull argued that pluralist proposals were unlikely to actually solve the species problem.[33]
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)[1]"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[52]
"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[18]
"The concept of a species is a concession to our linguistic habits and neurological mechanisms" Haldane (1956)[45]
"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)[48]
"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)[21]
"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)"[53]
"We show that although discrete phenotypic clusters exist in most [plant] genera (>80%), the correspondence of taxonomic species to these clusters is poor (<60%) and no different between plants and animals. ... Contrary to conventional wisdom, plant species are more likely than animal species to represent reproductively independent lineages."[54]
