Species problem Totally Explained

Everything about Species Problem totally explained

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". 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.

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's already named and described, is found to actually include two slightly different kinds of organisms, it's necessary to use the rank of subspecies.
   Even though it isn't disputed that species is a taxonomic rank, this doesn't 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 wasn't 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 (External Link

). The question of which species concept is best has occupied many printed pages and many hours of discussion
   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 . 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 .
   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're 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's 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 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 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 .

Philosophical aspects

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've 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 isn't 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, don't literally exist.
   If the nominalist view is correct then kinds of things, that people have given names to, don't literally exist. It would follow then that because species are named types of organisms, that species don't literally exist. This can be a troubling idea, particularly to a biologist who studies species. If species are not real, then it wouldn't 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 isn't 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 David Hull 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 isn't primarily an empirical one, but it's rather fraught with philosophical questions that require-but can't 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)"

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