“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 may be useless for species that reproduce asexually, such as bacteria.”
The problem of accurrately defining what a 'species' is goes back at least as far as the 1870s. “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.” : from H. A. Nicholson, A manual of zoology. 1872.
Even given the extreme accuracy of gene profiling now available to taxonomists, many are resigned to the fact that an accurate definition of what a species actually is may never be practical.
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