Sibling species poses a challenge to taxonomists. Morphologically identical, sympatric or allopatric populations, which are reproductively isolated, are called sibling species. Sibling species are common in insects.
A well studied example of sibling species is found in Drosophila in which most species complexes contain groups of sibling species. D. pseudoobscura and D. persimilis are established as sibling species. Anopheles maculpennis, A. atroparvas, A. labranchiae, A.sacharovi, A. messae and A. melanoon are described as A. maculipennis group of sibling species.
Second problem is posed to taxonomists by asexually reproducing organisms because criterion of interbreeding cannot be used in their cases. For example, some taxonomists recognize 200 species of blackberry, while others recognize only 20 species. Asexual reproduction in unicellular organisms makes their classification difficult.
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We are primarily concerned with large or conspicuous organisms of temperate regions, although biodiversity is reported to be the greatest among small, inconspicuous organisms inhabiting the tropics. Taxonomists have overlooked a number of species on land and in shallow water. Presently our knowledge about deep sea biodiversity at depths of 3250 feet are insignificant. In this connection a few lines are worth mentioning.
“The task of collection, inventorisation and description of unknown species is stupendous as we neither have enough resources nor expertise to cope up with this problem. Further, the taxonomists themselves are a “rare breed” or an “endangered” lot and their number is declining day by day.
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All over the world, the charm and glitter of modern biology has pushed taxonomic research to a very low level. The “old guards” of taxonomy have either retired or are retiring, many others have turned their backs on taxonomic research because of obvious and justifiable reasons, and unfortunately new talents are nowhere in sight mainly due to lack of employability of taxonomists. All these have made the problem of inventorisation very difficult” (Jairajpuri, 1996).