Conventional & Newer aspects in Taxonomy

M.RajAssociate ProfessorDarrang CollegeConventional & Newer aspects in Taxonomy

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Procedure in taxonomy, (conventional or recent) Based on taxonomic characters

Taxonomic charactersorganismal traits used as evidence for species discovery.

While on one hand the total number of species to mankind has increased by leaps and bounds from the time of Linnaeus, so have the methods by which we today characterize, recognize and identify species. Charactersbiochemical, molecular, morphological, behavioral, ecological

qualitative or quantitative

fixed statesor be polymorphic

Misof et al. (2005) hold the view that without any doubt, the incorporation of as many different character sets as possible into a species taxon description will improve the fit between species taxa and real evolutionary units.

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Morphological approachMorphological characters includeThe study of form Internal structure of organismsExternal structure of organismsCharacteristic shapes, size, coloration and pigmentation patterns, and special external featuresTechniquesScanning electron microscopy, Silver impregnation techniques, Ultraviolet reflection patterns etc.ProtozoaTests, shells, thecal plates, cystsEchinodermsOrientation types of calcite crystals in the skeletons Fishes and reptilesScale counts BirdsPlumage, Color pattern MammalsPelage characters , The skull (with teeth) ButterfliesColor pattern Genitalia of many animals

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Disadvantages: The use of morphological characters however has certain disadvantagesThere is always a subjective component when defining and interpreting character statesThe morphology in certain taxa does not supply sufficient charactersExternal morphology is sometimes misleading because of special adaptationsThey are unsuitable for some groups of organisms, either because speciation occurs without morphological change, which leads to morphologically cryptic species, or because morphological structures are labile or difficult to study--e.g. as in prokaryotesMorphology reflects only part of the genotype and may not reflect genetic relationship accuratelyMorphological characters of adult specimens however are still used more frequently than are any others, but they are supplemented to an increasing extent by other characters such as those derived from the study of proteins, chromosomes, behavior etc

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Embryological approachEmbryology strictly refers to the study of the development of the embryo and the structure of the mature embryoThe most significant and important feature is the high degree of correlation amongst embryological characters as well as the fact that embryological characters are conservative.Eggs Embryos Immature or larval stages

Egg structure Anopheles spp. complex into a number of sibling species. Animal groups whose morphological traits are less reliableSpongesPupaeClassification of the Aleyrodidae (whiteflies) Egg capsule morphology A valuable aid to the classification ofPhasmatodeaLarval and adult characters are visible manifestations of the same genotype. There can be different identification schemes for larvae and adults, but there can be only one classification for a given group of organisms, and this classification must be based on the proper weighting of both adult and larval characters. Sometimes it is the adults that have acquired specialized adaptations, and sometimes it is the larvae; only a biological analysis can lead to the proper evaluation.

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Ecological approachIt is now well established that every species has its own niche in nature, tending to differ from its nearest relatives in food preference; habitat selection, breeding season, tolerance to various physical factors, altitudinal distribution, resistance to predators, competitors, and pathogens; and other ecological factors.When two closely related species coexist in the same general area, they avoid fatal competition by means of these species-specific niche characteristics. Many aspects of the life cycle, such as life span, fecundity, and length or time of breeding season may be different in closely related species. Example 1: Kohn (1959) found that every species of the genus Conus a sea snail, a marine gastropod mollusk in the family Conidae in the Hawaiian Islands differs ecologically from related species. Two sibling species (C.ebraeus and C.chaldaeus) feed on nereid polychaetes, but had specific food preferences for different species of the polychaete as shown in Table.

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Example 2: The Anopheles maculipennis complex could also be broken down into six independent species on the basis of ecological differences

Example 3: Host differences may also signal species differences. There are several cases of cryptic species associated with different hosts. For example, Trioxys utilis Muesebeck, an aphidiine Braconidae introduced into California against the spotted alfalfa aphid, Therioaphis maculata (Bucktron), is very similar morphologically to T. pallidus (Haliday), introduced for control of walnut aphid, Chromaphis juglandicola (Kaltenbach). Both species were considered conspecific for a time (Rosen 1986). Example 4: A number of sibling species were discovered as a result of discrepancies in food preference (host specificity), for example the apple and blueberry maggot or habitat preference

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Behavorial approachBehaviour is one of the most important sources of taxonomic characters. Why?: Behavioural characteristics are the most important isolating mechanisms in most animals, and new adaptations are often initiated by changes in behavior.Aspects of behavior studied in taxonomy: Sound recordings, web patterns, bioluminescence patterns, use of extraneous materials in construction of nest etc.A great technical advance in the study of behavior has been the development devices for accurate sound recording and the translation of sounds into graphic patterns (the sonograph).Example 1: Fireflies of genus Photuris 18 sibling species discovered on the basis of height and length of the marks indicating intensity and pattern of flashes.Example 2: Crickets, acoustic mating signals or calling songs A. More than 40 species of North American crickets were either discovered or rediscovered by Alexander (1962) as a result of a careful analysis of their songs.

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B. Recently Jaiswara et.al. (2012) undertook cluster analysis of the acoustic data of calling songs of males of the field cricket Itaropsis sampled from four sites in peninsular India. They concluded that the field cricket currently described by only one valid species, Itaropsis tenella, with a broad distribution in western peninsular India and Sri Lanka could actually be represented by three morphologically cryptic incipient species in peninsular India.Example 3. Birds discovered because of their distinct vocalisationsA. Two new species of Andean pygmy-owls, Subtropical Pygmy Owl Glaucidium parkeri (Robbins & Howell 1995) and Cloud Forest Pygmy Owl G. nubicola (Robbins & Stiles 1999) B. The observation that there were two markedly different vocal types, and pronounced differences in display, in what was originally believed to be Suiriri Flycatcher Suiriri suiriri led to the discovery of the Chapada Flycatcher Suiriri islerorum Example 4. Other behavioral elements of taxonomic valueA. Use of extraneous materials in the construction of nests or of larval or pupal cases provides characters at various levels in the classification of caddisworms and hagworms.B .Various species of termite Apicotermis are separated on the basis of structure of nests

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C. Web structure is diagnostic for certain genera or even higher taxa of spidersD. Ability to attach foreign objects (shells, corals, sea-urchins fragments, man-made items, etc.) to the dorsal part of the shellDrawbacks: Even though behavioral characters are often clearly superior to morphological characters in the study of closely related species, particularly sibling species (Mayr 1963). The use of behavior in taxonomy is beset with certain drawbacks. Behavior cannot be studied in preserved material It is intermittent even in a living animal Some types of behavior occur only during the breeding season Other behavior types occur only during part of the 24-hour period Mollusca genus Xenophora

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Geographical approachEach taxon from the species to the highest taxon, has a geographic range. Since this range changes as the taxon evolves, an intimate relation exists between systematics and biogeography. Patterns of distribution often provide decisive clues for clarifying a confused taxonomic picture and for testing taxonomic hypotheses.Geographic characters are important on two levels. a. At the level of microtaxonomy: In microtaxonomy the sympatric-allopatric relationship of populations is often decisive in the determination of species status. Two sympatric populations in reproductive condition can never be conspecific. The mapping of populations, subspecies, and allopatric species is indispensable in the delimitation of polytypic species and superspecies. b. At the level of macrotaxonomy: In macrotaxonomy an understanding of the relationship of higher taxa is often helped by an analysis of their distributions. Two questions are particularly important: What is the distribution of the nearest relative (the sister group) of the studied taxon? If the taxon has a disjunct range, what is the probable cause of the range disjunction? A knowledge of the Earths history and the geological record often permits one to decide whether the isolation is primary or secondary and more important, for how long a time the two isolates have been separated. The distributions of most taxa display patterns that permit conclusions about their past history

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