Polytypic Species Concept

M.RajAssociate ProfessorDarrang CollegePolytypic Species Concept

IntroductionSpecies that contain two or more subspecies are called polytypic speciesSpecies that are not subdivided into subspecies are called monotypic speciesRecognition of the fact that many species taxa particularly widely distributed species are polytypic was one of the most important developments in taxonomy. To convert the nominal species of all groups of animals into well- delimited polytypic species taxa is one of the major tasks of taxonomy. A number of other technical terms such as formenkresis (Klein- Schmidt) and rassenkresis (Rensch) were formerly applied to polytypic species but did not become established

Importance of the Recognition of Polytypic Species Taxa The shift from the morphological (typological) to the biological species concept necessitated a drastic change in taxonomic practice.While previously every population that could be diagnosed morphologically was called a species, it became necessary to infer the reproductive isolation of populations. The discovery of the geographic variation of morphological and other characters led to the realization that mans previously recognized morphospecies were not reproductively isolated from other allopatric populations and did not qualify as separate biological species. They were consequently ranked as subspecies in more or less widely distributed polytypic species.

A major benefit derived from the recognition of polytypic species taxa is the considerable simplification it has made possible in the classification of such well-studied groups of animals as birds, mammals. butterflies, and snails. The reclassification into polytypic species of geographically representative forms that originally were separately described as monotypic species led to a great simplification of the system. The 19000 monotypic species of birds listed in 1910 together with thousands of species described since then, have been reduced to approximately 9040 species.A similar simplification has been reported for many other groups of vertebrates and invertebrates. This reorganization of classification on the species level is virtually complete in birds, mammals, and some groups of insects and land mollusks but has hardly begun in most other groups of animals. Importance of the Recognition of Polytypic Species Taxa

Of much greater significance than this practical benefit is the restoration of a definite biological meaning and homogeneity to the species category. Awarding species rank to every local population no matter how slight its difference, destroys the biological significance of the species category.

Importance of the Recognition of Polytypic Species Taxa

The assembling of local populations into polytypic species or more broadly, the sorting of large numbers of nominal species and varieties into polytypic species reveals many taxonomically and biologically interesting situations (Mavr 1963:343)It has provided the best available evidence for the process of allopatric speciation the frequent origin of evolutionary novelties in peripherally isolated populations and numerous intermediate stages in the evolutionary process, thus elucidating previously inexplicable discontinuitiesGeographic variation, particularly in peripherally isolated populations, helps bridge the gap between microevolution and macroevolution.Some of the best proofs of the occurrence of evolution have emerged from the study of polytypic species taxa. Importance of the Recognition of Polytypic Species Taxa

Difficulties in the application of Polytypic Species Concept In establishing polytypic species the taxonomist encounters three sets of difficulties. Polytypic species are composed of allopatric or allochronic populations that differ from one another. However, all populations of sexually reproducing organisms differ slightly, and certain standards must be met before subspecies can be recognized.A second difficulty is thatclosely related species with similar ecological requirements occasionally replace each other geographically and it is difficult to decide whether they are full species or subspecies.Finally, many isolated populations are in the middle in the process of evolving into new species and are on the borderline between subspecies and species status.

Occurrence of Polytypic species in Animal KingdomThe frequency of polytypic species differ from animal group to animal group. They occur most frequently where species are not contiguously distributed.Polytypic species are also scarce or absent in groups with slight species differences (e.g. group of sibling species).

In most well-studied group of animals between 40 and 80 percent of the species are polytypicSome highly specialized groups particularly certain host plantspecific insects do not readily form polytypic species.

ConclusionThe polytypic species is in a sense the lowest of the higher categories. Being multidimensional it lacks the simplicity and objectivity of the non-dimensional species.Most of the difficulties involved in delimiting species of animals occur when it is doubtful whether two allopatric populations belong to the same polytpic species. Among birds, such borderline cases are few. The claim that such difficulties are more common in other groups of animals awaits verification.