THE CONCEPT OF SPECIES AND “MICROSPECIES”

Abstract

Many of the large families of Angiosperms, including the Gramineae and the Leguminosae, are of considerable morphological complexity and are considered to be difficult for the taxonomist. Such families are broadly sub-divided into sub-families and tribes. The large, unwieldy genera, under the tribes, are further split up into smaller units such as sub-genera, species, sub-species, varieties and forms. The term 'species' has been variously defined. According to John Ray, probably the first to define species in his Historia plantarum of 1686, 'a species is group of plants which breeds true from seeds within its own limits' (with distincta propagatio ex semine). This concept of the species has been endorsed generally by students of taxonomy. The category ecospecies, widely accepted from the ecological standpoint, was defined by Turesson (1922a, b, 1925) as group of plants comprising one or more ecotypes whose members are able to interchange their genes without detriment to the offspring. An essentially similar definition was given by Clausen, Keck and Hiesey (1945). In general, the ecospecies are considered equivalent to the conventional, Linnaean species; they usually inhabit different, though often contiguous, ecological or geographical areas. The isolation mechanism preventing free gene exchange between ecospecies stems from ecological barriers which help them retain relative genetic purity. From the point of view of student of genetics, Emerson (1945) defined species as a genetically distinctive, reproductively isolated, natural population. He elaborates this by saying that the genetic difference may be morphological, physiological or behaviouristic. The isolation, by whatever mechanism, effectively prevents interbreeding with other populations. An almost similar definition has been given by Stebbins (1950), who considers species as natural units separated from each other by gaps of genic discontinuity. This genic discontinuity involves morphological as well as physiological characteristics which are maintained by the absence or rarity of gene interchange between members of different species. The cytotaxonomist, or the biosystematist, according to Lawrence (1951), strives to determine objectively whether particular taxon deserves the rank of species or is of infraspecific level. In this approach, apart from morphological and histological considerations, emphasis is placed mainly on crossability between different taxa and on the homologies of the chromosomes in the hybrid, with view to determining the reproductive isolation barrier, if any, and the degree of phylogenetic relationship. Some of the species are rather complex, heterogeneous assemblages and do not easily lend themselves to taxonomic treatment. Apomictic species introduce further complication in the task of the taxonomist. In the apomictic complexes, apart from morphological polymorphism, the occurrence of intra-specific chromosome races complicates their delimitation and taxonomic classification. Apomixis has been found to occur in widely different groups of plants, including the cryptogams (Miintzing, 1961). The Gramineae, cosmopolitan and one of the largest families among the angiosperms, are characterized by relatively high incidence of apomixis (see McWilliam, 1964). As regards the taxonomic treatment of the agamic, polyploid complexes, different viewpoints have been expressed. In such taxa the apomictic mode of reproduction not