Species Concepts and Speciation Genetics: Examples from Drosophila
Résumé
Speciation genetics explores the genetic mechanisms and processes underlying the formation of new species, focusing on how reproductive isolation and divergence arise through genetic changes. The term speciation was coined by Cook in 1906. Basically speciation involves two processes: Anagenesis and cladogenesis (true speciation). Anagenesis means phyletic change in the course of geological time or the transformation of old species into a new one in due course of time i.e. transformation in time. Cladogenesis or true speciation means origin of new species of organisms through splitting of the pre-existing ones. During the phyletic change in the process of anagenesis, there is no role of reproductive isolation because one species is transformed into another one. However, during cladogenesis, one species splits into two or more populations, they gradually accumulate genetic changes through the action of mutation, recombination, selection, genetic drift and migration and when they become geographically isolated, develop reproductive isolating mechanisms and acquire the status of new species. The origin and development of reproductive isolating mechanism are considered as pre requisite for the process of speciation. It has been rightly remarked that based on Biological Species Concept, the question should be asked how reproductive isolating mechanisms are established instead of asking how new species evolve. In the process of speciation, three components are involved: mechanisms generating genetic variability, origin of reproductive isolating mechanisms and geographic component (ranging from allopatry to sympatry). Species is a Latin word which means kind. Species is a basic unit as well as considered as basic category of biological classification which has been defined in different manner under different concepts of species. Initially the first species concept was morphological/typological/essentialist species concept which was explained by Linnaeus in eighteenth century but it was finally discarded because of morphological variations within the species and the occurrence of sibling species which are morphologically similar but reproductively isolated. Later, in seventeenth and eighteenth century, there was emergence of idea of reproductive relationship by Ray and Koelreuter which paved the way for Biological species concept. Darwin also gave importance to biological species concept because he also believed in the role of reproductive isolation in speciation. In twentieth century, the biological species concept was elaborated and developed in detail by Jordan, Mayr and Dobzhansky. Under this species concept, the species is defined as a group of potentially or actually interbreeding individuals which are reproductively isolated from other such groups. However, it has certain difficulties in its application. de Queiroz argues that there is single, primary species concept that is adequate –applying across biodiversity and that is the general lineage concept. Recently, a new species concept, genic species concept has been proposed by a molecular evolutionary biologist Wu who severely criticized biological species concept and suggested that it should be abandoned. According to genic species concept, it is not the whole genome, but a gene is unit of speciation. Thus speciation genes are responsible for creation of new species or cladogenesis. In Drosophila, hybrid male sterility has been frequently studied and the genes involved are known to play role in speciation. It has also been argued that there are two models, with the respect of number of loci involved in hybrid sterility. The first model states that there are a number of segregating units (polygenes) involved in hybrid sterility each with small effect. In the second architecture, it is suggested that there are one or a few genes of large effect involved in hybrid sterility leading to speciation. In this review, while taking the examples of hybrid sterility from Drosophila, an excellent biological model, speciation genetics as an emerging area of evolutionary biology is discussed keeping in view the biological species concept as well as genic species concept of species.