REPRODUCTIVE ISOLATION AND A NOTION OF SPECIES IN BIRDS

Biological diversity is continuous as a result of evolutionary processes, whereas the language for its description is symbolic and, therefore, discrete. So, the main question of the "species problem" is that: do species really exist in nature or not? The answer of this question depends on a "system of coordinates", which is used for the description of species. The species inhabiting the same territory ("one-dimension species") are usually discrete. However, when we begin to describe closely related forms within their ranges ("two-dimension species"), the determination of interspecific border becomes problematic. One can define speciation as a process of establishing the sympatry between diverging taxa and species, as morphologically distinguished groups of organisms that can coexist in the same territory for a long time. The diversity of the existing species concepts may be reduced to the two main types - biological and phylogenetic. The latter has a disadvantage as compared to the former because it is not deal with the problem of species reality. Within the framework of biological species concepts, the theoretical substantiation of species reality (the reinforcement hypothesis) has no reliable empiric support. Moreover, the practical use of biological concepts may lead to a misinterpretation of the speciation process. At the initial stages of diversification, forms can be described as independent species (for example, ecological races of the red crossbill, Loxia curvirostra), whereas at the final stage of speciation, they are regarded as conspecific ones due to their hybridization (yellowhammer Emberiza citrinella and pine bunting E. leucocephala). The "species problem" seems to be solved using the synthesis of biological and phylogenetic species concepts.