Reproductive isolation and the 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. Thus, the main question of the "species problem" is as follows: do species really exist in nature or not? The answer of this question depends on the "system of coordinates" that is used for the description of species. Species inhabiting the same territory ("one-dimensional species") are usually discrete. However, when we begin to describe closely related forms within their ranges ("two-dimensional species"), the determination of the interspecific border becomes problematic. Therefore, speciation can be defined as the process of establishing the sympatry between diverging taxa, and species can be defined as morphologically distinguishable groups of organisms that can coexist in the sympatry zone for a long time. The diversity of the existing species concepts can be reduced to the two main ones-biological and phylogenetic. The latter has a disadvantage compared to the former because it does not deal with the theoretical substantiation of the species reality. But within the framework of the concepts of biological species, the theoretical substantiation of species reality (the hypothesis of the reinforcement of isolating mechanisms in a hybrid zone) has no reliable empirical support too. Another substantial disadvantage is that the use of the biological concept in practice may lead to misinterpretation of the taxonomic status of forms at different stages of the speciation process. At the initial stages of divergence, taxa can be regarded as independent species (for example, ecological races of the red crossbill (Loxia curvirostra), whereas at the final stage of speciation, forms are regarded as conspecific due to their hybridization in the sympatry zone (the yellowhammer Emberiza citrinella and the pine bunting E. leucocephala). The "species problem" seems to be solvable using a synthesis of the biological and phylogenetic concepts of species.