The Concept of Bird Species: Theory and Practice

Abstract

As it has presently become evident, reproductive isolation can no longer be considered as the leading criterion for estimating the species limits, but it is still used for determining the status of the majority of morphs. The presence/absence of hybridization between individuals is not directly related to the degree of their evolutionary insularity. Hybridization can result in the origin of new morphs including those that have species status. The application of phylogenetic methods is justified in reconstructing the relation links within complex groups including the morphs of various evolutionary levels (from geographical races to “good” species), the relationships between which are exacerbated by hybridogenous polymorphism and/or the hybridization origins of their populations. Taking into account both new data and new conceptions, we suggest genuine interrelated definitions of concepts such as species, subspecies, and semispecies in birds. The definitions are based on two main criteria: biological, i.e., an evaluation of the reproductive relations of the particular morphs with each other, and phylogenetic, i.e., an evaluation of their evolutionary age and kinship. The main feature of a species as an evolutionary entity should be considered through its stability in time even when its reproductive isolation is periodically broken. Geographic intraspecies races show sustainable variations of different degrees, but they have no reproductive isolation; they breed upon contact and form intergradation zones. They are taxonomically denoted as subspecies. Descriptions of new subspecies are viable to the limits that reflect the species’ natural geographic structure to the fullest extent. For the young morphs that have reached the level of species insularity in the course of evolution, it seems appropriate to restore a semispecies category. Semispecies show significant morphological differences and distinguished ecological particularities, as a rule, but they are connected to closely related morphs by gene flows in contact zones. Distinguishing this category is not regulated by the International Code of Zoological Nomenclature and, as splitter tendencies prevail in modern systematics, semispecies are more often equated to species; i.e., they have binominal names. We propose to denote the attribution of a semispecies to a particular species group (superspecies) in parentheses between genus and species names. Thus, it would become possible to outline natural complexes and avoid a groundless increase in the taxonomic statuses of morphs in the stage of development. Representatives of distant phylogenetic lines (morphs that separated historically long ago) are not to be considered as semispecies even in the cases of their reproductive isolation being broken and a steady hybridization existing between them.