Hybridization as a result of spatial and reproductive interactions between species within the group of ‘yellow’ wagtails (Aves, Motacillidae) in areas of sympatry

Abstract

The area of overlapping ranges, contact and relationships of two closely related species—the western yellow wagtail Motacilla flava Linnaeus, 1758 and the yellow-headed wagtail Motacilla lutea (Gmelin, 1774)—in the Volga region are the subject of the present study. At present, under conditions of wide sympatry, there is spatial and reproductive interaction between these closely related species—the western yellow and yellow-headed wagtails. Crossbreeding of species of ‘yellow’ wagtails gives a light-headed form of hybrids, which is a marker of the hybrid zone in the Volga region. As a result of the study of the interaction between M. flava and M. lutea in their contact zone in overlapping areas of species ranges, the following was revealed. The concept of a system of traits of the light-headed hybrid form (M. f. beema × M. lutea), marking the hybridization zone of the model species M. flava and M. lutea, is postulated. The population characteristics of this system have been determined and the possibility of their use in population and evolutionary studies of a group of species of ‘yellow’ wagtails has been shown. As a result of the spatial and reproductive interaction of species in populations, the light-headed hybrid form (M. f. beema × M. lutea) is formed, which integrates the characters of the two original species and has its own, peculiar traits, which can mark the boundaries between the western (left-bank) and eastern (right-bank) groups of populations and the boundaries of their distribution. On the basis of this, differences are formed and divergence of these groups of populations occurs, which may reflect the processes of sympatric speciation. The revealed tendencies in the variability of groups of characters in the space of the ranges of model species have a clinal character and may indicate the directions of microevolution of species. The identified hybrid zones within the boundaries of species ranges have a central position. The results of spatio-temporal divergence are M. flava, M. lutea, and their hybrid light-headed form M. f. beema × M. lutea, which may later become an independent species. In the contact zone of M. flava and M. lutea, there is an unlimited hybridization of these species.