Genus Betula L.: Species-Specific Population-Genetic Features and Taxonomy Problems

The article summarizes and systematizes the results of studies by Russian and foreign authors related to the population-genetic features of the main members (silver birch Betula pendula L., downy birch Betula pubescens Ehrh., curly (or Karelian) birch Betula pendula Roth var . carelica (Mercklin) Hämet-Ahti) of the birch genus ( Betula L.) growing in the forest zone of Europe. Information is provided about the formation of birch populations in the region and about its migration pathways when recolonizing the territory after the Ice Age. We demonstrate that due to certain geographic and climatic affiliations there appeared zones of hybridization between different birch species, which had an effect on the subsequent evolution of this genus. Attention is given to the role of hybridization in shaping the genetic structure of the birch population in the northwestern part of continental Europe, where introgression has generated unusual genotypes and haplotypes, among which curly birch has probably become differentiated. We argue that the introgressive hybridization of species observed now and then in the birch genus may be of the main reasons for the problems with definite taxonomic identification of silver birch and downy birch. It is also remarked that curly birch, although meeting the conventional biological criteria of a species, is still regarded a variety of silver birch. Having analyzed the population-genetic features of members of the genus Betula L . , the authors conclude that the species status of silver birch and downy birch should be retained in spite of the identification difficulties and that instating curly birch as a separate biological species is advisable. We emphasize the importance and relevance of studying the population-genetic features of both common and rare members of the Betula genus to enable the development of efficient methods and practices of their selective breeding and reproduction of the most valuable genotypes as a solid scientific foundation for sustainable forest management.