The earlier proposed ecogenetic hypothesis on the origin of the curly (Karelian) birch is further developed and detailed based on the published data and our own data. This hypothesis is based on the concept of the interaction of specific natural climatic factors and the conditions that had been established in the northwest of continental Europe during the so-called Little Ice Age with a high population genetic polymorphism and adaptive character of many anatomical and morphological traits and specific physiological and biochemical features of the Betula L. species growing in this area. Various facts and observations favoring the ecogenetic hypothesis of the origin of the Karelian birch are described.