The Law of Homologous Series in Variation for Systematics

Abstract

The article discusses the significance of Vavilov’s law of homologous series in variability for plant systematics. According to this law, the Linnean species was proposed to be considered as a dynamic system of ecological and geographical races—hereditary forms, all of them are variable across time and space under the influence of environmental factors and after mutations and recombination of alleles during hybridization in either nature or experiments. In nature, species can differ in many traits, but the spectrum of potential variability of morphological and physiological characters hidden in the genotype is almost the same in related species. Related genera and species differ only in a few significant characters (so called, radicals); they are the only taxonomically significant characters. Radicals and the genetic affinity of species can be revealed in experiments by disclosing the latent intraspecific phenotypic and genetic diversity of the species. The farther the species are from each other, the less overlap the variability spectra of their phenotypes and genotypes. It took time and the emergence of new effective approaches for assessing the genetic affinity of plants (such as karyosystematics, immunosystematics, GISH, and molecular phylogeny), to put at the service of plant taxonomy not just the study of morphological similarity but the study of characters of relatedness. The current stage of the study of parallel genotypic variability in plants is characterized by the fact that, as a result of advances in molecular phylogeny and comparative genomics, there is a real opportunity to study the phenomenon of morphologically indistinguishable but genetically heterogeneous (cryptic) species and to study the degree of genetic diversity and kinship of natural populations of morphologically polymorphic species with wide area of distribution.