Homologous Series, Law of

law of variability of homologous series, a law, formulated by the Soviet scientist N. I. Vavilov. that establishes parallelism in the variability of organisms.

C. Darwin was aware of the far-reaching parallelism in the variability of allied species and genera of animals and plants as early as 1859–68. In the 19th and early 20th century a number of botanists and zoologists made special studies of the parallel variability of various plant and animal species (for example, the French scientist M. Duval-Jouve. 1865; the Swiss mycologist E. Fischer, 1896; the German botanist E. Zederbauer, 1907. 1927; and the Russian zoologist V. M. Shimkevich, 1906, 1921). The Soviet geneticist Iu. A. Filip-chenko summarized (1922) a number of these (mostly zoological) findings in an article on the parallelism of variability in living things, which he ascribed to the taxonomic and phylogenetic closeness of the genera and species in the groups studied.

Only Vavilov approached the question of the parallelism in variability of allied species and genera from a genetic standpoint and on the basis of a comparative study of very extensive worldwide data (under natural conditions, in cultures, and in experiments) of the variability of a number of plant families abounding in well-studied cultivated species (chiefly grasses). As a result of this work, he read a paper, “The Law of Homologous Series in Hereditary Variability,” at the Third All-Russian Congress of Breeders, held in Saratov in 1920. Vavilov showed that if all the variations known to exist in the most studied species of a given group are arranged in a table in a certain order, almost all the same variations in variability of characters will be found in other species as well. Moreover, as research continues on the species belonging to a given group, the “empty” places in the table will be filled and the parallelism in variability of the allied species will become increasingly complete. Fundamentally similar but less pronounced parallelism characterizes the variability of different genera within families and, even less completely, of different families within a group of higher rank.

Thus, the law of homologous series can be reduced to the statement that allied species, because of great similarity in genotype (almost identical sets of genes) posses similar potential hereditary variability (similar mutations of the same genes). The parallelism of hereditary variability becomes less complete with increasing evolutionary-phylogenetic distance between the groups studied (taxa) because of emerging genotypic differences. Therefore, parallelisms in hereditary variability are based on mutations of homologous genes and portions of genotypes in representatives of different taxa (that is, genuinely homologous hereditary variability). However, even within the same species, externally similar traits may be produced by mutations of different genes. Such phenotypically parallel mutations of different genes may also appear, of course, in different but fairly close species. Vavilov emphasized that the law of homologous series also necessarily embraces this not strictly homologous (in the genetic sense), but phenotypically parallel, variability.

After 1920 the members of Vavilov’s school in the USSR as well as foreign botanists and breeders collected a vast amount of factual material that confirmed the universality of the law of homologous series.

At first, research was directed mainly at morphological traits; it was later extended to biological, physiological, and biochemical properties. Numerous confirmations of the law of homologous series were obtained through studies of protozoans, lower plants, many families of higher plants, and animals.

The law of homologous series reflects a universal and fundamental phenomenon in nature. It has vast practical importance in plant raising and selection as well as in animal husbandry. On the basis of this law, plant and animal breeders can systematically seek and find the desired characters and variants in different species in the almost infinite world variety of cultivated plants and domestic animals and their wild relatives. This search, especially among cultivated plants and their wild ancestors, is greatly facilitated by Vavilov’s theory (1926) of the centers of origin of cultivated plants and by his work (1927, 1928. 1930) on the geographic patterns of distribution of genes of cultivated plants. In the 1930’s, Vavilov’s law of homologous series became a powerful stimulus to planned selection, the creation of new varieties of cultivated plants, and the establishment of a scientific basis for the introduction and acclimatization of these varieties. The law of homologous series still plays an important role in studying the mechanisms of the evolutionary process, in the interpretation of some biogeographic phenomena, and in working out the principles for a modern classification of the lower taxa.