BIOTYPE. — By b. is meant, in genetics, a group of individuals identical among themselves in their hereditary character traits. This concept arose from the investigations of Johannsen (1903), who, by causing bean plants to reproduce through self-fertilization, was able to isolate a certain number of so-called pure lines, i.e., groups of plants descended from a single parent and thus possessing the same hereditary endowment; by statistically examining various traits (such as, for example, the weight and dimensions of the seeds), he noted that these showed different average values for each pure line, which remained constant in subsequent generations as long as the different pure lines were not crossed with one another. In a similar manner, Jennings (1908), in certain protozoa (Paramaecium), selected, through asexual reproduction (fission), a certain number of clones, or groups of animals that, all descending from the same parent, were perfectly homogeneous from the hereditary point of view; the traits of these clones also had characteristic and constant average values. Pure lines (obtainable in plants or animals with sufficient hermaphroditism) and clones (obtainable instead in organisms that reproduce asexually or through parthenogenesis) are likewise biotypes, according to the definition given above.
Every natural population consists of many biotypes, mixed together as a result of sexual reproduction, which is accompanied (in fertilization) by the fusion of the hereditary endowments of the two parents, frequently belonging to different biotypes. Selection, however practiced, can separate the various biotypes from one another, which remain distinct if further heterogeneous crosses are avoided; but within a biotype, selection has no further effect. From the systematic point of view, the biotype may be considered the smallest unit of subdivision of an animal or plant species, whose races or varieties are also almost always mixtures of biotypes.