The Taxonomic Units in Relation to Cytogenetics and Gene Ecology

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Abstract
No universal definition of species is possible. This term, "species," cannot mean the same category in bacteria and mammals. Dobzhansky''s naming of microspecies of Drosophila as "species" even when there is no morphological difference, results from his use of reproductive isolation as the sole criterion of species. Intersterility may arise early or late or not at all in the development of spp. When a sp. produces another in geological time, no intersterility is involved. Its origin in contemporary spp. is incidental, and various forms of sterility can arise within a sp. In speciation, two independent streams of change can occur; (a) gene mutations, (b) transloca-tions and other chromosome changes, generally without phenotypic change. Both are concerned in evolution, as nature selects the whole organism. Various subspecific categories are considered, including subspecies and ecotypes, with numerous examples from recent transplantation experiments. Different definitions of species are given, as they apply to different plant genera. The role of isolating mechanisms is discussed, and cases cited where spp. separated for millions of years are still interfertile although they show many genic differences. Many cases of specific hybridization in plants and animals are cited, showing that intercrossing over long periods does not necessarily lead to merging of spp. Interfertility is not a sufficient reason for uniting two spp. which show many morphological differences.