Evolution, Genetics, and Man

By Theodosius Dobzhansky | Go to book overview

8
Species

Darwin great book was entitled The Origin of Species (the full title was rather more ponderous: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life). Darwin knew as well as anybody that formation of races and species is only a part of the grand story of evolution. The origin of species, however, was of crucial importance in Darwin's day, because of the current view that species were created entities which could not be produced by natural processes.

In a different way, species formation is also regarded as a critical stage of the evolutionary process in modern theories. Races of a species are populations which can, and often do, cross and exchange genes. Hybridization of races may, and sometimes does, lead to their fusion in a single population. This has actually happened, for example, to some human races, the members of which intermarried so frequently that the races disappeared as separate Mendelian populations. Races are genetically open systems, and the divergence of races is a reversible process; it can be undone by hybridization. Species, on the other hand, are genetically closed systems, since they exchange genes rarely, or not at all. Evolutionary divergence of Mendelian populations tends to become irreversible once the species level is reached. For example, man and chimpanzee are most unlikely ever to exchange genes or to form a hybrid population.

A minority of modern evolutionists, among whom Goldschmidt is most prominent, believe that the known factors of evolution--mutation, gene recombination, selection, and genetic drift--account only for "microevolution," which is usually equated with race formation. Other,

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