THE CHROMOSOMES AS FEEDBACK SYSTEMS IN EVOLUTION

All organisms carry their hereditary and evolutionary properties in the genetic code of their genotype. Changes arise in this genotype in unlimited variety and every change has effects at four levels: (i) the properties of the whole organism; (ii) its reactions with its environment; (iii) the differential or competitive survival of the organism in present or future environments; and (iv) by feedback, the survival of any other changes that may occur in the genotype. These are the principles of natural selection but they can operate efficiently only with regular means of propagation of the genotype. The necessary means have been provided during the last 1200 m. years by the chromosomes in two ways: (i) by mitosis giving the maximum certainty of propagation within each organism; (ii) by meiosis giving a controlled uncertainty of propagation through recombination of differences in the sexual reproduction of organisms. This recombination makes it possible for all changes which are not disfavoured to feedback through selection so as to favour other changes. If these arise or take effect at the same site they result in the well‐known evolutionary trends which are recognised either in the chromosomes or in the whole organism. In human evolution such trends seem to have arisen in the structure and behaviour of the organism as a result of genetic feedback from an increasing, and acceleratingly increasing, ability of man both to create and to destroy his environment.