Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

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442 POSTSCRIPT terms of comparative biochemistry, one must ask to what extent the evolution of these reactivities, reaction patterns and competences is con¬ ditional upon the evolution of methods of synthesis of new proteins, etc., and to what extent the proteins, etc., are always within the synthetic competence of an organism. In the latter case evolution is the history of changing uses of molecules, and not of changing synthetic abilities. Much the same view emerges from consideration of the chemical aspects of vision, discussed by Willmer. The fact that certain chromoproteins appear in the visual organs of certain groups of vertebrates is no evidence that the rate or course of evolution has been conditioned by the difficulties of synthesizing these compounds. Similar chromoproteins are found in a variety of organisms much more primitive than the vertebrates. Indeed, it is by no means certain that the production of such chromoproteins represents a difficult innovation at any stage of evolution. Many cellular activities appear to be brought about by contractile proteins, energy for an activity being made available by an enzymic centre associated with the contractile protein. The actomyosin-ATPase complex is at present the best explored example of such a system. The chromoproteins which act as photoreceptors may be no more than contractile proteins functioning with photon-absorbing groups instead of catalytic centres, so that light energy may be substituted for chemical energy. It is difficult to believe that this relatively minor chemical innovation could condition the rate of evolution. But the production of a physiological complex which can use chromoproteins might well be a rate-controlling process. II. CHEMICAL EVOLUTION OF GENES If organisms are able, in the course of their evolution, to produce ' at need ' all of a vast repertoire of compounds, it follows either (a) that the necessary genes are always available, or else (b) that the production of the necessary genes is more readily accomplished than is the evolution of reactivities, reaction patterns, morphological innovations, etc. It is only in such circumstances that the rate of evolution will not be controlled by the rate of evolution of chemical processes. These postulates appear to be com¬ patible with what we know of the function of genes. We cannot accept the 'one gene: one enzyme' theory in its simplest form, but it may be that in a form such as 'one gene:one protein' or 'one gene:one high polymer', or even 'one gene:one of a certain range of high polymers', we shall find a satisfactory theory. Thus in support of the hypothesis (a) we have the well-known fact that genes may be present without displaying their action, and in support of (6) we have evidence rapidly accumulating that genes