Evolution.

Society for Experimental Biology

Date:: 1953

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

Credit: Evolution. Source: Wellcome Collection.

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446 POSTSCRIPT at least as promising as a means of provision of new genes as the violent and abrupt changes attributed to raditions. Cellular changes leading to malignancy may well be produced in the same fashion. III. ADAPTATION AND EVOLUTION Resistance to the action of drugs may be attributed to at least two mechanisms which appear to be distinct. One is that occasional individuals have a high resistance to the drug, or readily mutate so as to become resistant, so that a resistant population develops from the multiplication of these few resistant individuals. This is the explanation which has been commonly invoked, and shown to be true in certain instances. The second mechanism involves a direct modification of the greater part of a cell population. Hinshelwood's article in this volume shows that such adaptation certainly occurs in some instances. The mechanisms involved in the adaptation of the greater part of a population to a chemical change in the environment may or may not be of direct evolutionary significance. But the possession of a high degree of ability to adapt in this manner must add greatly to the chances of survival, particularly for unicellular organisms, and so must constitute a mechanism of great indirect value in evolution. Biochemically adaptation may involve the use of existing high polymers (including enzymes) for new purposes, or the production of new high polymers to cope with the new situation. Or both factors may be involved. How can we envisage such changes taking place in a directed manner, rather than at random, so as to secure the adaptation of a large part of a population? There are so many ways in which adaptation could occur that I propose to limit my discussion to one example—the development of new active transport mechanisms. Active transport mechanisms play a vital part in pumping substances into, or out of, cells. For example, glucose and other sugars diffuse into cells only very slowly; for these substances to become available inside a cell in reasonable amounts, active transport across the cell membrane is necessary. If a cell living on glucose is transported to a medium in which galactose is present, it will not be able to use the galactose unless it already has, or develops, an active transport mechanism capable of moving galactose into the cell. All the necessary enzymes for metabolizing galactose may be present inside the cell, but they will be practically useless unless an active transport mechanism is available to move galactose across the cell plasma membrane. Thus in certain instances adaptation may consist in the development of a new mechanism, or a modified mechanism, for active transport. These mechanisms may be based upon the properties of specialized high polymers (Goldacre & Lorch, 1950; Goldacre, 1952; Danielli, 1952^:).