Evolution.

  • Society for Experimental Biology (Great Britain)
Date:
1953
Catalogue details

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

Credit: Evolution. Source: Wellcome Collection.

    15/484 (page XI)
    FOREWORD xi dangerous effects on pregnancy have been partly overcome. He did not speculate as to why natural selection has not abolished so dangerous a characteristic. Perhaps antigenic differences are necessary evolutionary steps, and their abolition would prevent evolution. It may also be that antigenic complexity within a zygote or more probably within a cell has physiological advantages which on the whole outweigh the disadvantages of antigenic diversity between zygotes. It is difficult to explain the bene¬ ficial effects of heterozygosis and perhaps of other results of outcrossing, except by the increased vigour due to the biochemical complexity produced by them. If Beale's conclusions on Protozoa are applicable to Metazoa, it might be better to say that heterozygosis confers biochemical versatility on a cell, so that it can perform different syntheses in different environ¬ ments. If so, Medawar was studying some undesirable consequences of biochemical versatility and the ways in which mammals have coped with them. In doing so he made the important generalization that 'endocrine evolution is not an evolution of the hormones but an evolution of the uses to which they are put'. From the genetical point of view genetically determined differences in the response of tissues to hormones are common and often harmless, while such differences in hormone production often lead to serious pathological changes. It would perhaps be truer to say that the evolution of the vertebrate hormones was nearly completed in Silurian times, and that the subsequent adaptive radiation of the vertebrates depended in part on changes in tissue responses to them. Hinshelwood followed with a valuable summary of his recent work. If anyone doubted that bacteria could adapt themselves to new metabolic processes, and that these adaptations could persist for hundreds of cell generations after the original stimulus to adaptation had ceased, I hope that his doubts were set at rest. The existence of adaptation does not, however, imply that mutation and selection do not occur. Nor, in my opinion, does it imply that the autocatalytic mechanism postulated by Hinshelwood is always responsible for the changes. The work of Monod and his colleagues suggests a different possibility. In the case of bacteria it is not obvious to me that either the adaptationists or the selectionists have discovered how and why bacteria evolve. Adaptation is complete in a few days or weeks at most; selection is very rapid in populations with enormous numbers and enormous rates of increase. But unless bacteria are exceptional, their evolution has a time scale measured in millions of years, like that of diatoms and other unicellular organisms whose palaeontology is known. Hinshel- wood's paper was useful in presenting results comparable with those on which Lysenko's criticism of Mendelism is based, in circumstances which permitted a cooler discussion than is often possible.