On evolution / John Maynard Smith.

  • Maynard Smith, John, 1920-2004.
Date:
1972
Catalogue details

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

Credit: On evolution / John Maynard Smith. Source: Wellcome Collection.

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    with Раь large and the other with Рав large. Suppose now that a third environment becomes available for colonization by both populations ( this could be a new area, or a transformation of one or both the existing environments ). Then in the coloniz¬ ing population Раь • Рав > Раь • Рав, and sexual reproduc¬ tion would enable Pab to increase more rapidly than could happen in an asexual population. In other words, if two genetically different populations are adapted to different environments, sexual reproduction makes possible the rapid evolution of a new population, carrying genes from both parental populations, and adapted to a third environment. Notice that the initial genetic adaptations were the result of natural selection. Thus another way of looking at the matter is to say that sexual reproduction makes it possible to utilize genetic variance generated by past natural selection to adapt rapidly to new circumstances. If existing genetic variance has been generated by mutation, as suggested in Fisher's argument, then sexual reproduction confers no advan¬ tage. The origin of sex It follows that sexual reproduction does confer a long-term advantage in enabling genes initially present in different in¬ dividuals to be brought together in a single individual, but only if the 'species' ( in this context, the group of individuals between which genetic recombination can take place ) is div¬ ided into populations genetically adapted to different environ¬ ments. Sex in this sense long preceded mitosis and meiosis; the processes of transduction and transformation achieve essen¬ tially the same end. Sexual reproduction requires first that DNA from different ancestors be brought together in the same cell, and second that there be some mechanism of genetic recombination. The latter seems always to depend on a process of pairing between identical, or at least very similar, lengths of DNA, and on some process functionally equivalent to breakage and reunion of DNA molecules. It seems therefore that the enzymes re¬ quired for genetic recombination could not have evolved because of the advantages conferred by sexual reproduction, because these advantages would not have existed until all the necessary enzymes had been perfected. In fact the same en¬ zymes are probably used in repairing damaged DNA. As is so 118