Evolution now : a century after Darwin / edited by John Maynard Smith.

Date:
1982
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Evolution now : a century after Darwin / edited by John Maynard Smith. Source: Wellcome Collection.

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    50 THE EVOLUTION OF THE GENOME more attention on the developmental implications than on the evolu¬ tionary ones. I believe that this was a wise decision on her part, although I do not know whether her actual reasons for this were the same as mine. Changes in DNA, ranging from point mutations through gross re¬ arrangements, are the raw material of evolutionary change. We cannot doubt that any mechanism that produces such changes must have occasionally contributed to evolution by creating adaptive gene combina¬ tions that have survived. But it is also true that at the time controlling elements were discovered, there was no obvious shortage of known mechanisms (understood at the same level as controlling elements could be understood at the time) that could generate the same end results. Evolutionists did not accord high priority to the search for new muta¬ tional mechanisms. The principal outstanding problems in their field did not appear to be at that level. Although some people may disagree, I believe that, for the most part, the situation remains the same today. We all agree that movable elements can cause deletions, inversions, translocations, etc. Few of us would assert that all deletions or all inversions require their participation. With a few possible exceptions, such as the distribution of dispersed, moderately repetitive DNA within related species, most observable evolutionary change can be adequately (though not necessarily correctly) explained by other mechanisms. Without in any way minimizing the value of some interesting evolutionary speculations that have been made recently, I believe that the case that movable elements provide much of unique explanatory value remains to be made. One example often cited of the role of specific events in evolution is the bacterial plasmids. Comparisons of the genomic organization of related plasmids show that frequently they differ from one another by insertion of transposons and by replicón fusions catalyzable by specific systems. There can be no reasonable doubt that these events have occurred in nature and have contributed to the diversity of the existing plasmid population. Accepting that such events have happened and are important, one may still ask whether they really constitute evolution. Let me illustrate the reasoning behind this position with an extreme case. If I survey Escherichia coU strains in nature, some are lysogenic for bacteriophage к and many are not. I could, therefore, speak of the acquisition of a Á. as part of the evolution of a strain such as E. coli K12. In a sense it is. However, perhaps because of conditioning, I generally regard the tiansition from phage to prophage and back again as defining the life cycle of the virus, rather than as constituting evolution, which implies (to me) some degree of progressive as well as cyclical change. I tend to consider the relocation of transposons and the insertion of one plasmid into another likewise as constituting part of the Ufe cycle of these elements. Unions are created when convenient and eventually dissolved when their selective value disappears. All this happens within a single bacterial species or a group of species belonging to some Exchanger List, and I suspect it happens at a frequency many times as great as events that lead to a degree of genetic isolation comparable to that separating distinct species of higher organisms.