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

Date:
1982
Catalogue details

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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    74 the evolution of the genome An analogous situation may obtain in certain American species of salamander. These often differ considerably in the rapidity of their development and of their life cycles, the tropical species tending to take longer than the more temperate ones. Drs David Wake and Herbert MacGregor (personal communciation) tell us that preliminary evidence suggests that species with the longer developmental times often have the higher С values. This appears to parallel the situation just described for the herbaceous plants. It remains to be seen if further evidence will continue to suppюrt this generalization. (See the interesting paper by Oeldorfe et al.^^ on 25 species of frogs. They conclude that 'genome size sets a limit beyond which development cannot be accelerated'.) Testing the theory The theory of selfish DNA is not so vague that it cannot be tested. We can think of three general ways to do this. In the first place, it is important to know where DNA sequences occur which appear to have little obvious function, whether they aro associated with flanking or other sequences of any special sort and how homologous sequences differ in different organisms and in different species, either in sequence or in position on the chromosome. For example, it has recently been shown by Young^'^ that certain intermediate repetitive sequences in Drosophila are often in different chromosomal positions in different strains of the same species. Second, if the increase of selfish DNA and its movement around the chromosome are not rare events in evolution, it may be feasible to study, in laboratory experiments, the actual molecular mechanisms involved in these processes. Third, one would hope that a careful study of aU the nonspecific effects of extra DNA would give us a better idea of how it affected different aspects of cellular behaviour. In particular, it is important to discover whether the addition of nonspecific DNA does, in fact, slow down cells metabolically and for what reasons. Such information, together with a careful study of the physiology and life style of related organisms with dissimilar amounts of DNA, should eventually make it possible to ex¬ plain these differences in a convincing way. Conclusion Although it is an old idea that much DNA in higher organisms has no specific function®^^, and although it has been suggested before that this nonspecific DNA may rise to levels which are acceptable or even advantageous to an organism®'^2, depending on certain features of its life style, we feel that to regard much of this nonspecific DNA as selfish DNA is genuinely different from most earlier proposals. Such a point of view is especially useful in thinking about the dynamic aspects of nonspecific DNA. It directs attention to the mechanisms involved in the spread and evolution of such DNA and it cautions one against looking for a special