Mendelism and evolution / by E.B. Ford.

  • Ford, E. B. (Edmund Briscoe), 1901-1988.
Date:
1931
Catalogue details

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

Credit: Mendelism and evolution / by E.B. Ford. Source: Wellcome Collection.

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    EVOLUTIONARY PROBLEMS 75 rence in animal communities, and that in many instances it is subject to a regular periodicity, giving rise to cycles of abundance and rarity. He has therefore pointed out that the opportunity for the spread of genes which fluctuation provides may be an important factor in evolution. In order to explain the non-adaptive nature of the characters which are sometimes known to separate closely related species, he suggested that this process may lead to non-adaptive evolutionary change. Such an idea must, however, be received with caution. It is apparent that when a mutation chances to occur in a population which is rapidly increasing, it will be enabled to spread in a way which would be impossible if the numbers had remained constant and each pair had on the average been giving rise to but a single pair in the next generation. Practically non-adaptive or even disadvantageous genes would thus be placed in a number of different gene-complexes, with which they would have the opportunity of reacting in various ways. Should one of these chance to be beneficial, the effect so produced would be favoured by selection. This would lead to the establishment of such a gene, and of the gene-complex with which it reacted usefully. When the increase in numbers ceases, the spread of non-adaptive genes would be checked, and those of a semi-lethal character quickly eradicated ; this process being, of course, even more swift when, as so frequently happens, periodic fluctuations take place, leading on to a decline in the population. Any advantageous com¬ bination which had arisen would, however, maintain its footing at this time, and lead to an adaptive change in the population as a whole. Fluctuation in numbers, then,, may give an oppor¬ tunity for normal evolutionary processes to take place more rapidly. For so long as the numbers remain con-
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