HETEROTHALLY AS AN OUT-BREEDING MECHANISM IN FUNGI

Abstract

ALL organisms are dependent on a supply of heritable variation for future adaptive changes ; but the presence of free variation means that some members of the population show a poor adaptation to existing circumstances. The balance between present and future adaptation is thus conditioned by the amount of heritable variation which the population can maintain. This, in its turn, depends on the system of breeding or mating in force in the species. Where close inbreeding is in force the individuals tend towards a high degree of homozygosity. Heritable variation is then reduced to a low level, with the result that present adaptation is good but the prospect of future adaptation to changed circumstances poor. When, on the other hand, outbreeding is the rule, great genetical heterogeneity is maintained and in consequence the chances of good future adaptation are improved at the expense of present fitness¹. In this way genes which control or affect the breeding system of a species have a profound effect on its genetical structure and evolutionary history. They have an adaptive value and will be subject to evolutionary changes.