HETEROSIS IN POPULATION GENETICS 151 



somes, linkage between genes, sterility mechanisms, mating discriminations, 

 devices favoring inbreeding, and other conservative forces that aim at the 

 preservation of certain constellations of genes over a large number of genera- 

 tions. On the other side we find mutation pressure, recombination between 

 chromosomes, recombination among genes due to crossing over, outbreeding 

 devices, migration pressure, and other revolutionary forces that aim at the 

 production of genetic novelt3^ 



It seems reasonable to maintain that, at any particular time, a species or 

 a natural population can be considered as a sort of compromise between the 

 two conflicting forces — a compromise that is brought about through the 

 action of natural selection. In other words, the fine adjustment or adaptation 

 of a population to its environment is the expression of such compromise. At 

 any particular time the terms of the compromise between the conflicting 

 forces are always different as compared to other moments, as the compro- 

 mise itself is a dynamic process. 



In order to reach the highest possible level of adaptation with respect to a 

 certain set of environmental conditions, natural selection is discriminating not 

 only for or against a certain individual genetic constitution, but for or against 

 a group of individuals, as w-ell. Sometimes selection acts at the level of the in- 

 dividual, sometimes it operates at some higher level. If we consider a genotype 

 that insures resistance against an infectious disease, present in a certain area of 

 distribution of a species, it wall be obvious that an individual carrying it shall 

 directly benefit by it. But if we consider a genotype producing fecundity 

 higher than the average of the population, this will be selected by the mere 

 fact that a larger number of individuals having such genetic constitution will 

 be present in the next generation. These, in their turn, shall have a chance of 

 being represented in the next generation greater than that of individuals 

 having a less fertile genotype. The individual itself, though, obtains no direct 

 advantage from such selection. 



The next extreme condition we can consider is the one occurring when the 

 advantage of the individual is in conflict with the advantage of the group. 

 This is the case, for instance, of a genotype that would extend the span of 

 life far beyond the period of sexual activity — or higher fertility linked with 

 antisocial attitudes in the case of man. In both cases, natural selection favor- 

 ing the preservation of the group will discriminate against the individual. A 

 similar mechanism must have played a great role in various critical periods 

 of organic evolution. When intergroup selective pressure is in the opposite 

 direction from intragroup selection, a sort of compromise has to be reached 

 between the two conflicting tendencies. This can be reached in many differ- 

 ent ways that are best illustrated by the great variety of life histories and 

 mating systems to be found in the living world. 



Those factors which we have classified as conservative tend to j)roduce 

 genetic homogeneity, or what is technically known as homozygosis. Factors 



