152 ADRIANO A. BUZZATI-TRAVERSO 



that we have named revolutionary tend to produce genetic heterogeneity or 

 heterozygosis. Thus we come to the conclusion that the mentioned compro- 

 mise brought about by selection consists of the pursuit of an optimum level 

 of hybridity with respect to the conditions under which the organism lives. 

 Such a hybridity optimum is the product, not only of the mutation rate and 

 selective value of single genes, but also depends largely upon the genetic sys- 

 tem and the mating system — the breeding system — of the considered species 

 or population. 



The genetic structure of natural populations cannot be solved only in 

 terms of individual variations observable in the group. Instead, it must be 

 integrated into a unitary research on changes in gene frequencies as related 

 to the underlying breeding systems. This is why we are justified in consider- 

 ing the natural population as a unit, since individual variations must be 

 referred to the genetic balance of the whole aggregate of individuals. 



What is that hybridity optimum I was speaking about but heterosis? How 

 else could heterosis be defined in population problems other than that type 

 and amount of heterozygosity that gives the population or the individual the 

 best adaptive value with respect to the conditions in which the organism 

 lives? With this view, then, it becomes feasible to analyze experimentally 

 what morphological and physiological characteristics of the hybrids produce 

 the better adaptation. 



MECHANISMS WHICH PROMOTE HYBRIDITY 



In studying how heterosis mechanisms are brought about in living crea- 

 tures, we may attempt a sort of classification of the devices present in plants 

 and animals insuring hybridity. Starting from the most complex and proceed- 

 ing to the less complex cases, we can distinguish three types of mechanisms: 

 (1) mating systems, (2) chromosome mechanisms, and (3) gene effects. 



We will not discuss in detail all the devices insuring hybridity found in 

 plants and animals. We will mention a few, in order to show how many differ- 

 ent paths have been followed in evolution to reach the same sort of results. 



Under the heading "mating systems" we may mention homo- and hetero- 

 thally among fungi; monoecism and dioecism, incompatibility mechanisms, 

 and heterostyly among flowering plants. Here, in some cases such as Primula 

 scofica, there is close relation between the variability of ecological conditions, 

 and, therefore, of selection pressure and the efficiency of the incompatibility 

 mechanisms. Other species of this genus present in England are character- 

 ized by heterostyly and incompatibility devices to insure the occurrence of 

 outcrossing, apparently necessary to meet the requirements of varied eco- 

 logical conditions. Primula scofica, living in a very specialized ecological 

 niche, shows that such a mechanism has broken down. In fact, it looks as if 

 the requirements of a constant environment are met better by populations 

 genetically less diversified. 



