ADRIANO A. BUZZATI-TRAVERSO 



Universiia, Isfituto di Gene/ico, Pavia, Ifaly 



Chapter 9 



Heterosis in 

 Population Genetics 



Population genetics is the study of the genetic structure of populations. 

 Such a statement may look at first to be a truism, a tautology. The subject 

 matter of our research becomes very intricate, however, as soon as we try to 

 specify what we mean by the above definition. The terms "genetic structure" 

 and "population" may have different meanings according to what we are 

 willing to indicate by such words. It therefore seems convenient to start 

 with an analysis of the terms we are using. Such discussion will give us a 

 chance to see how the problem of heterosis is intimately connected with the 

 general theme of population-genetical studies. A few experimental data will 

 be used to illustrate such points. 



Let us consider first what we mean by population. If we take a dictionary 

 definition, we find in Webster's that population is "all the people or in- 

 habitants in a country or section." It means, in this sense, the sum of indi- 

 viduals present at a certain moment over a more or less arbitrarily limited 

 territory. But this definition does not correspond to the requirements of our 

 studies, as I have tried to show elsewhere (Buzzati-Traverso, 1950). Such a 

 definition is a static one, while the population, as considered in the field of 

 population genetics, is a dynamic concept. We are interested not in the 

 number of individuals present at a certain time in a certain place and their 

 morphological and physiological characteristics. Instead, we are concerned 

 with the underlying mechanisms which bring about such characteristics, and 

 the particular size the population reaches at any particular moment. Since 

 such mechanisms depend upon the numerical dynamics of the population 

 and upon heredity, it follows that our concept of population is typically 

 dynamic. On this view, then, a population is an array of interbreeding indi- 

 viduals, continuous along the time coordinate. 



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