150 ADRIANO A. BUZZATI-TRAVERSO 



Consideration of a population as a phenomenon continuously occurring in 

 time makes it impossible for the experimental student of population genetics 

 to get a direct and complete picture of what is occurring within a population 

 at any particular moment. We can attempt to collect data on the population 

 under study only by freezing such flowing processes at particular time in- 

 tervals. Collecting observations on a population at different times gives us 

 a chance to extrapolate the direction and rate of the processes that have 

 occurred within the population during the time elapsed between two succes- 

 sive sets of observations. If the samples studied are large enough and give an 

 unbiased picture of the total population at the time when the sample is being 

 drawn, this experimental procedure ma}^ give us a fairly adequate idea of 

 what is going on within the array of interbreeding individuals continuous 

 along the time coordinate. That sum of individuals at a definite time, which 

 one usually means by population, is of interest to the population geneticist 

 only as an index of the particular evolutionary stage reached by the array of 

 interbreeding individuals. Since there are actual breeding and genetic rela- 

 tionships between the individuals of any such array, of any such population, 

 the population can be considered as the natural unit of our studies. 



If we consider now what we mean by "genetic structure," our task be- 

 comes much more complex. At first we could assume that the genetic struc- 

 ture of a population could be properly described in terms of the gene frequen- 

 cies present at a certain time within a population. But this is only part of the 

 picture. 



For the total description of the genetic structure of a population we have 

 to consider not only the frequencies of existing genes, but how these are 

 fitted within the chromosomes, how these allow the release of variability by 

 means of recombinations, how large is the amount of new variability pro- 

 duced by mutations, and several other factors which we cannot analyze now. 

 In a few words, the study of population genetics aims at the knowledge of the 

 breeding system of populations. This, as we shall see, is a rather difficult task 

 because of the complexity of factors responsible for the origin and evolution 

 of such systems. 



EVOLUTIONARY FACTORS INVOLVED 



When we take into consideration a species or a natural population at a 

 certain stage, we have to assume that such a natural entity is the product of 

 a series of evolutionary factors that have been at work in previous times and 

 that some, or all of them, are still operating on the population while we are 

 studying it. This means that we should try to explain the genetic structure of 

 the population in terms of such evolutionary factors. 



Now, if we are willing to examine the nature of the known evolutionary 

 agencies, we conclude that these can be classified into two types. On one side 

 we find, in sexually reproducing organisms, a limited number of chromo- 



