In the first part of this book, the origin of Hfe, the coding and trans- 

 fer of genetic information, the development of organisms, and some 

 features of populations have been discussed. We shall now begin to 

 deal with the very core of evolutionary theory— changes not in 

 individuals but of populations. This chapter will be concerned with 

 the theoretical aspects of the genetics of mendelian populations. A 

 knowledge of the basic ideas of population genetics is absolutely 

 essential to an understanding of how the mass of inherited informa- 

 tion possessed by a population changes from generation to genera- 

 tion. Familiarity with the simple mathematical ideas presented here 

 will permit the reader to comprehend the more complex situations 

 discussed in ensuing chapters. Although nonmathematical descrip- 

 tions accompany the various algebraic examples, a firm grasp of the 

 material will be facilitated by working through the simple algebra. 

 The examples in this chapter are gross oversimplifications. The 

 integrative aspects of the genotype, multiple alleles, simultaneous 

 operation of diflFerent evolutionary forces, and other complicating 

 phenomena are largely ignored. For the moment, it is assumed that 

 a single locus can be torn from its substrate and subjected to condi- 

 tions of our choice; complex interactions are left for later considera- 

 tion. 



MENDELIAN POPULATIONS 



Only sexual organisms comprise mendelian populations, which can 

 be defined loosely as aggregates of interbreeding individuals. A more 

 precise definition is neither possible nor desirable, for the word 

 "interbreeding" may refer to any situation from panmixis to almost 

 complete isolation. One might consider the potato beetles on a 

 single potato plant as a mendelian population, or the definition might 

 be broadened to include those in a single potato field, those in a 

 group of adjacent potato fields, or indeed those in a county or larger 

 area. It is therefore important to indicate the scope of a population 

 under discussion and to state what is known of its structure. 



Panmixis 



A population is panmicfic if the individuals within it mate at random. 

 Each individual is equally hkely to mate with every individual of the 

 opposite sex within the population as defined. The expected fre- 

 quency of any given kind of mating is the product of the frequency | 91 



