NATURAL SELECTION: I 469 



of the positive action of natural selection in favoring certain mutations 

 or characteristics. Now we can appreciate that the matter is really more 

 complex than such a statement implies. To a considerable extent the 

 Mendelian population, rather than the individual, is the unit upon which 

 natural selection operates. Such a population has a great variety of geno- 

 types present. Natural selection will favor the development of an aggregate 

 of genotypes which will react to produce for the population a high level of 

 adaptive and homeostatic properties, with resultant high efficiency in 

 reproduction. 



THE ROLES OF ISOLATION 



We have seen that large, random-breeding populations 

 have a tendency to maintain a genetic equilibrium which is the antithesis 

 of evolutionary change (pp. 427-436). We have likened such equilibrium 

 to an inertia which evolutionary processes must overcome if change is to 

 be effected. Since the equilibrium is connected with large population size, 

 any factors which tend to break up large populations into smaller ones are 

 likely to contribute to evolutionary change. Thus isolation, and factors 

 giving rise to it, are important in evolution. 



Types of Isolation 



Geographic isolation is the most easily visualized type. It exists when 

 two populations, or two parts of one population, are separated by some 

 geographic barrier (examples listed on p. 272). The effectiveness for 

 evolution of this, or any other, type of isolation resides in the fact that it 

 prevents, or greatly reduces, exchange of genes between the populations 

 so isolated. Such isolated populations are more or less completely "out of 

 touch with each other," genetically, and hence the occurrence of new 

 mutations, genetic drift, the action of natural selection, etc., in one 

 population has no effect on the other populations. 



As a corollary of the fact that isolation is important only as a means of 

 impeding gene interchange we should note that the only isolation of im- 

 portance is that concerned with the breeding of animals. Most species of 

 higher animals have definite periods of breeding; it is isolation during these 

 periods which counts. Many migratory birds, for example, collect into great 

 flocks and range over vast territories, yet when the breeding season ap- 

 proaches individuals return to the same locality, even the same dooryard, 

 where they themselves were hatched. The prolonged and hazardous 



