GENES IN CONTROL 4I 



hereditary changes into adaptive channels through differ- 

 ential reproduction. 



The modern concept of evolutionary change is based on 

 studies of this differential situation, shifting gene frequen- 

 cies in the population, new proportions and new combina- 

 tions of the genetic factors involved. In this concept, selec- 

 tion does not minutely work over every mutant in the 

 species. It will eliminate or favor some, but there will always 

 be changes of a more or less minor nature which remain 

 largely untouched. Favored genes will tend to spread in the 

 population; and in this sense selection is a positive force in 

 nature, not purely negative as was once claimed by its 

 critics. It is very important to remember, particularly since 

 this review is brief, that the characteristics of an organism 

 as an integrated whole depend on the combined effects of 

 many genes, not on the action of one gene, and that there 

 are endless ways in which the combined effects can vary. 

 The population geneticist points out that in the parental 

 population the frequency of the genes involved in a given 

 combination will be governed by natural selection and will 

 differ in effect according to many factors involving muta- 

 bility and size of groups of individuals. Some species are 

 much less mutable than others, at least as observed at pres- 

 ent. There are organisms which have undergone little 

 change in the past 250,000,000 years; the starfish is an ex- 

 ample. Other species are much more variable— man, for 

 instance. 



Obviously, there is a degree of mutability necessary to 

 change, and this is the situation in most organisms. The real 

 factor, then, is size of population. Theoretically, an indefi- 

 nitely large population, where breeding is completely at 

 random, will become stable. Variation will occur but the 

 variability will tend toward fixed and permanent ratios, and 

 evolution will cease. Actually, no populations are ever of 

 unlimited size; and interbreeding cannot be purely random 

 over the whole population, for localized situations are 



