NATURAL SELECTION: II 495 



II. Forces Operative upon the Variability Present in a Population 



A. Tendency to Equilibrium 



1. MECHANISMS OF MENDELiAN INHERITANCE tend to establishment and 

 maintenance of equilibrium in a population; this is expressed by the 

 Hardy-Weinberg formula (pp. 431-435). 



2. GENETIC HOMEOSTASIS, based upon heterozygote superiority, tends to 

 conserve genetic variability present in a population, while the popula- 

 tion itself remains in equilibrium (pp. 466-469). 



3. STABILIZING SELECTION is an aspcct of natural selection tending to 

 maintain the status quo of a population already well adapted to its 

 environment. Stabilizing selection (pp. 466-469) may operate by 

 (a) weeding out ill-adapted deviates and (b) favoring production of 

 a genotype which confers upon its possessors high adaptability to 

 environmental change, i.e., a genotype characterized by homeostasis 

 and the tendency of embryonic development to be canalized (pp. 

 421-422). 



B. Factors and Forces Tending to Disturb Equilibrium 



1. isolation. Populations are isolated from each other primarily by 

 geographic-environmental factors (pp. 469-471) involving some 

 element of isolation in space. Such isolation may be reenforced by, 

 and in later stages of speciation even supplanted by, various phys- 

 iological and behavioral adaptations which prevent interchange of 

 genes between populations. These mechanisms of reproductive isola- 

 tion include selective mating (pp. 471-472). Within isolated popula- 

 tions or subpopulations the following forces may be operative — 



2. genetic drift. Especially if the subpopulation is small, chance may 

 increase or decrease the frequency of a gene or genotype (p. 444). 

 If the genotype increased in frequency in this manner has elements 

 of superiority, the way is paved for the action of — 



3. natural selection. Natural selection will act in these subpopula- 

 tions upon the store of genetic variability submitted to it, tending 

 always toward production of populations having large capacity for 

 reproduction. This will mean in most, if not all, instances a popula- 

 tion adapted to its environment and well "buffered" to withstand 

 some range of environmental diversity. 



a. Post-adaptation. Insofar as the characteristics which become es- 

 tablished in a subpopulation are of value to their possessors, they 

 will usually have the effect of adapting the subpopulation to its 

 particular environmental niche. 



b. Tendency to expansion of range. A successful subpopulation will 

 usually tend to increase in numbers and hence to expand its range. 

 Doing so will bring it into competition with other subpopulations. 

 If reproductive isolation has been developed by two subpopula- 

 tions while isolated, interbreeding will not occur and the sub- 

 populations will have attained the species level of differentiation. 

 One species may then supplant the other or, alternatively, the 



