Changes in Populations 153 



bility to free genetic variability, permitting further selection without 

 loss of fitness. The whole situation can then be looked at in terms of 

 shifting states of balance. Strong selection at one or a few loci places 

 a stress on the balanced genotype. After the expression of the char- 

 acter has been shifted a certain distance, this stress will result in 

 loss of fitness, followed by either extinction or the attainment of a 

 new balanced state. If selection is relaxed before either of these 

 events, the line tends to regress to the control level. 



A tremendous volume of literature on artificial selection has ac- 

 cumulated, as work on economic problems (improvement of do- 

 mestic animals and plants, studies of resistance, etc. ) has produced 

 information of great value. Much of our understanding of such di- 

 verse problems as the origin of dominance, the integrative prop- 

 erties of genotypes, and the efficacy of selection under varying 

 conditions has been the direct or indirect result of investigations of 

 such prosaic matters as egg laying in chickens, the weight of swine, 

 rust resistance in wheat, the yield of corn and cotton plants, and the 

 productivity of bovine mammary glands. The reader wishing a well- 

 organized introduction to this vast and complex subject is referred 

 toLerner (1958). 



GENETIC HOMEOSTASIS 



At this point it would be well to mention an important steady-state 

 property of mendelian populations, the often-observed tendency of 

 populations subjected to directional selection to regress toward the 

 original mean. Lerner has called this phenomenon genetic home- 

 ostasis. A mendelian population has characteristics above and beyond 

 those of its component individuals. For instance, it would be mean- 

 ingless to say that an individual is in Hardy-Weinberg equilibrium. 

 Populations tend to retain a genetic composition that produces a maxi- 

 mum number of individuals with a high degree of fitness. In short, 

 there is selection in favor of maintaining the balanced "maximum-fit- 

 ness" genotype. This is essentially a stabilizing selection operating 

 against deviant individuals. A genotype showing a high degree of fit- 

 ness is adapted not only to the environment in the classical sense but 

 also to its genetic environment, that is, the gene pool in which the 

 genotype occurs. In other words, the frequency and distribution of 

 genes in the population help to determine the fitness of any genotype 

 within the population. One may summarize the subject of genetic 

 homeostasis by saying that selection organizes the gene pool of a 

 population in such a way that its various components are coadapted 

 and produce a maximum number of highly fit genotypes. Well- 



