Mutation, Selection, and Population Fitness 



C. C. LI 



Graduate School of Public Health, 

 University of Pittsburgh, Pittsburgh, Pa. 



The problems of mutation and selection in natural populations 

 are varied and complicated. Probably each case merits a separate 

 study, and it is doubtful if any attempt of abstract generalization is 

 warranted. This situation, however, is in principle no different from 

 that in physical sciences. A physical phenomenon may also be very 

 complicated and yet physicists start out from very simple models and 

 gradually build more and more realistic models as experimental 

 evidence accumulates. It is in this sense that biologists may formulate 

 mutation and selection models, granting and remembering that they 

 are only first approximations of natural situations. Simple models do 

 help us to understand certain biological phenomena and may serve 

 as starting points for further research. 



Before proceeding, it may be well to define the scope of our 

 discussion. The disciplines of population genetics, ecology, and evolu- 

 tion are interlocked in such a way that it is impossible to disentangle 

 one from the others. In a very general way, however, ecology deals 

 with population growth in size and the joint distribution of more 

 than one group of organisms under a variety of environmental condi- 

 tions. Evolution is concerned with long-term changes whatever the 

 causes may be. In the present discussion, I will limit myself to the 

 comparatively short-term changes in the genetic composition of a 

 population in the face of recurring mutation and persistent selection. 



Probably all genes mutate. The history of mutation must be as 

 old as the gene itself. Mutation is one of the most fundamental 

 properties of a gene. It is also probable that each allele produces a 

 different, however slight, effect on the organism in one respect or 

 another which may or may not be detectable by our present tech- 

 nique. As the title implies, the present discussion is further limited 

 to mutations which affect the reproductive capacity of the organism. 



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