110 Natural Selection 



Production of Advantageous Mutations 



Because many species, like the oppossum, have existed with httle 

 change for milhons of years, there is good reason to beheve that 

 each group has an optimum possible adaptedness to its environ- 

 ment. This optimum should not be confused with perfect adapted- 

 ness (as was explained previously in this chapter). The optimum 

 possible adaptedness is the best phenotype that results from the 

 mutational possibilities of the organism. If the species comes to 

 occupy a new environment in which the species was initially far 

 from its optimum possible adaptedness, the chances are that many 

 new mutations would be advantageous. As the mutations were 

 incorporated into the genetic structure, the species would evolve 

 closer to its optimum possible adaptedness. As this advance pro- 

 gressed, the chances are that fewer and fewer mutations would be 

 advantageous, hence, more and more mutations would be weeded 

 out of the species by natural selection. 



Observations on mutations in general indicate that the rate of 

 occurrence of mutations in a species is fairly constant. Because the 

 number of mutations occurring in a unit of time is relatively 

 constant, whereas the proportion of these which are advantageous 

 theoretically decreases with length of occupancy of a new habitat, 

 it therefore follows that theoretically the number of advantageous 

 mutations occurring per unit of time would decrease. On the basis 

 of this reasoning, the rate of occurrence of advantageous mutations 

 would be highest during the period of most pronounced centrifugal 

 selection. The rate would decline exponentially as the species ap- 

 proached its optimum possible adaptedness, which would represent 

 a state of centripetal selection. Simpson's (1944) analysis of rates 

 of change observed in the fossil record for the mammals fits this 

 reasoning. 



Selection of Mutations 



Strictly speaking, the "selection" part of centripetal or centrifugal 

 selection cannot and does not operate until a new mutant char- 

 acter has occurred in the species. After its occurrence the rate of 

 selection will be individual with regard to each new mutation and 

 will be expressed as changes in the proportions of the new allele in 

 the gene pool. Theoretically, if selection for the advantageous 

 mutation were relatively constant, the proportion of the new muta- 



