108 Natural Selection 



DIRECTION OF SELECTION 



Evolution takes one of two courses: toward stability of the existing 

 phenotype or change away from it. The selection pressures produc- 

 ing these courses are called centripetal and centrifugal, respectively, 

 by Simpson (1944). 



Centripetal Selection 



If an organism is well adapted to its environment, the chances are 

 great that any new mutations would produce phenotypes less well 

 adapted. Selection would remove these new elements and restore 

 the population to its original genetic composition. In this case of 

 selection the direction would be toward stability of existing pheno- 

 type. 



There seem to be considerable grounds for thinking that the 

 genetic system as a whole is sufficiently intricate to exert a strong 

 stabilizing selection pressure regardless of the environment. This 

 idea revolves around the concepts of genetic balance or homeo- 

 stasis (Lerner, 1954, 1958) and implies that any genetic change 

 will tend to create physiological imbalance even though it produces 

 a trait beneficial ecologically. Such a concept suggests that the 

 environment would have to exert a certain minimum selection 

 pressure toward change before the innate "inertia" of genetic 

 homeostasis would be counterbalanced. 



Theoretically centripetal selection over a long period of time 

 would result in an unchanged lineage of great age. The living Amer- 

 ican opossum Didelphis virginicus, which is remarkably similar to 

 its Cretaceous ancestor, might be such a product. 



Centrifugal Selection 



If an organism were in a new environment, the chances are great 

 that of the many mutations occurring, some would produce new 

 characters better adapted to the new environment than the old 

 ones. The classical example to illustrate this situation concerns the 

 teeth of the horse and its ancestors. The early horses had generalized 

 teeth, but later types of horses had cusped and furrowed teeth. 

 Presumably the early horses fed on many kinds of vegetation, and 

 later types fed primarily on grass. The grass-feeding habit sup- 

 posedly exerted a strong selection pressure toward teeth more 

 efficient for masticating grass and led to changes from the ancestral 

 types of teeth. 



