422 INTRODUCTION TO EVOLUTION 



the genotype may occur so that the heavier coat originally produced in 

 reaction to the environment becomes "hereditary," as we usually use that 

 term. We may think of it in this way: if it is desirable for the animal to 

 have a thick coat it will be desirable for the animal to be born with such a 

 coat, or with the capacity to develop it even before the stimulating effect of 

 cold weather is felt. Accordingly, natural selection will favor changes in 

 the genotype which will narrow the norm of reaction toward the upper end 

 of the range of coat densities originally possible. Waddington (1953-1960) 

 has spoken of the embryonic development of organisms as being canalized 

 or buffered "in the sense that even though it may become somewhat modi- 

 fied in response to an environmental stress, it also exhibits a tendency to 

 reach its normal end-result in spite of disturbing circumstances." In our ex- 

 ample the "normal end-result" is a thicker coat of hair; "canalizing selec- 

 tion" will favor genotypes which give rise to it (Waddington, 1953b). As a 

 result, remote descendants in the arctic might no longer be able to pro- 

 duce the sparser coats possible to their Temperate Zone ancestors. Heavy 

 coats have now become hereditary, in the sense that they are produced 

 without any action by the external environment. 



We have visualized this change as having been accomplished by what 

 is known as the "genetic assimilation of an acquired character" (Wad- 

 dington, 1953-1960). At first the heavier coat had to be acquired by each 

 generation, but eventually the genotype changed, narrowing or "funnel- 

 ing" the reaction range so that only heavy coats were produced. 



How does this "genetic assimilation of an acquired character" differ 

 from the inheritance of acquired characters in the Lamarckian sense? 

 There is no thought that the acquired character (denser fur) in some 

 manner changes the germ plasm directly; rather the idea is that genetic 

 variability already present is made use of in producing the hereditary 

 change. The range of variability already present is narrowed so that, in 

 our example, only the thicker coats are produced. 



What if still heavier coats, outside the original reaction range, might 

 prove advantageous to our arctic immigrant? A genetic change might oc- 

 cur making possible heavier fur than the original genotype could produce. 

 Such a genetic change would be to all intents and purposes a mutation or 

 series of mutations. Hence we see that genetic assimilation and mutation 

 are to be thought of as coexistent, and sometimes cooperating, means by 

 which genetic variability may be supplied to natural selection. 



Another instructive example is afforded by the fact that animals are 

 frequently born with thickened skin on areas of the body which will later 

 be subjected to friction in the course of the animals' normal activities. 



