GENETIC CHANGE AND EVOLUTIONARY CHANGE 421 



operating on chance mutations alone. As is repeatedly emphasized, the 

 mutation process is random in the sense that mutations occur without 

 reference to whether or not they are "needed." How could the delicately 

 adjusted adaptations that organisms are observed to possess actually have 

 arisen through such a haphazard process? That question disturbs many 

 thinkers on the subject. There are answers (the probability of even a 

 chance occurrence is high, given a long enough period of time), neverthe- 

 less some means by which the reaction between organism and environ- 

 ment could lead to the production of hereditary change would be welcome. 

 As indicated earlier (p. 341 ), this is the appeal of the Lamarckian idea of 

 the inheritance of acquired characters. Is it possible that there are indeed 

 means by which an adaptation at first individually acquired may become 

 hereditary? 



What does the genetical constitution (genotype) produce in an individ- 

 ual? It produces a bodily constitution (phenotype) which enables the in- 

 dividual to live in a certain range of environments. A species of mammal, 

 for example, may possess such a genotype that if the animal lives in a hot 

 climate the coat of hair will be relatively thin or sparse, while if it lives in a 

 cold climate the coat will be thicker. The range of coat densities possible 

 to the animal represents the hitter's "norm of reaction" (Dobzhansky, 

 1951) or "reaction range" (Simpson, 1953b). Some species have such a 

 genotype that the norm of reaction is broad; depending upon the environ- 

 ment, the coat may be very sparse or very thick. Other species have such a 

 genotype that the range is narrow, only relatively sparse or relatively thick 

 hair being possible. It is important to note that a change in the genotype 

 may produce a change in the norm of reaction. On the whole, dogs have 

 genotypes causing development of a certain range of densities of hair. In 

 some varieties, however, this range has been sharply restricted, with the 

 result that they either can not develop heavy coats in cold climates, or 

 they have such heavy coats that they experience discomfort in hot climates. 

 Without doubt both types arose by genetic change from an ancestor with 

 a broader norm of reaction, as their wild relatives the wolves presumably 

 have today. 



Now let us imagine a situation in which a species of mammal living in the 

 Temperate Zone finds open to it an opportunity to live in arctic regions. 

 It will be able to do so if its genotype is such that the reaction range is suf- 

 ficiently broad so that both the coat density appropriate to the Temperate 

 Zone and the coat density needed in the new environment can be produced. 

 We will suppose this to be the case and that the species extends its range 

 into arctic regions. What will happen then? As generations pass, changes in 



