Changes in Populations [ 157 



environmental stresses are highly similar, generation after genera- 

 tion. Developmental aspects of genetic assimilation are discussed in 

 Chap. 4. 



ADJUSTMENT TO THE ENVIRONMENT 



The diverse and ingenious ways in which organisms meet the prob- 

 lems of survival and reproduction are inferential evidence for the 

 great efficacy of natural selection. This adjustment to the environ- 

 ment is usually called adaptation, but for reasons discussed in the 

 final chapter this ambiguous term is avoided whenever possible. 

 One need not go into the details of the evolution of the bird's wing, 

 the giraffe's neck, the vertebrate eye, the nest building of some fish, 

 etc., as the selective origins of these and other structures and of 

 behavioral patterns may be assumed to be basically the same in out- 

 line as those, such as industrial melanism, which have alreadv been 

 discussed. Even a slight advantage or disadvantage in a particular 

 genetic change provides a sufficient differential for the operation of 

 natural selection. Thus the property of light sensitivity of unicellular 

 organisms provides a starting point for the development, through 

 selection, of the highly complex eyes found in vertebrates, insects, 

 and certain mollusks. 



The old antievolutionist argument that the vertebrate eye would 

 be useless unless present in its modern complexity is nonsense. 

 Many organisms with less complex and less specialized photore- 

 ceptors put them to good use, and it is easily seen that even a 

 human being would be better off with a non-image-forming photo- 

 receptor (one which gave information only on the amount of light 

 present) than without any photoreceptor at all. Similarly, any non- 

 detrimental variation in a highly edible butterfly, tending to make 

 it look more like a sympatric distasteful species, puts this deviant 

 at a selective advantage. 



The presence today of all degrees of refinement in the phenomena 

 of mimicry and protective coloration argues strongly against the 

 hypothesis that such resemblance must be virtually perfect before it 

 is effective. Experimental evidence at hand indicates that less per- 

 fect copies of certain distasteful model butterflies also enjoy a 

 degree of protection, though perhaps not as great as that of the 

 more nearly perfect mimics. It is interesting to note that mimetic 

 forms of various butterflies generally do not occur in areas where 

 the models are absent, indicating that a selection pressure favoring 

 mimicry is required to prevent regression to the wild type. Even 



