NATURAL SELECTION 



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Figure 96. Vestigial Hind Limbs of the Python. (From Romanes, "Darwin and 

 After Darwin," Open Court Publishing Co., 1902. ) 



adaptive value. Such characters can be understood on several bases. They 

 may be incidental eflFects of pleotropic genes— that is, genes with more 

 than one phenotypic effect— so that, while one effect of such a gene is 

 favored by selection, other effects are accidentally preserved along with it. 

 Or the gene for a character of no selective value might be closely linked 

 to one of definite selective value. Again, a gene may have become fixed 

 in a species as a result of genetic drift (see Chapter 16). Finally, a char- 

 acter might be present in vestigial form because it had been valuable in 

 the past, and has simply not been completely eliminated yet. Examples of 

 such vestigial characters are numerous. Snakes, which have no use for 

 limbs, are descended from typical reptiles with well developed limbs. 

 Some snakes still have traces of the hind limbs present (Figure 96). A few 

 of the many vestigial structures of man are illustrated in Figure 97. 



Schmalhausen, a distinguished Russian student of evolution, has em- 

 phasized the conservative role which natural selection may play in evolu- 

 tion. He points out that, as the majority of mutants which may arise are 

 disadvantageous, selection will tend to eliminate them and thus to conserve 

 the status quo. This is undoubtedly true. But the relationships between 

 natural species and their environments (i.e., the sum of the selective agen- 

 cies ) are continually in a state of flux, and so the creative effect of natural 

 selection, at least in the long run, may be expected to outweigh the 

 conserving effect. 



Cooperation end Selection. Part of the early opposition to Darwinism 

 was based upon Darwin's failure to discuss cooperative aspects of animal 

 behavior. All social organisms cooperate in some degree. In social insects, 

 like ants and termites, intracolony cooperation is the most striking aspect 



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