556 - Heredity and Evolution 



organs would seem to be inevitable on the 

 basis of mutation and natural selection. Mu- 

 tations are random and prone to decrease, 

 rather than increase, the size and perfection 

 of the affected parts. So long as the organ in 

 question has value in the survival of the 

 species, such detrimental mutations are 

 quickly eliminated by natural selection. But 

 if a structure has become nonessential to sur- 

 vival, detrimental mutations will tend to ac- 

 cumulate and spread throughout the popu- 

 lation. In fact, the displacement of a useless 

 organ will be accelerated by natural selec- 

 tion, because the whole organ would require 

 a greater share of the organism's food supply 

 compared to its partly degenerate counter- 

 part. Natural selection tends to reduce and 

 eliminate useless structures, although a long- 

 enduring persistence may occur as a result of 

 the random nature of the degenerative 

 changes, or as a result of hidden effects of the 

 persistent genes, which may be favorable in 

 relation to the internal adaptation of the 

 species (p. 543). 



EMBRYOLOGY AND EVOLUTION 



In any group of related species the em- 

 bryonic stages tend to resemble each other 

 much more closely than the adults (Fig. 29-2) 

 for reasons that are not very difficult to find. 



In the earlier embryonic stages, all organisms 

 are relatively simple as to their gross struc- 

 ture, so that little opportunity is afforded 

 for drastic differences to appear. Moreover, 

 many genes produce their phenotypic effects 

 relatively late in embryonic development, so 

 that differences induced by these genes are 

 added to the differential effects of earlier 

 acting genes. Thus in some cases the em- 

 bryonic stages of a species reveal genetic re- 

 lationships that would scarcely be suspected 

 on the basis of adult structure. This is par- 

 ticularly true of parasitic or otherwise de- 

 generate forms in which the adults lose many 

 of the characteristics of the group to which 

 they belong. Frequently the mutations re- 

 sponsible for these losses have their main 

 effects relatively late in development, so that 

 the early development of the species displays 

 an unmasked resemblance to the embryos of 

 the other members of the group. 



In some cases, however, the larvae of re- 

 lated animals do not resemble each other 

 more closely than the adults, but such larval 

 differences usually show a distinct adaptive 

 relation to the habitat in which the larvae 

 live. Highly distinctive larvae are particu- 

 larly characteristic of insects and other forms 

 that spend a considerable proportion of their 

 life span in the larval stage. In such cases, 

 apparently, natural selection has acted to 



FISH (SHARK) 



GILL 

 CLEFTS 



BIRO (FOWL) 



MAMMAL (MAN) 



Fig. 29-2. During early development the embryos of various vertebrates are strik- 

 ingly similar. 



