NATURAL SELECTION 



677 



may arise among plants because of self- 

 fertilization or vegetative reproduction, but 

 would appear rarely if at all among exclu- 

 sively sexually reproducing animals. Parthe- 

 nogenetic generations with an occasional 

 sexual generation might allow speciation 

 through polyploidy in certain animals 

 (Hughes-Schrader, 1948; p. 623). 



Examples illustrating the gradations to- 

 ward intersterihty or psychological impair- 

 ment of interbreeding between contiguous 

 populations are to be seen in circular chains 

 of subspecies such as the deer mouse, 

 Peramyscus maniculatus, and the Old 

 World warbler, Phylloscopiis trochiloides, 

 in which crossing occurs between each 

 contiguous subspecies, except that where 

 the two ends of the chain happen to oc- 

 cupy the same territory they are repro- 

 ductively isolated (Mayr, 1942, pp. 183, 

 184; see also p. 610). 



Porter (1941) experimented with frog 

 hybrids produced by fertilization of enu- 

 cleated eggs. The embryos showed abnor- 

 mal development when the eggs of northern 

 forms of Rana pipiens were fertiUzed by 

 spermatozoa of southern forms, or the eggs 

 of southern forms were fertilized by sper- 

 matozoa from northern forms. The amount 

 and direction of the abnormality were cor- 

 related with the amount and direction of 

 the difference in climatic adaptation of the 

 respective parental species. MuUer (1942), 

 in discussing this case, states "that the 

 genotypic difference responsible for the hy- 

 brid incapacitation did not arise as a conse- 

 quence of selection for that eflFect itself, but 

 for something quite different, namely, in 

 the given case, adaptation to development 

 at higher or lower temperatures, respec- 

 tively." 



In some instances, selection against a 

 character that has become harmful may 

 occur (pp. 637, 673). At the same time, 

 direct positive selection pressure does not 

 account adequately for the evolution of a 

 great many cases of loss of function, al- 

 though a slight selection in favor of econ- 

 omy of growth is possible. 



Pertaining to this economy theory of re- 

 gression. Walls (1942) states: 



"An old idea was that where the eye had 

 become useless, there was a positive incentive 

 for eliminating the organ, since tliis would save 

 energy both in adulthood and— especially— dur- 

 ing growth. This notion seems ridiculous nowa- 



days, for the proportion of a growing animal's 

 food-intake which goes to enlarge the eye 

 is negligible. Most of the energy released from 

 food goes lor motor and secretory activity, and 

 only a very small part of the food is converted 

 into new protoplasm. Nor does the disappear- 

 ance of an eye leave a hole in the head— its 

 volume is occupied by tissues (mainly muscle) 

 which consume just as much energy as the eye 

 had done." 



A number of other theories have been 

 advanced to explain regressive evolution 

 (see Breder, 1944, for a review). In some 

 cases the character undergoing reduction 

 might be harmful in a new habitat (p. 

 673). The eye of a mole might be a source 

 of infection in a subterranean burrow, but 

 the eye is hkewise reduced in burrowing 

 snakes in which the ocular scale prevents 

 any danger of infection. 



Needham (1930) claimed that all vestig- 

 ial organs have an embryological function 

 through induction of growth of other parts. 

 In some organs, such as the notochord in 

 vertebrate embryos, embryological func- 

 tions have been demonstrated, but this 

 theory places a tremendous burden upon 

 investigators to establish embryological 

 utihty for all the relict adaptations known, 

 such as the embryonic teeth and pelvic 

 bones of whales, or the showy flowers of 

 some species of dandeUons (Taraxacum) 

 that produce their seeds through obligatory 

 apomixis** (Huxley, 1942). 



Simpson (1944, p. 39) says "that degen- 

 erating structures are highly variable" and 

 "this may be advanced as an empirical evo- 

 lutionary generaUzation." The comparative 

 variation of functional molars and nonfunc- 

 tional wisdom teeth in man is an example. 

 If selection pressures were operating upon 

 a functional degeneration, variabihty would 

 probably be less. 



Several generahzations, each the result 

 of considerable experimental evidence, 

 seem, in combination, to give an adequate 

 understanding of the mechanisms of regres- 

 sive evolution. (1) Each gene or genetic 

 factor affects many characters. Genes with 

 manifold effects are said to be pleiotropic 

 (Dobzhansky and Holz, 1943). (2) Each 

 character is affected by many genes. These 

 characters are said to be multiple factor or 

 polygenic characters (Mather, 1943). (3) 

 Mutation of any single gene may occur at a 



" Apomixis is development without fertiliza- 

 tion, but with retention of the sexual structures 



