678 



ECOLOGY AND EVOLUTION 



statistically predictable rate. This tendency 

 is referred to as mutation pressure. (4) 

 The effect of the majority of mutations on 

 a functional character is commonly dele- 

 terious or degenerative (Timofeeff-Res- 

 sovsky, 1940; Mather, 1943; Silow, 1945). 

 (5) Selection acts upon the whole organ- 

 ismic unit or population as a system as well 

 as upon the parts somewhat independently 

 (Sturtevant, 1938; Emerson, 1939). 



It follows that elimination or weakening 

 of a selective pressure may in time result 

 in degeneration of the functional character 

 through the action of mutation pressure (p. 

 696). Also, if selection favors an increased 

 development of one character while an- 

 other character has a diminished survival 

 value in a given habitat, there will be a 

 shift in the alleles in many gene systems 

 with a consequent degeneration of the 

 character that is losing importance. This 

 secondary effect of positive selection pres- 

 sure is probably responsible for all rapid 

 regressive evolution of a harmless charac- 

 ter, because the effect of mutation pressure 

 in the absence of selection would be a slow 

 process (Wright, 1929, 1932; Fisher, 1930, 

 p. 20). Through the combined action of 

 these principles, we have a reasonable ex- 

 planation of nonfunctional vestigial struc- 

 tures and some recapitulative development 

 (see Robb, 1937; Wilson, 1941; Holmes, 

 1944a; see also p. 636). 



At the same time, we can understand 

 why relict adaptations or vestigial struc- 

 tures are commonly still visible after their 

 function has ceased. The complex genetic 

 system basic to the development of a com- 

 plex adaptive character cannot be elimi- 

 nated suddenly without affecting many 

 other vital characters and processes. Large 

 numbers of the genes have become so much 

 involved in the development of other adap- 

 tive characters through the action of selec- 

 tion over long periods of time that much of 

 the gene complex activating the growth of 

 a character that has lost survival value may 

 be retained, even though portions of the 

 gene pattern may have become modified as 

 the selection pressure fluctuated. 



Proof that the genes may be largely in- 

 tact even when a given character has un- 

 dergone evolutionary degeneration may be 

 demonstrated in the segmented, sexual, and 

 social forms. If one segment has legs re- 

 duced, while other segments retain legs. 



obviously the genes for legs are not lost, 

 and the reduction must depend upon a 

 threshold of development. This threshold, 

 in turn, may be determined by other genes 

 and thus be inherited (Wright, 1934, 

 1934a). If one sex has lost its wings, while 

 the other sex retains them, the loss is not 

 through the loss of the gene complex, but 

 rather in developmental thresholds under the 

 influence of genetic, physiologic, or ecologic 

 factors. An apterous worker ant must have 

 the gene pattern for the functional wings 

 of its parents. Similarly, bUnd and wingless 

 soldier termites must have the basic genetic 

 system that produces functional eyes and 

 wings in their parents, even though this sol- 

 dier type is characteristic of its genus, fam- 

 ily, and order (Emerson, 1947). We are 

 led to the conclusion that organisms re- 

 tain ancient genes that have been selected 

 and incorporated into complex interrelated 

 systems and that gave rise to adaptive 

 characters in ancient environments, even 

 though the visible vestiges of these relict 

 adaptations may have disappeared entirely. 



Inasmuch as genes not only initiate the 

 development of morphological attributes 

 within a proper physiological and ecologi- 

 cal environment, but are also foundational 

 to the development of physiological pat- 

 terns (Needham, 1930) and behavior 

 (Emerson, 1938), we should expect to find 

 vestigial activities and recapitulative tend- 

 encies in the physiology and psychology 

 of organisms (p. 636). 



The interrelationship of the genetic, 

 physiologic, psychologic, and ecologic in- 

 fluences in regressive evolution has often 

 been misunderstood. The Ozark cave sala- 

 mander (Typhlotriton spelaeus) demon- 

 strates some of these interrelationships. The 

 eggs of this salamander are laid in pools 

 outside the caves and develop into eyed 

 and pigmented larvae (Noble and Marshall, 

 1929; Bishop, 1944). Normally these larvae 

 move into the caves. In the dark, the eye- 

 lids become fused and much pigmentation 

 is lost, but, if experimentally kept in the 

 light, these cave modifications do not 

 develop. A closely related and possibly an- 

 cestral species (T. nereus) lives in the 

 same vicinity, but usually not near caves. 

 If it enters a cave it also loses much of its 

 pigment, but its eyeUds do not fuse. In 

 both cases the genes for eyes and pigment 

 are obviously present. Physiologic thresh- 



