\i)n-hcritahli' variations 



Nut all \ariatioiis of organisms are of direct 

 significance in spcciation. \'on-lieritai)le changes in 

 hocly structures, functions, and behavior are coninion. 

 If muscles are used continually and intensively, they 

 become thicker and stronger ; if one kidney is re- 

 moved, the other becomes hypertrophied ; skin sub- 

 jected to frequent rubliing or pressure thickens and 

 becomes horny : and so on through a lengthy catalog. 

 Animals progressively exposed to ever more severe 

 temperatures or lower oxygen concentrations will tol- 

 erate extreme conditions, which, had they been sud- 

 denly presented, would have been fatal. Insect larvae 

 transferred to a new type of food often become so 

 conditioned to it that they produce a strain that pre- 

 fers that food to other more usual food of the species. 

 . Chimney swifts in wilderness North America nested 

 in hollow trees, but with settlement of the country 

 and the construction of chimnied buildings during 

 the last two centuries, the species has changed its 

 behavior almost completely, accepting chimneys as a 

 satisfactory nest-site location. 



Many phenotypic adaptations persist genera- 

 tion to generation, either as similar responses made 

 by each generation to constant environmental condi- 

 tions : as the result of imitation of parents, condi- 

 tioning of young, or imprinting (Gushing 1941, 1944, 

 Thorpe 1945) : or because the particular genes re- 

 sponsible for these characteristics have been sorted 

 out (canalized) from the general gene pool of the 

 sjiecies (^^'addington 1957). One would expect a 

 change in behavior or function, arisen in consequence 

 of exposure to the new conditions, usually to presage 

 the evolutional development of a new structure, for 

 natural selection cannot bring about the structural 

 adaptation or perfection of an organ unless the organ 

 is already being used for the new purpose (Prosser 

 1957). According to the so-called Baldtvin effect, a 

 mutation which afTects established behavioral or 

 functional adjustments is more likely to become per- 

 manently fixed in the germplasm than if the popula- 

 tion were not already so adjusted phenotypicallv 

 (Baldwin 1896, 1902, Simpson 1953a). 



Polymorphism 



When individuals of a population can be 

 grouped into several color phases, distinct body sizes, 

 or other character variations, the population is said 

 to display polymorphistn. Polymorphism of a char- 

 acter arises in a species when heterozygotes persist 

 in an environment in the face of natural selection and 

 homozygotes are reduced or eliminated. For instance, 

 individuals with certain characteristics may be better 



pic to 

 ^oregonensis 



ntergrodation areo 



eschscholtzi- 



FIG. 19-2 Speciatlon through distance In the salamander species 

 Emafina eschicholfzli In California. The coastal subspecies pic/o 

 nnay represent the ancestral type and demarcate the center of 

 dispersal from which cllnal lines became dispersed southward in 

 the coastal and Interior mountains separated by a barrier. Re- 

 cently, the subspecies xanthoptico crossed this barrier. Inter- 

 breeding occurs between adjacent subspecies, but partial re- 

 productive Isolation obtains between xanthopfico and plafensis, 

 and complete isolation similar to that of species obtains between 

 eschscholhli and klauberi (Stebblns l?49). 



adapted to environmental conditions during the 

 spring, while individuals with other characteristics 

 are superior in summer or autumn. This results in a 

 mixture of types in the population more or less segre- 

 gated by seasons. Likewise populations may vary in 

 characteristics as adaptations to local habitat condi- 

 tions. An increase or decrease in the frequency of 

 a given characteristic appears a result of variations 

 in the selective pressure of the environment, permit- 

 ting individuals with certain characteristics or gene 

 combinations to survive at one time or place ; other 

 individuals, at other times and in other places. In an 

 environment that presents the same set of selective 

 pressures year after year, there commonly occurs a 

 stability in the ratios of the different forms in which 

 a character is manifested (Ford 1940, Dobzhansky 

 1951, 1956, 1958, Sheppard 1958). 



Most species differ not by single genes but by 

 hundreds certainly, possibly by thousands, of genes. 

 When panmixia (free interbreeding) obtains in a 



Speciation 261 



