642 



sible for an evolutionary process. Under 

 biparental reproduction, however, a limited 

 number of mutations which are not too in- 

 jurious to be carried by the species furnish 

 an almost infinite field of possible varia- 

 tions through which the species may work 

 its way under natural selection." 



PREADAPTATION AND HABITAT 

 SELECTION 



Before taking up the positive and nega- 

 tive aspects of selection, it is well to dis- 

 cuss the chance adaptive effects of varia- 

 tion (preadaptation), and the role of selec- 

 tion of a favorable habitat by the organism 

 (called organic selection by Baldwin, 



ECOLOGY AND EVOLUTION 



called its prospective function, in contrast 

 with its realized function (Parr, 1926; 

 Simpson, 1944, p. 184; Fig. 234). The en- 

 vironment also has its prospective and real- 

 ized functions or fitness for organisms (p. 

 73). Preadaptation has been defined by 

 Simpson as "the existence of a prospective 

 function prior to its realization." 



Salaman (1936) studied a wild tuber- 

 bearing solanum (Solanum demissum) that 

 lives in Mexico more than 8000 feet above 

 sea level. Cultivation in England showed 

 that the plant carried a recessive gene 

 producing resistance to wart disease (Syn- 

 chitrium endobioticum) . By selection, an 

 immune pure-breeding stock was produced. 



REALIZED FUNCTIONS 



f777Z) WHALES 

 1?^^^ SHARKS 

 R^?^ FIELD OF REALIZED CONVERGENCE 



Fig. 234. Convergence and radiation in evolution of phylogenetically prospective functions 

 of organisms through overlap with the prospective functions of their environments. A, Con- 

 vergence in evolution through overlap of some of the same prospective functions of an en- 

 vironment by phylogenetically prospective functions of two diflFerent groups of organisms. B, 

 Divergence in evolution through overlap of prospective functions of two different environ- 

 ments by the phylogenetically prospective functions of a single group of organisms. (Redrawn 

 from Simpson.) 



1896). Such adjustment precedes the sub- 

 sequent action of selection through ecologic 

 survival and elimination. 



The majority of mutations studied by 

 geneticists are not of the type that would 

 survive under natural ecologic conditions, 

 and many of them, such as the so-called 

 recessive lethals, do not permit homozygous 

 individuals to survive even under controlled 

 laboratory conditions. There is no doubt, 

 however, that among the myriad variations, 

 some mutations and combinations by 

 chance may produce characteristics that 

 have survival value in special habitats 

 (Gates, 1936). The potential adjustment of 

 the organism to the environment may be 



The same plant was also found to be resist- 

 ant to the bhght fungus (Phytophthora 

 infestans) through the action of a domi- 

 nant gene, although another form of Sola- 

 num demissum recently found in Mexico 

 is susceptible to bhght. The gene initiating 

 resistance to blight has been transferred to 

 the common potato by crossbreeding. In 

 both these cases the evidence indicates 

 that the mutations producing resistance to 

 these diseases arose independently of the 

 presence of the diseases, which do not oc- 

 cur in the native habitat of the plant in 

 which the variations originated (see also 

 Snyder, Baker, and Hansen, 1946). 



The fact that selection must act upon 



