ECOLOGY AND EVOLUTION 



634 



heritance. To these principles may be 

 added regeneration, regressive evolution 

 and convergence illustrated by other ter- 

 mite nests (Emerson, 1938). The only 

 conclusion possible is that the evolution of 

 behavior results from similar forces and fol- 

 lows patterns similar to those characteristic 

 of morphological evolution, and that both 

 depend upon physiological development 

 and genetic interactions. 



At times it is well to distinguish adapta- 

 tion to a general situation from speciahzed 

 adaptation. The term "adaptation" is some- 

 times used to denote capacity to succeed, 

 rather than functional adjustment. Gen- 

 erahzed organisms sometimes may outlive 

 specialized forms in geologic time. Simp- 

 son (1944, p. 31) states that "reduction 

 of adaptabihty with increase in specializa- 

 tion" is a leading paleontologic and evo- 

 lutionary empirical principle today. He also 

 says (p. 180) that the best criterion for 

 adaptation is the increase in numbers of 

 the better-adapted group in comparison 

 with the less-well adapted. Although this 

 criterion is sometimes valid and has the 

 advantage of being a testable and quantita- 

 tive concept, in the sense of increasing fit- 

 ness, numbers may be detrimental (or 

 beneficial) to the population as a whole, 

 particularly with an increase in population 

 density (pp. 274, 332, 396). It is also true 

 that a rare animal or plant may be well 

 adapted to an environment with narrow 

 limits, while an abundant organism may be 

 less well adapted to an extensive environ- 

 ment. The slave-making ant, Polyergus, is 

 a rare ant in comparison with the slaves, 

 Formica. The structure and behavior of the 

 slave-making species, however, are indica- 

 tive of a high degree of adaptive speciali- 

 zation (see p. 424). 



Fitness in terms of adjustment to special 

 factors affords a highly important measure- 

 ment of adaptation, although such fitness 

 may be complex and difficult to compare 

 quantitatively. For example, it may be 

 stated that an extinct bird such as the pas- 

 senger pigeon was better adapted for ffight 

 than the common chicken or grouse. It is 

 therefore plain that adaptation is a com- 

 plex phenomenon that cannot be reduced 

 to a single type of measurement without 

 grossly oversimplifying the concept. Num- 

 bers are highly important in a considera- 

 tion of adaptation, but must be placed in 



their proper perspective. Numbers should 

 also be considered in relation to biomass 

 (pp. 525-528). Biomass alone, however, 

 would give an oversimplified conception of 

 adaptation. It would be like judging the 

 importance of a tissue in the body by 

 measuring its relative bulk. 



The combination of adaptations in a 

 single organism to various factors in the 

 habitat indicates an evolutionary history in 

 the particular environment as a whole. For 

 example, Stebbins (1944) fists several 

 adaptations of the fizards of the genus Uma 

 to their desert habitat. These include over- 

 lapping of the free edges of the eyelids, 

 which prevents sand particles from in- 

 vading the eye; a translucent area in the 

 lower eyefid, which permits the perception 

 of fight changes when the eyefids are 

 closed; a nictitating membrane over the eye, 

 which expels mucus-encapsulated sand; 

 fringe-scales on elongated toes, which as- 

 sist in locomotion over the sand and in 

 "sand-swimming" during submergence be- 

 low the surface; coloration approximating 

 the sand background with special markings 

 that mask the eye and render the animal 

 inconspicuous in the shadows of bushes; 

 and behavior activities associated with fife 

 on the sand, feeding adjustments, and 

 escape from enemies; together with adjust- 

 ment to temperature variations (Cowles 

 and Bogert, 1944; Parker, 1948, p. 308). 

 Adaptation is evidently not to a single 

 factor in the environment, but to a com- 

 bination of factors, and this combination is 

 often characteristic of special habitats. 

 When adjustments to one factor involve 

 the impairment of adjustments to another 

 factor, selection will guide evolution either 

 toward adjustment to the more important 

 factor or to a compromise between the two. 

 No organism can survive in a habitat in 

 which any single factor essential for its ex- 

 istence is lacking (Wolcott, 1942; pp. 198, 

 635), but organisms often— one might say 

 always— survive with only partial fitness to 

 each ecologic factor in the complex envi- 

 ronment (Bacot and Martin, 1924). 



In addition to its adjustment to the im- 

 mediate habitat, an organism often needs 

 to be adapted to rare and extreme condi- 

 tions in order to survive— conditions that 

 may not occur in the fifetime of an individ- 

 ual. Between 1886 and 1936, nine sudden 

 cold periods or "freezes" occurred in the 



