664 



ECOLOGY AND EVOLUTION 



genetic capacity to organize through ex- 

 perience could be selected and result in 

 the evolution of social organization based 

 upon nongenetic individual or group dif- 

 ferences (pp. 631, 632, 639, 686, 691, 

 693). 



Simpson (1944, p. 31) says: 



"From every point of view there is an 

 essential difference between variation of in- 

 dividuals within a group and variation between 

 groups, but the two are often inadequately 

 distinguished. Natural selection, for instance, 

 acts on both, but its action on intergroup 

 variation can produce nothing new; it is purely 

 an eliminating, not an originating, force. 

 Despite its critics, the action of natural selection 

 on intragroup (or interindividual) variation is 

 essentially an originating force: it produces 

 definitely new sorts of groups (populations), 

 and the interbreeding group is the essential unit 

 in evolution. Action on intra-individual varia- 

 tion also occurs, but, again, can only eliminate, 

 not originate, types of individuals or of in- 

 dividual reactions." 



One may take partial exception to 

 Simpson's conclusion. Elimination of in- 

 dividuals may affect the genetic composi- 

 tion and adaptational response of the group. 

 Elimination of groups may aflEect selection 

 pressures on competing or dependent groups 

 with consequent changes in gene incidence 

 and patterns. It is admitted that reproduc- 

 tive isolation interferes with further ex- 

 change of genes between two populations 

 with consequent inability to produce new 

 characters through reassortment. Selection, 

 however, guides the development of heredi- 

 tary patterns, whether operating on intra- 

 species subgroups or on the whole species 

 in its relation to other species, and conse- 

 quently is largely responsible for progres- 

 sive evolution. Otherwise, it would be dif- 

 ficult to explain both intraspecies and inter- 

 species adaptations by any known princi- 

 ples, and both have doubtless evolved (see 

 pp. 684, 695, 728). 



Selection may be interpreted as exerting 

 a pressure toward the determination of new 

 genetic patterns by a succession of choices. 

 Selection may thus produce a balance be- 

 tween species and between individuals 

 within a species (p. 684), as well as be- 

 tween parts of an organism. Wright (1945, 

 p. 416) concludes that selection between 

 partially isolated groups is perhaps the 

 greatest creative factor, making possible 

 selection of genetic systems as wholes 



rather than the mere selection of allele 

 effects. He also states that drastic elimina- 

 tion of famiUes and orders, and the com- 

 pensatory adaptive radiation of successful 

 groups, are highly creative. 



We conclude that selection is the only 

 mechanism that adequately explains the 

 evolution of complex endoadaptation and 

 exoadaptation and that competition exerts 

 a strong but not exclusive influence in en- 

 hancing selection pressures. Competition 

 usually has an optimal value in exerting 

 selection pressures resulting in progressive 

 evolution. Comp'^tition may be either above 

 or below the optimum in any given case 

 and is thus itself subject to modifying 

 selective pressures (see Hutchinson, 1948, 

 for a discussion of circular causal systems). 



ADAPTIVE RADIATION AND 

 CONVERGENT EVOLUTION 



The student of phylogeny of any division 

 of the animal or plant kingdom has long 

 been aware that the main branches, at 

 least, represent adaptive adjustments either 

 to internal efficiencies or particular environ- 

 ments. For example, the subclasses of mam- 

 mals are distinguished by their embryonic 

 development— within an egg, a marsupium, 

 or a uterus. An adaptation increasing the 

 internal efficiency of the organism may be 

 termed a general adaptation. Secondarily, 

 the orders of mammals seem to be asso- 

 ciated with special adjustments to the re- 

 spective environments, such as food special- 

 izations, locomotor specializations, or special 

 means of defense. Intraspecies aggregation 

 relations are important (Chap. 23), stem- 

 ming in part from the sex and family ad- 

 justments of all mammals and moving 

 toward special herd, flock, or pack organi- 

 zations in some of the orders and lower 

 taxonomic categories. Thus, an adaptive ra- 

 diation of internal characters, intraspecies 

 characters, and interspecies characters is 

 exhibited by the mammals (Fig. 234). 

 Every large group of organisms shows simi- 

 lar radiate evolution. (Rapid or "explosive" 

 radiation is discussed on pp. 600, 662). 



Adaptive radiation is illustrated by many 

 examples among the vertebrates. Mesozoic 

 reptiles and Tertiary mammals are oft-cited 

 cases. The bills of birds show remarkable 

 adjustments to food types and food posi- 

 tion (Amadon, 1947; Lack, 1947). 



With their vast number of species, adap- 



