NATURAL SELECTION 



663 



niches were being created by the evolution 

 of the plants and other terrestrial animals, 

 and no competitors had arisen to prevent 

 the rapid evolution into widely divergent 

 habitats. For both mammals and insects, 

 competition was weak during the period of 

 greatest adaptive radiation. Contrariwise, 

 strong competition forces the extinction of 

 primitive forms or allows their survival only 

 as rehcts in regions or niches with weak 

 competition (p. 679) and prevents adaptive 

 evolution into occupied niches. 



Oparin and Morguhs (1938) explain the 

 contemporary absence of intermediate 

 forms between inorganic systems and hving 

 organisms through the possible elimination 

 of incipient Ufe by highly adapted types. 

 Before the present organisms originated, 

 however, a slow transition from the inor- 

 ganic to the organic might have taken 

 place (see also p. 75). 



Much subtle evolution in small niches 

 probably occurs in the absence of competi- 

 tors. Worthington (1940) gives an account 

 of the speciation of a genus of cyprinid 

 fishes of open water, Engraulicypris, which 

 exhibits topographical isolation in the Afri- 

 can lakes (Victoria, Tanganyika, Rudolf, 

 Nyasa, and Rukwa). In Lake Edward, 

 however, no species of Engraulicypris oc- 

 curs, but the same niche has been filled by 

 a cyprinodont, Haplochilichthijs pelagicus. 

 Most cyprinodont fishes are inhabitants of 

 shores and swamps, and this is the only 

 species of the family that has become 

 highly modified toward plankton feeding in 

 open water. Worthington thinks that the 

 evidence points to a rapid evolution of 

 these adaptations since the Pleistocene arid 

 period, which separated the two major 

 pluvial periods in central Africa, and draws 

 the conclusion that "where a good niche 

 exists, vacant for reasons of isolation, some 

 species will fill it rapidly, even though con- 

 siderable structural alterations are involved 

 in the process." The time involved in this 

 case is considered to be of the order of 

 15,000 to 40,000 years. The last pluvial 

 period in Africa seems to have coincided 

 with the last glacial period in Europe. The 

 retreat of the continental glaciers began 

 30,000 to 40,000 years ago (p. 81). 



Some authors (Bertalanfi^y, 1937; Friel- 

 ing, 1940) express the opinion that major 

 evolution of the larger categories has come 

 to an end, and only minor speciation or ra- 



ciation is now possible, particularly because 

 of the presence of man. Just (1944) thinks 

 that such generahzations are unwarranted. 

 Occupied niches probably prevent adaptive 

 evolution of forms that would otherwise 

 radiate, but the evolution of new organisms 

 creates new biotic niches. The advent of 

 man may result in evolutionary spurts of 

 certain organisms at the same time that ex- 

 tinction or reduction of populations occurs 

 for many other species. To some extent 

 future evolution will be directed or pro- 

 foundly influenced by man, but it seems 

 safe to say that much will take place in 

 spite of or because of his influence. 



Kropotkin (1902) concluded that intra- 

 species competition is always harmful. His 

 view is in contrast with the Spencerian 

 concept that progressive evolution depends 

 almost wholly upon competition, whether 

 intraspecies or interspecies. Both these 

 early interpreters of biological theory based 

 their assumptions upon a meager accumu- 

 lation of data, and their generahzations 

 were oversimphfied and somewhat subjec- 

 tive. In organic evolution, intraspecies and 

 interspecies competition doubtless have a 

 strong effect through selective elimination; 

 on the other hand, selection hkewise may 

 tend to mollify both types of competition. 

 Competition usually seems to have an op- 

 timum, too little and too much both acting 

 to the detriment of the species (p. 395) 

 and thus creating selective pressures that 

 guide the evolution of the competitive 

 system itself. The effect of competition may 

 grade all the way from a slight population 

 pressure, which would tend to space com- 

 peting individuals in relation to territory, 

 food, or mates (see p. 413), to a drastic 

 lethal elimination of the loser of a dual 

 combat. It is pointed out elsewhere (pp. 

 692, 706) that death of individuals is not 

 necessarily harmful to the species. The 

 elimination of the genetically unfit individ- 

 uals results in adaptive evolution. 



Competition between individuals within 

 a group, or between groups, may select 

 genetic capacities for learning and for 

 cooperative social organization. Guhl and 

 Allee (1944) compared organized and un- 

 organized flocks of hens that were presum- 

 ably without important genetic differences. 

 The organized flock had less individual 

 combat, consumed more food, and laid 

 more eggs. It is quite conceivable that a 



