NATXmAL SELECTION 



683 



upon the various levels of gene integration 

 and gene pattern, sometimes eliminating 

 the new and less effective variations, some- 

 times eliminating the older and less effec- 

 tive systems. Effectivity will vary with 

 changes in the organismic system and in 

 the habitat. If a form is highly adapted to 

 a durable stable environment, genetic 

 modification is more Ukely to be selectively 

 eliminated than if the organism is less well 

 adapted (Simpson, 1944, p. 141, 149), and 

 a rapid adaptive evolution may be followed 

 by a retardation of the further evolution of 

 both endoadaptations and exoadaptations. 



Lack of ecological opportunity results in 

 a retardation of evolution (pp. 600, 662). 

 The more developed the special adaptation, 

 the more likely will be the extinction of the 

 form when the environment changes. The 

 more rapidly the environment changes, the 

 greater are the number of forms that be- 

 come extinct. 



For purposes of analysis, experimental 

 control of different factors, or observation 

 of natural instances in which only one or 

 a few factors vary and all others remain 

 constant, is desirable. Evolution usually oc- 

 curs as the result of a large number of 

 variable pressures influencing the popula- 

 tions at the same time and at different 

 rimes, and the isolation of all such factors 

 may be practically impossible in any given 

 case. Not only is there a multiplicity of fac- 

 tors with varying quantitative effects, but 

 these factors strike a certain balance in 

 successfully evolving organisms, so that 

 variation in one may influence a recipro- 

 cal variation in another (Fig. 229; see also 

 Mather, 1943). Wright (1932) summarized 

 this viewpoint as follows : 



"The most general conclusion is that evo- 

 lution depends on a certain balance among its 

 factors. There must be gene mutation, but an 

 excessive rate gives an array of freaks, not 

 evolution; there must be selection, but too 

 severe a process destroys the field of variability, 

 and thus the basis for further advance; prev- 

 alence of local inbreeding within a species 

 has extremely important evolutionary con- 

 sequences, but too close inbreeding leads 

 merely to extinction. A certain amount of cross- 

 breeding is favorable but not too much. In this 

 dependence on balance the species is like a 

 living organism." 



Balance between opposing systematic 

 pressures may produce stability. The tend- 

 ency toward the selective elimination of 



a gene may be opposed by its reintroduc- 

 tion by mutation or immigration. The fre- 

 quency of two alleles of the same gene may 

 be maintained by selection favoring the 

 heterozygote over either homozygote. There 

 may be little chance of particularly favor- 

 able mutations occurring if they can arise 

 only through a succession of unfavorable 

 mutations. Wright (1948) says that equihb- 

 rium frequencies of genes are restored 

 through such balanced pressures resulting 

 in the stability of subspecies and species in 

 spite of continuing mutations, continual in- 

 terbreeding at a low rate with neighboring 

 populations, and continuing selection. 



An examination of the factors negating 

 evolutionary change gives perspective on 

 the mechanisms of evolutionary advance. 

 Many existing organisms have evolved 

 slowly or have stopped evolving. Some are 

 on the verge of extinction. As is true in 

 evolutionary progress and regression, re- 

 tardation results from numerous factors, 

 often acting in comphcated interrelation- 

 ship (Wright, 1948a). 



ORGANISMIC LEVELS AND SELECTION 



Biologists have become increasingly 

 aware that there are different levels of or- 

 ganismic integration and that these levels 

 may be arranged in phylogenetic order. 

 Higher levels incorporate lower levels 

 (Schmid, 1941; Gerard, 1942; Needham, 

 1943). Such levels are discussed under a 

 number of terms, the gene and virus with 

 molecular organization; the cell with pro- 

 toplasmic parts; the cell colony and multi- 

 cellular organism with cellular parts; the 

 metamorphic organism with larval and 

 adult stages; the colonial organism with 

 zooids; and the metameric organism with 

 segments. 



Where direct protoplasmic connections 

 are broken, intraspecies population units at 

 various levels of integration are recognized 

 and referred to under various terms: the 

 species composed of individual organisms; 

 the cyclomorphic population with periodic 

 polymorphism; the aggregation, school, 

 flock, and herd with gradations of popula- 

 tion integration (p. 393); the sex pair v^dth 

 sexual dimorphism; the family with recipro- 

 cal adjustments between parents and off- 

 spring; and the society in the strict sense 

 (Chap. 24) with marked division of labor 

 between adults of the same sex (p. 687). 



