NATURAL SELECTION 



641 



and of its conscious misapplications in 

 human affairs, both in economics and in 

 interclass and international warfare, can 

 hardly be overemphasized (Hofstadter, 

 1944). Experimental as well as observa- 

 tional studies of survival are numerous in 

 recent biological Hterature, and the mathe- 

 matical analysis of the process is at present 

 being actively prosecuted (Chap. 18, p. 

 271; alsopp. 646, 654, 657). 



The concept of the internal integration 

 of the individual organism and of other 

 unitary biological systems has been and is 

 now a field of investigation occupying the 

 attention of thousands of biologists, in- 

 cluding cytologists, physiologists, embryolo- 

 gists, psychologists, population biologists, 

 and students of speciation. Competition be- 

 tween individuals seems to be essential to 

 Darwin's original theory (see Alice, 1940). 

 Modern investigation certainly indicates 

 that selection often operates through com- 

 petition, and that release from competition 

 allows forms to survive that would other- 

 wise be eliminated (Salisbury, 1936). This 

 is particularly well illustrated in animals 

 under domestication. Selection, however, 

 may also operate on internal factors of 

 viability in the absence of competition, 

 and we might expect some adaptive evolu- 

 tion in an ideal world with overabundance 

 of all the necessities of life. On the other 

 hand, unrestricted competition may easily 

 result in harm and even extinction to both 

 competitors (p. 699). Competition in its 

 relation to evolution, both biological and 

 social, may be beneficial at an optimum 

 and deleterious both at a minimum and at 

 a maximum population density (p. 395). 

 Fitness involves cooperation, and adapta- 

 tions leading to coordination of parts of 

 organisms and of individuals in populations 

 are the result of evolution through natural 

 selection (p. 683). 



Reproductive isolation and its essential 

 role in evolutionary diversification were not 

 adequately understood by Darwin; but, 

 starting with the work of Wagner (1868a), 

 Wallace, and GuHck, this concept has ex- 

 panded in the hands of geneticists, ecolo- 

 gists, and biogeographers. Darwin probably 

 overemphasized natural selection as a fac- 

 tor in the origin of species. A better name 

 for his great book might have been "The 

 Origin of Adaptation." 



Pearl (1930a) outlined the basic and 



minimal logical requisites to a proof that 

 natural selection has altered a race. These 

 he lists as: (A) proof of somatic difference 

 between survivors and eUminated (p 

 649); (B) proof of genetic differences be- 

 tween survivors and eliminated (pp. 652, 

 653, 687); (C) proof of effective time oi 

 elimination (p. 692); (D) proof of somatic 

 alteration of race (pp. 653-655); (£) 

 proof of genetic alteration of race (p. 654). 

 Proofs on these points are summarized in 

 the following pages. 



Natural selection cannot be construed as 

 a causative force producing genetic muta- 

 tion in the strict sense, but only as a sieve 

 that eliminates the unfit and allows the fit 

 to pass tliiough. In another sense, natural 

 selection is a causative force that deter- 

 mines the pattern of hereditary units 

 through selective sorting after recombina 

 tion. 



The basic factors initiating the evolution 

 of adaptation must be sought in the causes 

 of genetic variability. Mutation as seen in 

 the laboratory nearly always reduces a 

 structure or function. We must remember, 

 however, that geneticists tend to choose 

 the mutations that exhibit striking charac- 

 teristics in order to treat them statistically 

 with greater ease. Slight morphologic, be- 

 havioristic, or physiologic changes are 

 usually not noticed, but these are just the 

 types of mutation most liable to be involved 

 in the evolution of natural populations. 

 More drastic mutational effects are Ukely to 

 be less viable (p. 275). Both initial muta- 

 tion and recombination, however, are pre- 

 dominantly random so far as any adjust- 

 ment to the environment is concerned, and 

 complex adaptation must thus be conceived 

 as evolving through selection acting as a 

 slow sorting process that ultimately ar- 

 ranges the genes in a harmonious func- 

 tional assemblage. 



Recombination, rather than mutation, is 

 often the most important factor producing 

 the hereditary variation within a popula- 

 tion that is essential to evolution, both 

 nonadaptive and adaptive (Simpson, 1944, 

 p. 36). Wright (1932) has succinctly 

 stated the relation between haphazard gen- 

 etic variation and adaptive evolution. 

 "The observed properties of gene mutation 

 —fortuitous in origin, infrequent in occur- 

 rence and deleterious when not negligible 

 in effect— seem about as unfavorable as pos- 



