NATURAL SELECTION 



681 



Mesozoic times (Fig. 243). The habitat of 

 these intestinal flagellates is also extremely 

 stable, and adaptation may be so high as 

 to prevent the survival of genetic modifi- 

 cations. 



Mutation is a basic cause of variability 

 upon which evolution depends (see pp. 

 600, 601, 638, 662). Genes can be de- 

 tected only through their mutation. If a 

 gene has not mutated, the geneticist is un- 

 able to gather evidence of its existence. 

 General effects of groups of genes may be 

 postulated from a study of chromosome 

 deletions (McClintock, 1944). 



Differences in mutation rate suggest a 

 differential in gene stabihty. The relative 

 stability of plasmagenes, plastogenes, nu- 

 clear genes, and chromosome systems may 

 also differ (Darlington, 1944; see also p. 

 602). We may thus expect some hereditary 

 imits to maintain chemical structure over 

 long periods of time, ages that would make 

 genetic homology conceivable through long 

 geological intervals. In some cases it may 

 be assumed that the whole gene maintains 

 stability with constant physiological effects 

 (Gushing, 1945), while other genes mutate 

 to produce divergent physiological effects 

 in the development of the same character. 

 In other instances the gene may mutate to- 

 ward a series of alleles while the basic 

 homologous gene structure is maintained to- 

 gether with certain of its physiological ef- 

 fects. 



From the idea that the observed genes 

 constitute the whole of the genetics of a 

 structure, some authors have assumed that 

 homology does not necessarily rest upon 

 the genetic constitution (Harland, 1933; 

 de Beer, 1938), a conclusion not in accord 

 with the probable stability of many genes 

 or the general effects of the genes in an 

 allelic series. In conformity with the theory 

 of the high mutation pressure of every 

 locus during geological time, the explana- 

 tion of the continuance of a homologous 

 organ would be based upon the selective 

 incorporation of each mutation. In an or- 

 gan like the vertebrate eye, seemingly 

 homologous in all vertebrates since Ordo- 

 vician times, it would not be assumed, ac- 

 cording to this theory, that the basic genes 

 initiating the development of the eye are 

 the same or similar in fish and mammal. 

 Rather, it would be assumed that all had 

 mutated many times, but that each muta- 



tion was selected in terms of its function in 

 the eye and gradually replaced the older 

 genes that may have served a similar func- 

 tion somewhat less efficiently in the an- 

 cient eye (or organism). This concept 

 places the burden of the explanation of 

 homologous structures maintained through 

 long geological ages upon selection rather 

 than upon genetic stability. 



Trivial unadaptive structures may be 

 characteristic of higher taxonomic groups 

 through millions of years of speciation. 

 Emerson (1942a) cites the case of a use- 

 less subsidiary tooth in the mandible of 

 certain primitive termites (Stolotermes: 

 Hodotermitidae) that is also characteristic 

 of an entire somewhat advanced family 

 (Rhinotermitidae). If secondarily produced 

 by a favored gene complex, such charac- 

 ters may be explained by gene stability, 

 while it would be difficult to think that 

 direct selection could be sufficiently strong 

 to maintain them. If homology is based 

 upon constant selection, there is no reason 

 to assume that the secondary, nonadaptive 

 effects of genes would remain stable while 

 the selection of numerous mutations is 

 causing a shift in the gene pattern. With 

 this evidence of the stability of genes, we 

 may still rest the concept of homology 

 upon some degree of constancy of the 

 genetic system through geological time. 



Inbreeding in small populations may re- 

 duce the field of variability through the 

 homozygous fixation of genes and the pre- 

 vention of reassortment (Wright, 1940a, 

 p. 167; see also pp. 407, 602). 



As a consequence of the Mendelian 

 mechanism. Hardy (1908) pointed out 

 that the relative frequencies of various 

 genes in the population are maintained 

 from generation to generation, regardless of 

 the absolute values of their initial fre- 

 quencies. This equilibrium is to be ex- 

 pected only in a sexually reproducing, ran- 

 dom breeding population in which the 

 genotypes are equivalent with respect to 

 natural selection, in which immigration 

 does not occur, and in which mutation 

 pressure is zero. This concept has been re- 

 considered and somewhat modified by 

 Wright (1931), Haldane (1932), Fisher 

 (1930), and Kollross (1944). Hardy's 

 theory pertains to the average condition 

 only without disturbing factors. Under 

 natural conditions it would be expected 



