adaptation to the physical coiuhtions of tlic habitat, 

 avoidance of predation, more efficient physiological 

 functioning, and new behavior patterns. Xatitral se- 

 lection favors those variations that are adaptive, and 

 thereby fosters the continued existence and improved 

 reproduction of the species. If the population under- 

 going these changes is geographically isolated so that 

 the favored changes do not spread throughout the 

 species, tlien differentiation of characters leading to 

 speciation occurs. The actual mechanics of natural 

 selection are disputed, although understanding of the 

 general processes involved is steadily increasing. 



When the ratio of one character to another 

 changes from 1.00:1.00 to 1.01 :1.00 in each genera- 

 tion, the character is being selected for : more indi- 

 viduals with the character are surviving than are in- 

 dividuals without it. With a selective advantage of 

 1 in 100. a dominant character will become estab- 

 lished in 99 per cent of a population in about 1200 

 generations (Huxley 1943). This rate is considered 

 rapid evolution. .A selection pressure of even 1 in 

 1000 represents fairly rapid change, but when selec- 

 tion is decreased to 1 in 1,000,000 or more, evolution 

 is relatively slow. A good mathematical analysis of 

 selection pressures is given by Li (1955). 



The action of natural selection is evident in the 

 following examples. Scale insects, flies, and other in- 

 sect pests are commonly controlled by hydrocyanic 

 gas or DDT sprays. However, after many applica- 

 tions, surviving populations display immunity. Ap- 

 parently, normal populations are mixtures of resistant 

 and non-resistant genotypes. In the absence of the 

 fumigant, there is no selection between the two geno- 

 types, but with continuous fumigation, the resistant 

 types survive and reproduce until they become pre- 

 dominant in the population (Dobzhansky 1951). 

 Xatural selection presumably functions in much the 

 same manner to make the color of local populations 

 of small mammals and snails match the color of the 

 soil or vegetation as protection against predators 

 (Dice and Blossom 1937, Rlair 1951a, Sheppard 

 1954) : to correlate the pigmentation of butterflies 

 with local differences in temperature, moisture, and 

 solar radiation (Hovanitz 1941): to bring parallel 

 variants in many kinds of fish when in the same kind 

 of environment (Hubbs 1940) ; and generally to 

 establish the many other adjustments and adaptations 

 of organisms to their particular niches. 



Xatural selection is relatively less effective in 

 small populations than in large ones. Small popula- 

 tions may be locally restricted, come into conflict 

 with few competing species, and experience only a 

 favorable habitat. Because of the low selection pres- 

 sure, chance combination of genes (genetic drift) 

 may produce characters of little or no adaptive value 

 that yet have a good chance to persist while really 



;nla|)tive characters may be lost. In the Hawaiian 

 and Society Islands, there is a great variation in the 

 characteristics of snails that variously occur on the 

 different islands and locally in different isolated val- 

 leys or regions on the same island (Gulick 1905, 

 Crampton 1932). This is apparently a result of the 

 lixatioii of random variations in small populations 

 not subjected to any considerable selective pressure. 

 In ]jopulations that are increasing rapidly in size, in 

 the upswing of cycles, say, there is little selection, 

 and non-adaptive variations may persist as well as 

 adaptive ones. As populations come to saturate hab- 

 ited niches and disperse into new or less favorable 

 habitats, competition, predation, and parasitation in- 

 creases, and individuals become exposed to evermore 

 severe physical and climatic conditions. Natural se- 

 lection then functions, and characteristics that are 

 adaptively advantageous will tend to persist while 

 those less favorable or even harmful are eliminated. 



Mutations upon which natural selection works are 

 often recessive. They do not become fully expressed 

 in the phenotype except when the individual is homo- 

 zygous for the character. Nevertheless they are im- 

 portant, and in stabilized populations tend to persist 

 indefinitely in constant proportion to the dominant 

 alleles (Hardy-Weinberg law). A recessive charac- 

 ter will become more prominent in a population if 

 that particular gene continues to mutate toward the 

 recessive, or if the homozygous recessive phenotype 

 has adaptations that give it selective advantages. In 

 this latter case, natural selection may ultimately re- 

 sult in complete suppression of the dominant allele so 

 that the hitherto recessive allele becomes perma- 

 nently fixed in the population as the only gene for 

 the character. 



Since emergent species usually entrain adapta- 

 tions to new environments, it would appear that those 

 characteristics by which we distinguish species and 

 higher taxonomic categories generally are such as 

 serve some useful purpose to the organisms either 

 structurally, functionally, or in point of behavior. It is 

 often very difBcult to determine a useful function for 

 all distinctive characteristics of a species, yet one can 

 seldom be positive that a seemingly minor character 

 does not serve, say, as a releaser for some critically 

 necessary behavior or is not vitally important in 

 other unsuspected ways. However, not all characters 

 that distinguish species or higher taxonomic catego- 

 ries are necessarily adaptive (Robson and Richards 

 1936, Simpson 1953). Some characters originally 

 adaptive may have lost their usefulness, although 

 they persist in the organism. With natural selection 

 no longer acting on them, they usually retrogress and 

 may eventually disappear as have skin pigmentation 

 and eyes in many cave animals, for instance. Other 

 characters may even have a slightly unfavorable 



Speciation 265 



