654 



ECOLOGY AND EVOLUTION 



was noted in 1914, and since then the scale 

 has increased its resistance and the range 

 of the resistant population. The resistance 

 varies directly with population density, 

 large populations being more resistant 

 (Knight, 1932). Under experimental con- 

 ditions, the diflFerence in resistance of 

 populations from diflferent areas has been 

 adequately demonstrated (Lindgren and 

 Sinclair, 1944; Lindgren and Dickson, 

 1945). Experiments also indicate differ- 

 ences of resistance to methyl bromide and 

 ethylene dioxide. Resistance depends upon 

 a single sex-Unked gene or group of closely 

 linked genes in the X cliromosome. The 

 variation probably arose by mutation and 

 spread by selective elimination of the non- 

 resistant strain. Resistance has been main- 

 tained for sixty generations under experi- 

 mental conditions. Crosses between resist- 

 ant and nonresistant strains show interme- 

 diate resistance of the population. Resist- 

 ance is a physiological character of the 

 living insect, and not of the scaly covering. 



The recent evolution of insecticide-resist- 

 ant strains has also been reported for such 

 insects as the San Jose scale {Quadraspidio- 

 tiis perniciosus) to lime-sulfur spray; for 

 the black scale {Saissetia oleae) to hydro- 

 cyanic acid fumigation; for larvae of the 

 codling moth {Carpocapsa pomonella) to 

 arsenical and other sprays; for the citricola 

 scale (Coccus pseiidomagnoliarum) to hy- 

 drocyanic acid fumigation; for the screw 

 worm (Cochliomijia americana) to pheno- 

 thiazine; and for the citrus thrips (Sciiio- 

 thrips citri) to tartar emetic-sucrose spray 

 (Quayle, 1943). The evolution of insecti- 

 cide-resistant races of insects within a few 

 years indicates the speed with which sim- 

 ple adaptive changes may take place under 

 constantly applied selection, and parallels 

 the speed of evolution under artificial selec- 

 tion, being in fact a negative type of arti- 

 ficial selection. 



Adaptive evolution without selection 

 through the agency of man is usually a 

 much slower process, but in some cases 

 may occur within a few thousand years. 

 Subspecies adaptations to soils left by the 

 Quaternary Lake Lahontan in Nevada have 

 doubtless evolved since the late Pleistocene 

 in five species of rodents and a species of 

 fox (Hall, 1946, p. 61; for recent adaptive 

 evolution, also see p. 611), Simpson (1944, 

 p. 19) states that moq^hological differen- 



tiation of subspecies of rodents may take 

 place in even less than 300 generations. 



Mathematical analysis indicates how se- 

 lection may influence the incidence of a 

 gene. If a new dominant gene has an ad- 

 vantage of 0.001 and appears by mutation 

 with a frequency of 10"", it must appear 

 347 times before the odds favor its spread 

 (Haldane, 1932, p. 200). This would re- 

 quire 347,000,000 individuals. In most in- 

 sects and even in man, the new gene could 

 thus start to spread in a single generation, 

 but in the Indian elephant, with a popula- 

 tion of 20,000 and a generation on an aver- 

 age of every forty years (male elephants 

 mature at twenty years of age and females 

 at the age of sixteen), it would be nearly 

 a milUon years before such a new gene 

 could spread to a large enough fraction of 

 the population to be sure of spreading far- 

 ther. A new recessive gene would have 

 much less chance of spreading. In a small 

 inbreeding population, chance would favor 

 the continuation of such a recessive gene, 

 thus allowing selection to begin to operate 

 (pp. 602, 603). The time required for a 

 novel mutation to reach high frequencies 

 is probably less important than the mech- 

 anisms that keep it and its alleles at me- 

 dium frequencies for long periods and thus 

 make it an element in the store of varia- 

 bility (p. 641). 



Simpson (1944, p. 66) says that a muta- 

 tion of definite selective advantage (0.01) 

 arising at the rate 0.000001 in a population 

 of 10,000,000 is sure to become established 

 within 25 generations, but in a population 

 of 10,000 may require 25,000 generations— 

 a time so long that in many cases the selec- 

 tive advantage or other limiting factors are 

 Ukely to change. Other conditions being 

 equal, the selection advantage would 

 probably be different in populations of 

 markedly different size. Wright (1940a, p. 

 178) says that it is probable that most mu- 

 tations important in evolution have much 

 smaller selection coefficients than can be 

 demonstrated in the laboratory. It is even 

 more difficult to demonstrate minute selec- 

 tion coefficients in the field, and yet these 

 may be of great evolutionary importance. 



Observations on the percentage incidence 

 of a deleterious gene in a population may 

 be indicative of selection pressure. The 

 gene producing hemophilia in man is a 

 sex-linked recessive to the normal allele. 



