NATURAL SELECTION 



653 



tions do not exhibit such seasonal cycles. 

 The flies of bombed-out urban districts 

 showed a decrease in chromosome-inversion 

 frequency. Divergent urban and rural varie- 

 ties seem to have resulted from a rapid en- 

 vironmental selection of only one of these 

 populations. Surviving liibernating female 

 flies were more fecund than nonhibernating 

 controls and transmitted this fecundity to 

 their female offspring. Natural selection 

 evidently operates rapidly during liiberna- 

 tion, with resultant higher fecundity of the 

 survivors. 



Toleration of heat and radiation by ani- 

 mals through color adjustments (Cole, 

 1943; Brown and Sandeen, 1948; Parker, 

 1948) indicates that color adaptation is 

 not only a response to selective pressure by 

 predators and selection of species and 

 sexual integration mechanisms, but is also 

 die result of selection by factors in the phys- 

 ical enviromnent. 



Another approach to the action of selec- 

 tion pressures may be made through the 

 study of tension zones or ecotones (pp. 

 476-478) between contrasting biota (Al- 

 bertson and Weaver, 1945, 1946; also p. 

 634). The common factors on the margins 

 of the range of a species often give an in- 

 dication of the primary barriers to further 

 distribution (Hall, 1946, p. 48). Griggs 

 (1942, 1946) gives data indicating that a 

 long period of climatic change has moved 

 the tree line down on Mount Washington, 

 New Hampshire, and that trees are now 

 occupying a wide zone on the White 

 Mountains that they could not colonize 

 under present conditions. Such evidence 

 does not indicate adaptive evolution, but 

 does indicate the ehmination of the unfit 

 by environmental selection. Such selection 

 by the physical environment may influence 

 the evolution of physiological adaptations 

 within the organism, irrespective of com- 

 petition between organisms (pp. 641, 

 656). 



White clover {Trifolhim repens) raised 

 from imported seed at Scalof, Sweden, 

 gave 100 units of green matter in the first 

 crop, and 129 units and 137 units in the 

 next two generations (experiments by N. 

 Sylven reported by Gregor, 1944). The 

 less hardy genotypes were probably elimi- 

 nated in each generation by the climatic 

 conditions in Sweden. Inbred relatively 



homozygous stock did not change under 

 similar conditions. 



In Australia, races of the subterranean 

 clover {Trijolium subterraneum) vary in 

 earliness of maturity. Early and midseason 

 races were mixed and sown both in the re- 

 gion of Adelaide and in the higher Ade- 

 laide Hills. At Adelaide, conditions allowed 

 the early plants to seed, but the midseason 

 genotypes were unable to seed normally 

 because of moisture conditions. After a few 

 years, only the early races were found in 

 Adelaide because of selective elimination of 

 the midseason races, while in the Adelaide 

 Hills only the midseason genotypes sur- 

 vived. Though both races in the latter area 

 were able to seed normally, many more 

 flowers were produced by the midseason 

 races in the higher environment (experi- 

 ments performed by C. M. Donald and dis- 

 cussed in Gregor, 1944). 



Twelve strains of side-oats grama grass 

 (Bouteloiia curtipendula) on the average 

 respond to photoperiods (p. 121) by 

 flowering most vigorously in their normal 

 latitude, although a few individuals in 

 most strains grow and flower over a wider 

 range. The species probably originated in 

 low latitudes with short days and became 

 secondarily adapted to the longer days of 

 higher latitudes (Olmsted, 1944). 



Wilkes (1942) found that a chalcid 

 parasite (Microplectron juscipennis) intro- 

 duced for the biological control of the 

 European spruce sawfly (Gilpinia poly- 

 toma) in Canada showed wide variation 

 in its ability to establish itself in different 

 temperature areas. Experiments demon- 

 strated different modaUties (preferenda) in 

 a temperature gradient, and by selective 

 breeding, strains were procured, one of 

 which did best at 25° C. and another at 

 9° C. Recovery of parasites from natural 

 regions with a low mean temperature 

 (Parke Reserve, Quebec) indicated that 

 natural selection modified the percentage 

 modality in a manner similar to that pro- 

 duced by artificial selection in the labora- 

 tory. 



Several species of insects seem to have 

 rapidly developed populations resistant to 

 certain insecticides since control measures 

 were instituted (Quayle, 1943). Hydrocy- 

 anic acid tree fumigation has been used on 

 the California red scale (Aonidiella aiiran- 

 tii) since 1886. Resistance to the insecticide 



