NATURAL SELECTION 



Additional Examples. Another often-quoted example is that of the 

 crab, Carcinus maenus, reported by Weldon in 1899. The building of a 

 breakwater in an English sound resulted in a higher silt content of the 

 water. During a period of five years, the mean diameter of the carapace 

 of the crabs in the sound was observed to decrease. Weldon captured both 

 narrow-carapaced and broad-carapaced crabs and kept them in silty 

 aquaria. The narrow-carapaced crabs survived, while the broad-carapaced 

 ones died. Weldon attributed this to accumulation of silt on the gills of the 

 broad-carapaced crabs. He assumed that the same thing happened in 

 nature, and so constituted a selective force. The actual silting of the gills 

 was not demonstrated, but it is plausible. 



The relationship of wheat and wheat rust is also illustrative. One of 

 the primary objectives of plant breeders is to develop disease-resistant 

 varieties of commercially valuable plants. To a disease-producing para- 

 site, however, scarcity of susceptible hosts is obviously a severe selective 

 force. The history of rust-resistant wheats is monotonous in its repetition. 

 When a new resistant wheat is introduced, it gives excellent, disease-free 

 crops, and is widely adopted. After a few years, however, a few fields 

 show some active rust infection. Then the successful rust spreads rapidly, 

 and a new resistant wheat is again needed. It is evident that mutation has 

 simply by chance produced a rust variety which is adapted to the new 

 wheat, and it spreads rapidly because it is favored by a strong selection 

 pressure. This has been the subject of much study by Stakman. 



Quayle has reported an interesting case of selection in the scale insects 

 which parasitize citrus trees. The standard method of combatting the 

 insects is to cover each tree with a tent and fumigate with hydrocyanic 

 acid. But in each of the three species concerned, cyanide-resistant varie- 

 ties have appeared and have replaced the original cyanide-sensitive va- 

 rieties. One of these resistant species has subsequently disappeared for 

 unknown reasons. A similar development has more recently occurred with 

 respect to DDT poisoning. When DDT first came into general use late in 

 1945, it gave promise of being an almost perfect insecticide for the control 

 of household pests, such as flies. But soon DDT-resistant flies began to 

 appear. Under the strong selective force of DDT poisoning campaigns, 

 these resistant strains soon became well established, and in many localities 

 they have largely replaced the original DDT-sensitive flies. 



An interesting case in the fire-ant, Solenopsis saevissima, has been 

 studied by Brown and Wilson. A large, dark variety of this ant was acci- 

 dentally introduced into Alabama from South America about 1918. It 

 spread very slowly and caused no alarm. During the 1930's, however, a 

 smaller, lighter-colored form appeared, probably as a new immigrant from 

 South America. This proved to be far more invasive and aggressive than 

 the dark form, the nests of which it destroys. It is evidently favored by 

 selection, for it is rapidly replacing the dark form and extending its range. 

 It has become a major pest in the southeastern United States. 



Polymorphism. Selection does not lead to uniformity: it leads to high 

 frequencies of those genes which contribute to the most successful geno- 

 types. However, heterozygosity itself has considerable value, for it pro- 



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