104 Natural Selection 



The widespread North American water snake Natrix sipedon 

 has a population on a small island in Lake Erie which differs from 

 the heavily banded mainland populations in that it ranges from a 

 less conspicuously banded condition to a uniform limestone grey 

 or beige color. The snakes are active on the limestone rocks which 

 line the shore of the island. Camin and his colleagues found that 

 a high proportion of the young snakes were banded, but that the 

 proportions of the different color classes of the mature snakes had 

 shifted towards the unhanded type; about twice as many adults 

 were unicolorous as compared with the young. Because the uni- 

 colorous specimens are relatively inconspicuous on the limestone, 

 whereas the banded types are seen much more readily, these authors 

 believed that predation has been greater on the more conspicuously 

 banded individuals and thus that natural selection has favored the 

 unicolorous types (Camin, Triplehorn, and Walter, 1954; Camin and 

 Ehrlich, 1958). 



In the vicinity of various cities in Europe, melanistic individuals 

 of several species of moths occurred in increasing numbers. Experi- 

 ment showed that on the dark surfaces in and near industrial areas 

 these darker forms had a marked cryptic protection from predators. 

 The melanistic individuals of at least the species Boarmia repandata 

 have a greater fecundity than the light form. In non-industrial areas 

 the moths frequent light-colored and mottled tree trunks. Here 

 the preponderance of light forms is maintained by a strong selec- 

 tion pressure for protective coloration, in spite of their reproductive 

 disadvantage ( Goldschmidt, 1940; Ford, 1955; Kettlewell, 1955, 

 1956^,^7). 



Darwin (1859), Dobzhansky (1951), Simpson (1953), Stebbins 

 (1950), and others have summarized many additional examples of 

 known or inferred natural selection. The gist of this mass of in- 

 formation is that every factor in the environment may produce some 

 selective action on the organism, and that in the long run this action 

 leads to the survival of the better fitted phenotypes in a population. 



The "long-run" product, however, must be an average of the 

 devious and complex pattern of selection which surely occurs. Tak- 

 ing a hypothetical example, an abundant insect population might 

 strip its food supply so that progeny of later emerging and reproduc- 

 ing females died of starvation; only the progeny of the earlier 

 emerging females reached maturity. This would result in a positive 

 selection pressure for genotypes associated with early appearance. 

 If late lethal spring frosts occurred the next year, the earlier emerg- 

 ing individuals would be killed, and selection would be for late 



