Changes in Populations 131 



countryside appears to be unpolluted. Two reasons may be given 

 for this. First, pollution is often greater than meets the eye. Smog 

 clouds tend to drift a long distance, and, in spite of its overall green 

 appearance, the countryside may actually have a considerable layer 

 of soot and industrial chemicals. A second reason is that man's other 

 activities alter the countryside, and these changes (e.g., decimation 

 of the predators in an area ) may be enough to shift the balance to 

 the melanics. 



Although recessive melanics are known, the spread of industrial 

 melanism is due to the spread of dominant genes. The possible 

 reasons for this were discussed in Chap. 3 in relation to the origin 

 of dominance. 



Industrial melanism is an example of transient polymorphism. Re- 

 member that polymorphism is the occurrence in the same habitat of 

 two or more distinct forms of a species in such proportions that the 

 rarest of them cannot be maintained by recurrent mutation. Tran- 

 sient polymorphism is the situation in which the two forms coexist 

 while one is in process of replacing the other. Another example of 

 what is probably transient polymorphism involving melanic moths 

 has been studied in an old woods in Scotland. In these woods, which 

 are essentiallv free from pollution, one species of moth (CIcora 

 reparulata ) had a population in which 10 percent of the individuals 

 were melanic. At rest on lichen-covered trunks (light background) 

 the typical forms were very inconspicuous. On dark trunks the 

 melanics were inconspicuous, but their protective coloration was (to 

 the human eye) not as eflFective as the camouflage of the light form. 

 In flight, in the dark woods, the light forms were much more con- 

 spicuous. (Three were observed to be taken on the wing by birds in 

 a period during which no melanics were observed to be eaten. ) If 

 the advantages were the same both at rest and in flight, progress 

 toward fixation would be more rapid than in the situation described. 

 Nevertheless fixation will still occur unless conditions change. 



Mlcroevoiution in British Lepidoptera 



British lepidopterists have pioneered in studies of other types of 

 evolutionary changes in populations of butterflies and moths. Long- 

 term studies of the gene frequency at a single locus in the scarlet 

 tiger moth, Panaxia domimdn, have been made by Fisher, Ford, and 

 Sheppard. In the only colony (near Oxford) where the gene in 

 question has been detected, its frequency has been estimated every 

 year since 1939, and population-size estimates have been made for 

 all years since 1941. Three phenotypes occur in the colony: dominula 



