300 AN INTRODUCTION TO MODERN GENETICS 



A wood in Yorkshire was divided in two in about 1800 by a wide 

 avenue, and in 1885 one of the two portions was planted with birches, 

 while the other part remained mainly a pine wood. In 1907, 85 per cent 

 of the moths Oporahia autumnata in the birch wood belonged to the 

 light variety, while in the dark part of the wood there were only 4 per 

 cent of the light forms, and the other 96 per cent were of a dark variety. 

 Presumably this divergence in genes ratios had been brought about 

 since the wood was replanted in 1885, and certainly it must have been 

 produced since the wood was divided in 1800. Moreover, the change is 

 clearly related to the environment and is therefore probably due to 

 natural selection. In fact, evidence was found that in the dark wood the 

 light forms were at a selective disadvantage, since the proportion of 

 lights among the dead moth wings left by predatory animals (bats, 

 birds, etc.) was much higher than 4 per cent. 



Another well-known example of natural selection is that described 

 by Sukatschevji who cultivated together various clones of apomictic 

 dandelions, and studied their fertility and viability. There were con- 

 siderable differences in fitness between the varieties, but these differ- 

 ences depended on the environmental conditions; the forms which 

 were best adapted under conditions of crowding did not always do 

 better in less crowded cultures. Similar evidence is provided by Tures- 

 son's^ studies of genetical varieties of plants from different ecological 

 situations; when they were all cultivated under the same conditions, 

 their fitnesses were not identical. 



In recent years several rather complete investigations have been 

 made on the selective value of "protective colouration." Simmer^ 

 reported experiments in which fish {Gambusia) were allowed to adapt 

 to dark and Ught backgrounds, and the pale and dark specimens mixed 

 and exposed to predators (herons or penguins). With a mixed popula- 

 tion in a Hght-coloured tank, about 60 per cent of the fish eaten during 

 the experimental period were dark coloured, while if the experiment 

 was made in a dark tank, only about 40 per cent of those eaten were 

 dark. Similarly, Isely* allowed birds to attack different coloured grass- 

 hoppers placed on backgrounds on which they were either concealed 

 or conspicuous. Within a given period, about 85-95 per cent of the 

 conspicuous grasshoppers were eaten, but only 35-45 per cent of the 

 concealed. Although these experiments deal with colour variations 

 which are not hereditary, they demonstrate that apparently protective 



^ Sukatschev 1928. ^ Turesson 1925, 1930, 1931. 



^ Sumner 1935. * Isely 1938. 



