370 INTRODUCTION TO EVOLUTION 



Additional evidence that coloration is, in part, at least, controlled by 

 predators using the sense of sight is afforded by the studies of Cain and 

 Sheppard (1954; and Sheppard, 1955) on bright-colored, polymorphic Eu- 

 ropean snails of the genus Cepaea. In this case the predator is a song 

 thrush. The investigators found that the color patterns of snails most com- 

 monly found and eaten varied with the background. Interestingly enough, 

 no such correlation existed in localities in which snails were preyed upon 

 by rabbits rather than by birds. Apparently color as such was not in- 

 volved in the locating of snails by rabbits. 



Mimicry 



Another situation in which natural selection can be seen at work in the 

 predator-prey relationship is that in which organisms resemble, not their 

 backgrounds or surroundings, but each other. The simplest example is that 

 in which an edible species resembles an inedible one, a situation empha- 

 sized by Bates and hence called Batesian mimicry. If one species of butter- 

 fly is unpalatable to birds, another species which is palatable would find it 

 of advantage to resemble the unpalatable one and thus be spared from 

 predation by birds. This being so, natural selection would favor the ac- 

 quisition by edible species of markings and behavioral traits which 

 would cause them to resemble inedible species. 



Sometimes two or more inedible or unpalatable species resemble each 

 other; this is called Miillerian mimicry. Fig. 16.3 presents a striking exam- 

 ple; the insect has remarkable resemblance to a wasp, yet is a moth. Beebe 

 and Kenedy (1957) reported that they found this moth unpalatable to a 

 lizard, a bird, and three species of ants. Wasps are also highly inedible. 

 Of what advantage is it for two inedible species to resemble one another? 

 According to the theory of Miillerian mimicry the advantage stems from 

 a reduction in the number of "lessons" required by a young bird in learn- 

 ing to avoid inedible species. Once a bird has learned not to eat wasps it 

 has automatically also learned not to touch the Ctenuchid moth shown in 

 Fig. 16.3. Thus moth individuals are not destroyed by the bird in learning 

 that they are inedible. Conversely, if a bird learns that the moth is un- 

 palatable it will also avoid wasps, and such avoidance will be of advantage 

 to wasps. Thus Miillerian mimicry is of advantage to both, or all, species 

 concerned, while Batesian mimicry is of advantage only to the edible 

 species (the mimic) which resembles the inedible one (the model). In 

 actuality the sharpness of distinction between Batesian and Miillerian 

 mimicry is reduced by the fact that there are all degrees of edibility and 

 palatability. 



