THE DYNAMICS OF EVOLUTIONARY CHANGE 



studies of mimicry in butterflies. The phenomenon of mimicry, 

 or resemblance in outward appearance between a distasteful or 

 harmful species and an unrelated, harmless species, was used 

 even in Darwin's day as evidence for the power of natural se- 

 lection. Although some biologists in more recent times have 

 been skeptical of the existence of this phenomenon, the recent 

 experiments of Brower (1958a, b, c) have established its validity 

 beyond reasonable doubt. She offered specimens of the distaste- 

 ful monarch butterfly to jays under carefully controlled condi- 

 tions and showed that birds "trained" to avoid the monarch 

 butterflies would also refuse to eat its mimic, the viceroy, al- 

 though this butterfly was shown to be completely palatable 

 to birds. 



Sheppard (1959) has recently studied the genetic basis of 

 mimicry in African butterflies of the swallowtail genus (Papilio) 

 by extensive hybridization between the different mimicking 

 forms which occur in a single species (P. dardanus), as well as 

 crosses between mimetic and non-mimetic forms. These studies 

 have sh3wn clearly that mimetic forms will become more com- 

 mon and their mimicry will become more perfect in regions 

 where the distasteful models are the most frequent. Where 

 models are rare, mimicry is imperfect and variable within the 

 same population, while in regions where the distasteful species 

 is absent, the species consists entirely of non-mimetic forms. 

 Crosses between mimetic and non-mimetic forms show that the 

 general features of the mimetic pattern are determined by a 

 single gene with a large effect, but that perfection of the mimicry 

 is brought about by the action of many modifying genes, each 

 with a small effect. In Papilio dardanus of central Africa there 

 exist half a dozen or more different mimetic forms, each of 

 which mimics a different species or subspecies of distasteful 

 butterfly, with three different genera represented among the 

 models. The different mimetic races are all determined by 

 different allelomorphic genes at a single locus, with each of 

 these major or "switch" genes giving the butterfly a superficial 

 resemblance to a particular model. In regions where the model 

 is abundant, the "switch" gene responsible for mimicry of that 

 model has gathered around itself a collection of modifiers at 

 many different loci, which have perfected the resemblance be- 



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