LECTURES IN BIOLOGICAL SCIENCES 



tween mimic and model to a remarkable degree. The evolution 

 of mimicry in these butterflies has, therefore, been brought about 

 by natural selection of many different genetic changes, some 

 with large and others with small effects. These have all been 

 brought together into a harmonious combination which adapts 

 the population to a special situation (i.e., the presence of a 

 particular distasteful model). There is good reason to believe 

 that this type of progressive action of natural selection on a 

 number of highly specific genetic changes has been responsible 

 for all of the extraordinary adaptations found in nature, such 

 as the remarkable shapes and color patterns of such flowers as 

 orchids and milkweeds, the unbelievably complex instincts of 

 such animals as spiders and solitary wasps, and the social be- 

 havior of bees, ants, and primitive men. 



The Origin of Species 



The fifth and sixth basic postulates of the modern synthetic 

 theory of the causes of evolution concern the problem of the 

 origin of species: 



Fifth, the continued separation of new adaptive lines of ev- 

 olution from related lines with different adaptations requires 

 the origin of barriers of reproductive isolation, preventing 

 or greatly restricting gene flow between them. This separation 

 is essential for maintaining the diversity of adaptations which 

 exists in any one habitat, and so should be regarded as the basis 

 of species formation. 



Sixth, the origin of reproductive isolation, like that of new 

 adaptive types, requires the establishment of many new genetic 

 changes, including structural alterations of the chromosomes 

 and cytoplasmic changes as well as gene mutations. 



These two postulates place the origin of species on a differ- 

 ent level from the origin of different adaptations or races of a 

 single species. Both processes are based upon natural selec- 

 tion of genetic differences, but for the origin of species natural 

 selection must sort out differences of a very special kind. These 

 affect the behavior of a population toward certain specific re- 

 lated populations rather than its adaptation to the environ- 

 ment as a whole. 



A recent symposium on the species problem (Mayr, 1957) 



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