CHAPTER VI 

 THE EFFECT OF GEOGRAPHIC ISOLATION 



The area inhabited by a species is in general surrounded on all 

 sides by barriers which prevent its further dispersal, and the species 

 is thus limited in its range. As we have seen, the nature of these 

 barriers may be very diverse. Barriers of different kinds combine to 

 produce an isolated area, as is done by the mountains and ocean on 

 the west coast of South America, or the desert and sea in North 

 Africa. Cave animals are isolated by their negative reaction to light. 

 The border of the range of a superior competitor or of an enemy or 

 of a rich fauna may form a limiting barrier. Climatic barriers also 

 exist which usually show a less definite boundary than do physio- 

 graphic barriers. A fairly marked climatic barrier is furnished by the 

 20-inch isohyet in the United States which approximately coincides 

 with the hundredth meridian. 



Barriers to dispersal have a twofold effect. In the first place, they 

 may separate closely related forms on the two sides of the barrier 

 and thus prevent interbreeding. This makes possible independent dif- 

 ferentiation of the two groups, since newly acquired characteristics 

 will be restricted to that one in which they appear and not become 

 the general property of both. The earlier belief that variations would 

 be suppressed by crossing with invariant individuals, i.e., that "Pan- 

 mixia" would entirely nullify variation, has become very questionable 

 with the growth of our knowledge of heredity. In two isolated related 

 groups, however, the individuals within each group will, in general, 

 resemble each other more than they will members of the other group. 

 Thus arise races, varieties, subspecies, etc. In the second place, the 

 presence of barriers operates to protect the species within their isolated 

 range from the competition of rival forms or enemies which might be 

 dangerous to their survival, and thus enables the protected forms to 

 take full advantage of all opportunities presented by their environ- 

 ment. 



There is both direct and indirect evidence of species change asso- 

 ciated with geographic isolation. The experience of animal breeders 

 shows that, even without intentional selection, isolated groups of 

 domestic animals develop in different directions in a relatively short 



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