THE ORIGIN OF SPECIES 



tion are both generally based upon quantitative characters which are 

 influenced in a small degree by each of manv difi^erent pairs of genes. 



It is interesting to note that, if Goldschmidt was right in his opinion 

 that systemic mutation, in contrast to gradual accumulation of small mu- 

 tations, is the basis for speciation, then only genetic isolation is important 

 for speciation. For other types of isolating mechanism, while they would 

 then be important for the biology of a species, could not affect its evolu- 

 tion above the subspecific level. The decisive chromosomal changes, 

 which at once would be the basis both for genetic isolation and for 

 phenotypic differentiation, could arise as well in a continuous population 

 as in a population which was already partially isolated. And even with 

 very long isolation by other means, the decisive chromosomal isolation 

 might fail to occur. 



FAILURE OF ISOLATING MECHANISMS 



Regardless of the manner in which one evaluates the role of isolating 

 mechanisms, one of their important properties is their occasional failure. 

 Subspecies and species which are ordinarily separated by one or several 

 isolating mechanisms may occasionally produce hybrids. And these may 

 be most instructive. If the barrier is geographical, it is essential that this be 

 spanned before hybridization can occur. This may happen because of 

 natural events or through the agency of man. In the first class are the 

 many cases in which populations were separated during the glacial ages, 

 during which time they diverged, perhaps to a specific degree, perhaps 

 only to a subspecific degree. With the recession of the glaciers, the diverg- 

 ing populations have again migrated into their former range, and now 

 hybridization may occur. Mayr has assembled many such cases from the 

 fauna— particularly the avifauna— of central Europe. During the Pleisto- 

 cene glaciation, Scandinavian and Alpine ice caps approached within 

 about 300 miles of each other in central Europe, with the result that the 

 temperate flora and fauna which had formerly inhabited this zone were 

 forced to take refuge either in southern France and Spain, or in the 

 Balkans. Thus segments of formerly continuous populations were isolated 

 from each other at opposite ends of the Mediterranean Sea. While thus 

 isolated, the two populations of the various species diverged. But, with the 

 recession of the ice, the eastern and western populations again moved into 

 their original territory. 



The behavior of these once isolated but now sympatric populations is 

 very different in different cases. In some, no interbreeding occurs, indicat- 

 ing that the divergent populations have reached the status of distinct 

 species. In others, such as the hedgehogs, Erinaceus europacus (western) 

 and E. roumanicus, hybrids are rare, but they do occur. These are treated 

 as good species, but their status may be debatable. In still other cases, for 

 example the crows Corviis corone and C. comix, there is a rather stable 

 hybrid population where the eastern and western groups meet. These are 

 now treated as a Rassenkreis, under the former name. 



In the case of the several species of plane trees (Platanus) the great 



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