SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 381 



III. Historical Factors. 



A. The time of origin of the species or group, as a result of which it has been 

 subjected to the effects of a longer or shorter sequence of changes. 



B. Changes: 



1. In the physical environment, both as regards ecological conditions 

 within the range, and the spatial relations of barriers and highways. 



2. In the biotic environment, as modified by evolution and migration. 



The present geographic and ecological distribution of any species or 

 higher group is the resultant of the operation of all these factors through- 

 out the period of its existence. In so far as differences in the geographic 

 location of species and in the extent and form of their ranges are not the 

 direct result of the present distribution of their habitats, they have been 

 determined by the spatial and historical factors listed above. 



Place of origin. It is a fundamental assumption in biogeography that 

 each species originated in a single place and only once. This is simply a 

 working hypothesis; but it is borne out by the observed facts of distribu- 

 tion and has a basis in genetic theory. If new species were formed by single 

 mutations, there would be no reason why such mutant species might not 

 arise in distant portions of the range of the parent species. Such an origin 

 is conceivable for polyploid species of plants, but polyploidy is practically 

 nonexistent in animals. Most plant and animal species differ not in one 

 or a few but in many genes and gene groups. This implies an accumulation 

 of mutational differences between populations, and species formation by 

 changes in whole populations rather than by mutation of single indi- 

 viduals. It is in the highest degree unlikely that the gene complex in any 

 two separate parts of an ancestral species population would undergo the 

 same additions, subtractions, and alterations, thus giving rise to the same 

 new species. 



The place where a species originated is often called the center of origin, 

 but it should not be thought of as a point in space. Instead it must have 

 comprised the range of the population that became the new species. This 

 area need not even be included within the present range of the species, 

 though doubtless it usually is. Many once widespread species are now 

 restricted to a small part of their former range, and the center of origin 

 may have been in a region where the species no longer occurs. Determina- 

 tion of a center of origin is often difficult or impossible, but its geographic 

 position is one of the main factors influencing location of range. A species 

 that arose in North America is much more likely to be there still than 

 it is to occur in Africa. 



Dispersal in relation to barriers and highways. Pressure of popula- 

 tion makes every successful species tend to expand into more and more 

 territory — to extend its range. The gradual expansion of range that results 

 from the normal activities of animals and plants is called spreading. In 



