DISTRIBUTION OF SPECIES 



Willis concluded that the age of a species ( or higher group ) must in gen- 

 eral be proportional to the area now occupied. World-wide species are 

 thus presumed to be very old, and endemics are presumed to be young 

 species. To this he added what he called the principle of evolution by 

 differentiation. This means that evolution proceeds from higher groups to 

 lower, rather than from lower to higher as is most generally supposed. By 

 this he means that a large mutation may produce at a single step a new 

 class, order, or other higher group. The new order, let us say, is at first 

 monotypic, but successive large mutations produce subdivisions, the fami- 

 lies. These then break up into genera, and the genera into species. As the 

 oldest section of any group will be that one which is at the place of origin 

 of the group, this should also be the center of diversity for the group, for 

 here there will have been a maximum amount of time for differentiation. 



The major thesis of Age and Area is a truism, simply a restatement of 

 the expanding phase of the history of any group, as exemplified above in 

 the history of the mastodonts. But the major question is, is the expansion 

 of a species or higher group likely to be slow enough that the present dis- 

 tributions of species and larger groups can give a good indication of their 

 relative ages? Or are present distributions perhaps more generally maxi- 

 mal, and indicative of the limits imposed by physiographic or climatic 

 barriers? Fernald has pointed out that many cases are known in which 

 spread of plants has been very rapid, too rapid at least for Age and Area 

 to have useful application when time is available on a geological scale. 

 For example, it has been less than 25,000 years since the glaciers receded 

 from northern United States and Canada, yet in that brief interval a large 

 and not unbalanced flora has occupied the vast glaciated area, amounting 

 to about one fourth of the continent. This includes such trees as the white 

 spruce, Picea caimdemis; the canoe birch, Betula papyrifera; the white 

 cedar, Thuja occidentalis; and the mountain maple, Acer spicatum. Smaller 

 plants of this association include burreeds of the genus Sparganium, pond 

 weeds of the genus Potamogeton, Iris, Viola, and thousands of other plants 

 of many types. All of these plants have, in less than 25,000 years, colonized 

 a vast area, stopping only when stopped by barriers to their dispersal. As 

 most species are much older than 25,000 years— a mere moment of geo- 

 logical time— it appears that age must generally play a minor role in deter- 

 mining the distribution of species. 



Another serious defect of the Age and Area theory was Willis' emphasis 

 upon the nature of endemics as young species. He recognized that en- 

 demism could be produced also by extinction of a species over all but a 

 small part of its range. But he regarded this as an unusual situation. He 

 seems to regard extinction ( which is likely to be preceded by endemism ) 

 as a more or less unusual phenomenon. Yet it is clear from the paleon- 

 tological record that extinction has been the fate of the overwhelming 

 majority of species. As mentioned above in the discussion of endemics, 

 most endemics are regarded as relics rather than young species by most 

 biogeographers. His idea of evolution by differentiation is based entirely 

 upon statistical evidence, and it has never been favorably regarded by 

 most biologists. 



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