Origin of Biomes and Succession 



247 



Fig. 107. The region (shaded area) where twelve fishes of the Berends 

 Pleistocene fauna now occur together. The black dot indicates where these 

 species occurred together during an earlier and presumably cooler period of 

 the Pleistocene. (From C. L. Smith.) 



having 25 species all confined to a small part of the climax deciduous 

 forests of eastern North America (Keeton, 1959; Hoffman, 1959), 

 In this forest occur many genera of trees, each having a number 

 of closely related sympatric species, suggestive of past community 

 divisions and fusions. Corroborating evidence is found in many 

 groups of aquatic organisms which are restricted to streams within 

 the eastern deciduous forest or its northern ecotone areas. The stone- 

 fly genus Allocapnia contains about 15 sympatric species (Frison, 

 1935, 1942; Ricker, 1952), and the caddisfly genera Pycnopsyche, 

 Neophylax, Hydropsyche, and many others contain comparable 

 numbers (Ross, 1944; Betten, 1950). The sympatric eucalyptus tree 

 species of eastern Australia suggest successive community splitting 

 and fusing, as do the many closely related sympatric species of 

 caddisflies occurring in the same region (Mosely and Kimmins, 

 1953). 



Every community fission would not result in the fission of all 

 its contained species. As was pointed out in Chapter 7, a complex 

 of factors influences the rate of change between isolated popula- 

 tions. We would therefore expect that if the divided parts of the 

 community reunited, some of the original phylogenetic lines \A'Ould 

 have evolved into separate daughter species; others would not. This 

 circumstance seems to explain the fact that, in the temperate decid- 

 uous forest of eastern North America, the sycamore genus Platanus 

 contains only a single species, but the monophyletic cluster of Ery- 



