322 



Comparative Evolution of Biomes 



associations of marine species. Too little is known about marine 

 biomes, however, to attempt an analysis of their regional phylogeny. 

 The phylogeny of existing terrestrial biomes can be outlined in 

 a moderately comprehensive fashion. The oldest terrestrial biome 

 is the tropical evergreen rain forest, occurring in continuously warm 

 and humid areas. Through geologic time and by various evolu- 

 tionary mechanisms, plants colonized other areas, certain coloniza- 

 tions proceeding toward adaptation to drier conditions, other col- 

 onizations proceeding toward adaptation to colder conditions. 

 Through such developments the major biomes evolved, possibly 

 in the order tentatively suggested in Fig. 142. Certain branches of 



"*— 4— 





V 



i- vi *- 



O "O •,- «- 



0(=0 0— — — fcf»t=.^ — 



• -— — 4j a> o (JO -ca "~ ^— tj ^ 



o <^ Q. CL >- - - ^ «• = — )r. 



(ua> 3 ^ ^ ^ *- .d ^ v 



"O Q> (/I 



Z3 O 



OD O 1— 



CO o 



Colder 



Ancestral Vegetation 

 in worm humid climate 



Fig. 142. 



biomes. 



Tentative suggestion of the phylogeny of the major terrestrial 



the diagram seem to be well founded. The careful and detailed 

 analysis of Axelrod (1958) indicates clearly that the semi-desert 

 woodland-chaparral biomes of southwestern North America evolved 

 from a tropical subhumid forest. 



The youngest terrestrial biome is probably the tundra or paramos. 

 It is characterized universally by the short growth form of its plant 

 life, including trees, shrubs, and herbs. Few if any genera occur 

 only in tundra. Many of the tundra plant species in both northern 

 and southern hemispheres belong to widespread genera such as 



