300 Comparative Evolution of Biomes 



have had an individual rate of change and an individual dispersal 

 pattern. In a biome progressing through time, therefore, all the 

 phylogenetic lines do not change and disperse in unison, but each 

 line may do so differently from the others. The result of this situa- 

 tion is that many of the data for the study of the comparative 

 evolution of biomes are drawn from studies of emergent evolution 

 and biogeography. 



The emphasis in these two fields is on the evolution and dispersal 

 of taxonomic units, such as a genus, family, or class of plants or 

 animals. Because the larger taxonomic units include species in- 

 habiting many biomes, the analysis of these taxonomic units gives 

 comparative information on floras or faunas as a whole but informa- 

 tion which cuts across rather than coincides with ecological units. 

 The communities making up the biomes contain species from many 

 unrelated taxonomic groups. Hence the evolution of the biomes 

 must be reconstructed from ecologically similar but frequently phylo- 

 genetically unrelated species whose evolutionary history is in some 

 measure known. 



When such reconstructions are attempted, it is apparent that the 

 evolution and dispersal of many species in a biome have been af- 

 fected either synchronously or similarly by certain ecological fac- 

 tors. Thus the comparative evolution of biomes is a complex mix- 

 ture of similarities and differences in the response of their com- 

 ponent species to changes in ecological and geographic events oc- 

 curring through time. 



As a result of the interaction of a dynamic biota with a dynamic 

 geomorphology, each biome continues to evolve along its own dis- 

 tinctive pattern which is determined by ( 1 ) a mixing effect caused 

 by alternating divisions and fusions and (2) a differential evolu- 

 tion of the isolated parts. 



THE MIXING PROCESS 



Applied to biomes as a whole, the mixing process has three im- 

 portant variables: intercontinental connections, climatic zonation, 

 and dispersal availability. 



Intercontinental Connections 



Geologic and biogcographic evidence proves that in the past cer- 

 tain continents have alternatel)' been connected by land bridges 

 and separated by seas. From the standpoint of the past history of 



