324 Comparative Evolution of Biomes 



Interwoven Biome Phylogeny 



Should ecological conditions in areas between the New and Old 

 World cactus-like deserts change in such a manner that the two 

 deserts came into contact, the two associations would merge and 

 form a single biome of greater taxonomic heterogeneity than that 

 of either present association. This presupposes that species com- 

 prising the two associations have similar or widely overlapping 

 ecological tolerances and that most of the mixing species would 

 become adjusted to coexistence rather than undergoing wholesale 

 extermination of species. Even in case of considerable extinction 

 it is likely that some species from each original association would 

 survive, increasing at least the taxonomic heterogeneity of the 

 mixture as regards genera and families. After such a mixing had 

 occurred it would be impossible to determine its history from a 

 study of the biome itself. It would be necessary to trace the phy- 

 logeny and dispersal of the individual species of the biome in order 

 to unravel the history of the mixing. 



The temperate deciduous forest is an example of a biome follow- 

 ing such a mixing. This forest consists of three major, well-separated 

 units, one in western Europe, one in eastern Asia, and one in east- 

 ern North America. The close taxonomic affinities of plants in all 

 three areas, with many genera such as Quercus, Acer, Ulmiis and 

 Tilia represented by related species in all three, plus abundant 

 fossil records show that at one time these units were connected 

 as a single system. Since the last separation of the regional units 

 many different species have evolved in the isolated regions, but 

 except for a few groups such as the mammals no great evolutionary 

 changes seem to have occurred. 



With present knowledge it is impossible to ascertain how many 

 times this temperate deciduous forest biome has divided and re- 

 united in the past, or how many of its components mixed at each 

 contact. During this time the components of the biome undoubtedly 

 became interwoven in intricate fashion because species would be 

 associated or separated because of ecological rather than phylo- 

 genetic affinities. 



A comparison of the Old and New World regions of the tropical 

 rain forest draws attention to features arising from long regional 

 isolation (probably since Cretaceous time) following many pre- 

 vious mixings. Many of the slowly changing genera such as the 

 plant genera Piper and Fictis occur in both regions. More com- 

 monly the same families occur in both regions, but, as in the 



