Origin of Biomes and Succession 



261 



no effect on a shrub species, a salamander, or a cockroach hving 

 beneath these trees. Conversely, the shrubs, salamanders, and 

 cockroaches could change either into new types or into a larger 

 number of species, and this would ha\'e no direct effect on the 

 evolution of the dominants. 



If the various components of a community did evolve in unison, 

 theoretically it would be the result of either pure coincidence or 

 some intimate biological connection between particular species in 

 the community. 



For example, the demise of a tree fern species which was the 

 sole and obligatory host of an insect species would automatically 

 remove that insect; reduction of dominant species having branches 

 below the crown would reduce understory epiphytic ferns and 

 other plants; the evolution of some special kind of seeds would 

 set up selection pressures in understory organisms leading to utiliza- 

 tion of these seeds as food. 



The long geological history of certain biomes may be associated 

 with the evolution of dominant species which became extremely 

 well adapted to their environment, were seldom outcompeted, and 

 persisted virtually unchanged for long periods of time. These almost 

 changeless dominants are encountered in many biomes. Chaney 

 (1954) and Pierce (1957) believed that Pinus resinosa (Fig. 

 115/?), one of the dominant pines of northeastern North America, 

 is almost exactly like its Cretaceous counterpart, Pinus resinosipites 

 (Fig. 115«). Chaney (1940) believed that the sycamore Platanus 



(a) (b) 



Fig. 115. Lateral views of grains of pine pollen, (a) the Cretaceous species 

 Finns resinosipites; (b) the recent species P. resinosa. (From Pierce.) 



occidentalis of eastern North America is virtually identical with 

 the species which occupied the same general area in Oligocene. 

 Pierce (1958) showed that pollen characteristics of many Creta- 

 ceous trees are remarkably similar to those of existing species ( Fig. 



