Populations 87 



It is obvious that the different populations of organisms found in 

 a given place are not a random sample of organic diversity. Caribou 

 are found with Cladonia (reindeer moss), and wolves with caribou. 

 If a butterfly collector seeks the larvae of Battus philenor in the 

 Arizona desert he must find the decumbent pipe vine, Aristolochia 

 watsonii. He soon learns that this often grows in the shade of other 

 desert plants, especially along the edges of depressions. From the 

 searching behavior of adult female butterflies, it seems obvious that 

 they use similar associations in their search for an oviposition site. 

 Oaks and hickories are often found together, and oak-hickory 

 forests are a good place to hunt the Virginia deer. Wheat, 

 dogs, house flies, body lice, and Treponema pallidum, as well 

 as many other organisms, are often found in association with 



man. 



Most of the ecological analyses that have been made are descrip- 

 tive. The results, on the whole, have been disappointing, especially 

 for the evolutionist. A recently developed ecological school is at- 

 tempting to formulate mathematical descriptions of the structure 

 observed in communities. It is perhaps still too early to decide 

 whether these efforts will be successful, but the results thus far are 

 encouraging. At any rate, it seems certain that no special mech- 

 anisms are necessary to account for "community evolution." Ex- 

 tremely complex interactions of those processes described in this 

 book seem to explain all community phenomena that have been ob- 

 served. Some of the complexities of dealing with aggregates of popu- 

 lations are considered in the final chapter. 



SUMMARY 



Although for some organisms the concept of individual is difficult 

 to define, in most organisms aggregations of individuals referred 

 to as populations arise. These, or aggregations of these, may form 

 interbreeding populations; the latter are variously combined to form 

 taxonomic groupings. The functioning of individuals is physiological 

 and is determined by the genotype and epigenotype that set the 

 ranges of tolerance of the organism to intensity spans of complexly 

 varying environmental factors. The organic functions of the popula- 

 tion are genetic and determine the genetic constitution of the zy- 

 gotes. Communities are aggregations of diverse populations which 

 form part of each other's environment, and they become structured 

 with respect to energy relations. There are no biological functions be- 

 tween populations in a community. Communities owe their existence 

 only to the mutuality of the tolerance ranges of the constituent 

 organisms at a particular period of time. 



