242 The Evolution of Commtinities 



in species having rigidly cyclic life histories, and perhaps certain 

 results following hybridization. That these new species may be 

 a potent factor in the community ecology is well illustrated in 

 eastern North America by the Erythroneiira leaf hoppers. Four spe- 

 cies of this genus are the only serious etiolating insects on the 

 honey locust Gleditsia triacanthos, an important tree of the forest 

 ledge community. They constitute a monophyletic cluster whose 

 ancestor must have become established on Gleditsia by a host trans- 

 f er from some other host. Ilex decidua, an important shrub of the 

 floodplain community, likewise is etiolated chiefly by three or four 

 species of Erythroneiira, but each of these undoubtedly resulted 

 fifom separate transfers from some other host (Ross, 1953). 



In the plants polyploidy may also give rise to small species flocks 

 living and competing in the same community, but more frequently 

 the new polyploid species occur in different communities or at 

 miost overlap only slightly with the parental types (Manton, 1950). 



This brief summary directs attention to the fact that, even while 

 members of a community are evolving ecological specialization be- 

 cause the community is already a complex mixture of species, the 

 niLimber of species in the community is continually increasing, thus 

 creating greater taxonomic complexity with its attendant changes 

 in many types of selection pressures. 



SUMMARY 



Th(3 material outlined in this chapter depicts the probable events 

 whi'ch take place when geomorphic changes bring about the con- 

 gregation of species. From the first mixing of these different or- 

 ganisms, selection pressures of various types resulted automatically, 

 and these pressures led inexorably to various interspecific relation- 

 ships typical of present-day communities, including food chains, 

 food and habitat specializations, mutualism, and protective devices. 

 These interrelationships become more intricate as the communities 

 become more complex. The species composition of the community 

 may increase in a limited number of ways while the community 

 remains geographically intact, thus generating additional com- 

 plexity. 



The increase in the complexity of interspecific relationships, es- 

 pecially those leading to an elaboration of food chains, leads to a 

 community structure which is largely dependent on the dominant 

 speci(is of the community and which tends to perpetuate its tax- 

 onomic diversity. 



