The Evolution of Communities 235 



color discrimination in their insect pollinators, hence also the idea 

 that in many parallel lines of plants changes at the same evolu- 

 tionary level were occurring at the same time. 



Among the insects, adults in each of several families of flies, 

 moths, butterflies, and bees had concurrently become highly spe- 

 cialized for feeding on nectar and/or pollen of flowers. Following 

 Leppik's theory, it is highly likely that many phylogenetic lines 

 of plants and many phylogenetic lines of insects evolved mutual- 

 istic specializations at more or less the same rate because of their 

 interdependent nature. This complex plexus of correlated evolu- 

 tionary lines is a remarkable example of the action of selection 

 pressures engendered because difi^erent kinds of organisms were 

 living together. These are truly community selection pressures. 



The intricacies of these community pressures can be realized 

 more fully when one understands that most present-day com- 

 munities contain many hundreds of species of flowering plants, rep- 

 resenting all of Leppik's types plus many uniquely specialized spe- 

 cies, and many hundreds of insect pollinators, representing all the 

 major pollinating types. In whatever manner these difl^erent types 

 of flowers and pollinators evolved, both primitive and specialized 

 forms live together as a group of coexisting species in the same 

 biotic community. 



The two outstanding instances in which large wind-pollinated 

 groups (the sedge-grass complex and the amentiferous dicotyle- 

 dons) have arisen from insect pollinated ancestors surely are the 

 product of the cessation of previous community selection pressures 

 through a change in habit or habitat on the part of the plants. 

 The amentiferous dicotyledons, exemplified by the oaks and beeches 

 (Fagaceae) and birches (Betulaceae), bloom early. As blooming 

 began to take place earlier and earlier, presumably fewer and fewer 

 insects would be active, and, as a result, wind pollination became 

 more effective than insect pollination. It is likely that progenitor 

 species of grasses and sedges invaded areas having a small or in- 

 significant biota so that insect pollination would have been less 

 reliable than wind pollination. These events would explain the 

 selection pressures leading to the loss of conspicuous floral parts 

 which is presumed to have occurred in both groups (Lawrence, 

 1951; Tippo, 1938). 



An interesting sequel is the reverse evolution which has taken 

 place in the sedge Dichromena ciliata. A denizen of Central Amer- 

 ican forests, this sedge is now pollinated by insects, and in response 

 to the selection pressures attendant to this situation the upper leaves 



