The Evolution of Communities 233 



because of the predominance and importance of dominants, many 

 units of the community have a starkly real dependence un them. 

 From an evolutionary standpoint the conditions superimposed by 

 the dominants have been just as important in creating selection 

 pressures as have been conditions due to other causes. 



Mutualism 



The many recorded cases of commensalism and symbiosis certainly 

 arose in response to community relationships and hence followed 

 the evolution of communities. The manner in which each instance 

 of mutualism evolved is usually simple to deduce. Emerson (1949) 

 has explained many examples. 



An excellent example of mutualism is the combination of the 

 flowering plants and the animals which pollinate them. Because 

 of complementary and synchronous selection pressures, complex 

 flowering structures evolved in plants simultaneously with spe- 

 cialized pollen- or nectar-feeding and flower-visiting habits in in- 

 sects and certain vertebrates. The result is a mutualistic co-adapta- 

 tion in which the plant benefits by being pollinated and the animal 

 by having a source of food. Undoubtedly the pollination relation- 

 ship started with a situation such as that found today in the saw- 

 flies. Adult sawflies may feed on leaf pubescence or on other insects, 

 but all of them feed also on pollen. In the northern temperate re- 

 gions, early season forms feed especially on pollen and nectar of 

 Alniis and Salix, although the larvae of these sawfly species may 

 feed on other plants. Although these plants are considered prima- 

 rily as wind pollinated, there is no doubt that much pollination is 

 accomplished by the feeding of the sawfly adults. It is a small step 

 from this situation to one in which most of the pollination would 

 be done by the insects, resulting in selection pressures which would 

 favor any changes in the flower that would increase or insure visits 

 of insects. 



Many botanists (for example, Bessey, 1915; Hutchinson, 1926; 

 Cronquist, 1951) point out that the evolution of dift'erent flower 

 types has followed parallel lines in many entirely separate evolu- 

 tionary lines of plants, including such developments as radial sym- 

 metry of petals, fusion of petals into a flower tube, insertion of 

 stamens on the tube, and bilateral symmetry of flower such as 

 orchids, snapdragons, and peas (Fig. 100, numbers 3, 8, and 14). 

 Leppik (1956, 1957fl, b) presents the interesting idea that these 

 flower changes were correlated with the evolution of pattern and 



