342 The Organization of Matter and Life 



communities, they have the same general type of dominant species. 

 If these daughter communities later come in contact, the various 

 new pairs of species may intermingle sufficiently that all trace 

 of the original daughter communities is lost. Thus, although the 

 number of communities does not increase, the species mixtures 

 in the communities become more random. However, an ecological 

 ordering effect is superimposed on the community because only 

 those non-dominant species can exist that are adapted to the eco- 

 logical conditions produced by the dominant species. 



Natural selection among dominant species has effected another 

 ordering influence. Only a limited number of types of dominants 

 have persisted, each type producing a different landscape or its 

 aquatic equivalent. Each of these landscape aspect areas is called 

 a biome, and each is maintained solely by the competitive ad- 

 vantages of its dominants. 



Within each biome, especially terrestrial ones, past congrega- 

 tions of communities have led to the evolution of community suc- 

 cession, representing the order in which denuded areas are re- 

 habilitated by various communities of organisms. 



Changes in soil or climate may result either in a randomizing 

 effect because of the separation of certain species of one biome 

 into several new biomes or in an ordering effect through the union 

 of previously separated biomes. If habitable new climates come 

 into existence, some of the species in a neighboring biome will 

 colonize the new area, and if the dominants which become estab- 

 lished in these new areas are of a different physical character 

 than those in older areas, a new biome will result. These develop- 

 ments constitute a randomizing agent because of the evolution of 

 new communities and biomes. 



Each of the randomizing and ordering processes has a complex 

 action, and all of them operate constantly and simultaneously. As 

 a result, individuals in a modern ecological community come under 

 a reticulate set of selection pressures, acting on the individual, 

 respectively, as a member of a local population, as a member of 

 a species, as a member of an ecological community, and as a 

 member of a biome. A change produced by evolution in any one 

 of these roles may affect the response to selection pressures in 

 other roles or may affect the evolution of other populations or 

 species which have any direct or indirect relationship with the 

 changing unit. As a result the biota of the world is now extremely 

 diverse, and the inter-relationships within and between the parts 



