EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



695 



ability of selection pressure, largely but not 

 exclusively the result of competition, to sort 

 genetic variations through their somatic 

 eflFects is demonstrated by both observation 

 and experiment. Adaptive mechanisms en- 

 able organisms to maintain their ecological 

 position and partially to control their en- 

 vironment. Ancient complex and currently 

 less valuable adaptations secondarily re- 

 gress through positive selection of other 

 functions, but because of their genetic re- 

 lationship to evolving adaptations and 

 beneficial functions, the genetic pattern is 

 maintained in part and is exhibited by ves- 

 tigial characters. Interbreeding between 



populations may regress, leading to per- 

 manent reproductive isolation. Extreme re- 

 duction or extreme increase of variation, 

 isolation, or selection leads to the retarda- 

 tion or cessation of evolution. Selection 

 operates upon organismic and population 

 (supraorganismic) systems. Circular evolu- 

 tionary eflFects are the rule (see Hutchinson, 

 1948). Thus, variation and isolation set 

 the stage for the guiding action of selection, 

 and selection in turn guides the mecha- 

 nisms of variation and isolation. Evolution- 

 ary trends are in the direction of increased 

 homeostasis within the organism, the spe- 

 cies population, and the ecosystem. 



35. EVOLUTION OF INTERSPECIES INTEGRATION 

 AND THE ECOSYSTEM 



INTRODUCTION 



The evolution of the ecosystem, more 

 particularly the evolution of interspecies 

 integration, culminates in the balance of 

 nature within the community. The ecosys- 

 tem may be defined as the interacting en- 

 vironmental and biotic system. 



The activities of the community may be 

 summed up in the action of the physico- 

 chemical habitat upon the organisms, the 

 reaction of organisms upon the physical 

 factors, and the coaction of the organisms 

 upon each other (Clements and Shelford, 

 1939; see also p. 348). 



The evolution of interspecies integration 

 involves the genetic modification of the 

 ecologically associated organisms in rela- 

 tion to each other, in the aggregate result- 

 ing in the evolution of the community as a 

 whole. 



Progression of the community through 

 succession and development of contem- 

 porary species associations is discussed 

 elsewhere (Chap. 29). Succession and 

 development may be conceived as the 

 ontogeny of the community and its parts. 

 The evolution of interspecies integration 

 may be thought of as the phylogeny of the 

 definitive grouping of species within the 

 community. 



As an example, the prairie community of 

 the Miocene had much taxonomic and 

 ecologic similaritv to the prairie community 

 of the present day, although the species 

 composing these associations were different 



in the Miocene. In some cases, the species 

 have gradually evolved within a similar 

 habitat, and there is a genetic continuity 

 between the organisms of the Miocene and 

 present prairies (see Stirton, 1947). Some- 

 times a species evolves with changes in its 

 habitat. In other instances, species have 

 entered the prairies from other contiguous 

 habitats and have gradually become ad- 

 justed. On occasion, a distant species may 

 have entered the region and later evolved 

 toward a balanced adjustment with the 

 older estabhshed species. Selection pres- 

 sures within the whole community grad- 

 ually produce a balanced relation of the 

 species to each other and to their physical 

 environment. 



Species in a given association have often 

 come from several different regions, and 

 their origin may be traced by their taxo- 

 nomic relations. Mayr (1946) finds ele- 

 ments in the North American bird fauna 

 that may be classified according to their 

 ancestral origin in any one of the following 

 categories: Pantropical, Panboreal, Old 

 World, North American, Pan-American, and 

 South American. 



Weir (1946) discusses the environmen- 

 tal influence in releasing genes from their 

 adaptive function so that establishment of 

 mutations with new adaptive functions may 

 not be prevented by selection. He points 

 out that favorable mutations of genes al- 

 ready possessing vital catalytic functions 

 are improbable. Also, there is a size limit 



