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ECOLOGY AND EVOLUTION 



another species in the interspecies system. 



A convincing case of biotic antagonism 

 between related species is seen among the 

 house rats. The black rat (Rattus rattus), 

 originating in tropical Asia, was the com- 

 mon house rat of Europe during medieval 

 times until it came into competition in the 

 eighteenth century with the Norway rat 

 (R. norvegictis), originating in temperate 

 Asia. After the spread of these species 

 around the world, each became established 

 in the climatic zone of its origin and each 

 in general prevented and still prevents the 

 successful invasion of its area by its com- 

 petitor. Local conditions produce some ex- 

 ceptions to the general correlation of cli- 

 mate and relative abundance of the two 

 competing species. 



Elton (1946) studied eighty- two animal 

 and plant communities from diflFerent parts 

 of the world and found that 86 per cent of 

 the animal genera and 84 per cent of the 

 plant genera included only a single species. 

 The corresponding average number of 

 species per genus was 1.38 and 1.22. In 

 the faunal lists of large regions, such as 

 Britain, 50 per cent of the genera have 

 single species, and the average number of 

 species per genus is 4.23. The explanation 

 of this diflFerence seems to be competition 

 between closely allied species within the 

 same association (Crombie, 1947). 



A multiplicity of such biotic antagonisms 

 together with biotic limitations is probably 

 the explanation of biotic barriers in general. 

 The barriers often consist of closely related 

 and ecologically equivalent species, genera, 

 or families, but in some cases convergent 

 ecological equivalents may form a com- 

 petitive barrier. The absence of proper food 

 also may prevent the spread of specialized 

 herbivores, carnivores, or parasites, and 

 predators may prevent the establishment 

 of an unadjusted prey species. 



It may be concluded from these data 

 that the community maintains a certain 

 balance, establishes a biotic border, and has 

 a certain unity paralleling the dynamic 

 equilibrium and organization of other living 

 systems. Natural selection operates upon 

 the whole interspecies system, resulting in 

 a slow evolution of adaptive integration and 

 balance. Division of labor, integration, and 

 homeostasis characterize the organism and 

 the supraorganismic intraspecies population 

 (p. 435). The interspecies system has also 

 evolved these characteristics of the organ- 



ism and may thus be called an ecological 

 supraorganism (Emerson, 1946). 



Objections to the concept of the com- 

 munity supraorganism (p. 698) are largely 

 the result of (1) the handicaps in gather- 

 ing phylogenetic data on population num- 

 bers, (2) the failure to recognize that 

 coaction often creates selective pres- 

 sures on genetic patterns, and (3) the 

 failure to comprehend that the unit upon 

 which selection acts may be either an inte- 

 grated intraspecies or interspecies popula- 

 tion. 



The evolution of populations paral- 

 lels some aspects of the evolution of 

 organisms. When parallels are recognized, 

 they are sometimes dismissed as "mere 

 analogies" without realizing that these 

 analogies may not always be chance simi- 

 larities, but may be convergent as the re- 

 sult of similar evolutionary pressures. Be- 

 cause primitive organismic or supraorganis- 

 mic integration does not exhibit the special- 

 ization and cooperative interdependence of 

 the most highly integrated systems, basic 

 coordination may not be recognized. 



Because genetic continuity is often 

 broken and is replaced by environmental 

 continuity, the community is fundamentally 

 diflFerent from intraspecies populations or 

 individual organisms, but it also partakes of 

 certain aspects of organismic integration, 

 division of labor, and structure, and main- 

 tains ecological homeostasis. The concept of 

 the interspecies supraorganism has some 

 real scientific basis and is useful both in 

 relating many facts in ecology and in 

 directing our investigations toward the re- 

 lations of the parts of the coordinated 

 whole (Lotka, 1944). 



SUMMARY AND CONCLUSIONS 



Interactions between diflFerent species of 

 organisms and interactions between organ- 

 isms and their environment produce selec- 

 tion pressures. Reciprocal genetic patterns 

 evolve by means of such selection and pro- 

 duce interspecies adaptations, interdepend 

 ence, and integration. Harmful disoperation 

 between species eliminates itself. Exploita- 

 tion tends to evolve toward toleration and 

 mutualism. The evolution of mutualism be- 

 tween species has not progressed so far as 

 cooperation between parts of an individual 

 or between individuals in an intraspecies 

 population. The evolution of division of 

 labor and integration between species re- 



