696 



ECOLOGY AND EVOLUTION 



of the nucleus for an increase in gene num- 

 ber. If other organisms in the biocoenose, 

 however, supply certain physiological 

 needs, genes in a given organism may be- 

 come adaptively neutral, and are thus free 

 to mutate toward other functions. The 

 evolution of greater interdependence be- 

 tween organisms is correlated with progres- 

 sive evolution (see pp. 676 and 679). 



Species may be adapted to more than 

 one community. The larvae and pupae of 

 mosquitoes are adjusted to aquatic com- 

 munities, while the winged adults fit ter- 

 restrial and aerial conditions. The herbiv- 

 orous larvae are eaten by pond fishes, 

 while the predaceous adults suck the blood 

 of terrestrial vertebrates and may transmit 

 malaria or yellow fever to their vertebrate 

 prey. Thus, the same species of mosquito 

 is incorporated into diflFerent communities 

 that must be spatially close to each other, 

 but it takes a different place in the food 

 web in these different associations. 



The borderline between contiguous com- 

 munities is important in the hfe of a mos- 

 quito adapted to each. Hess and Hall 

 (1943) report a high positive correlation 

 between the population density of Anoph- 

 eles qiiadrimaculatus larvae and the "in- 

 tersection value," defined as the number 

 of meters of "intersection line" per square 

 meter of water surface. The "intersection 

 line" is the junction between water-air, 

 water-plant, and plant-air, such as the 

 perimeter of a floating leaf where it inter- 

 sects the water surface. 



Swynnerton (1940) has reported that 

 each of the twenty-one species of tsetse 

 flies has different behavior and ecological 

 adjustments. Each requires more than one 

 vegetational type at a time, and these 

 types must be in contact with each other. 

 He calls this "concurrence of requirements." 

 "Glossina morsitans, for instance, needs 

 savanna wooding to rest and breed in, and 

 vleis (temporary marshes) to search food 

 in. Continuous uniform savanna wooding 

 will not support it, while ant-heaps (ter- 

 mite mounds) with heavy vegetation, near 

 or at the contact of this and the vleis, add 

 much to the suitability of the general vege- 

 tational concurrence." Beecher (1942) also 

 found that certain bird species nest in 

 greatest density at the junction line (eco- 

 tone) between two communities. He re- 



fers to such distributions as producing an 

 "edge effect" (pp. 476-478). 



The adjustments of an organism to two 

 or more associations obviously influence the 

 evolution of the species assemblage. As an 

 example, the crayfishes of the Allegheny 

 Mountains are different species in the 

 streams of the Atlantic and Mississippi 

 drainages because, although the head 

 waters may be close and ecologically 

 nearly equivalent, there is no way for most 

 of the species to cross the divide. In con- 

 trast, the insect species of these same 

 streams, which have an adult flying stage, 

 are usually found in both drainage systems 

 because the divide is no barrier to their 

 distribution. 



Possibly even more striking are the com- 

 munity relations of migratory birds during 

 their annual flight from wintering to breed- 

 ing areas, the pelagic feeding and littoral 

 breeding activities of penguins and seals, 

 the marine feeding and fresh-water spawn- 

 ing of various Pacific salmon, and the 

 fresh-water feeding and marine breeding 

 of the migratory eels of Europe and North 

 America. Obviously, these animals must be 

 adapted to different sets of factors at the 

 same time or in different periods of their 

 life cycles, and the evolution of the differ- 

 ent associations of which they form an 

 element is influenced by their adaptations 

 and their periodicities. 



Occasionally, species at the base of food 

 chains may also be found in communities 

 of different types. Waksman (1945) points 

 out that many soil micro-organisms are also 

 characteristic of littoral sea waters and 

 must influence the pattern of these distinc- 

 tive communities. 



These complexities make biocoenoses and 

 communities less definable than are most 

 individual organisms, and there is no doubt 

 that considerable interdependence between 

 associations occurs— so much so, that a de- 

 gree of evolutionary integration and 

 balance exists that brings all life together 

 with its environment into an ecosystem 

 with some unity (Egler, 1942). Even 

 while many dynamic changes are affecting 

 portions of the total pattern, long-term 

 relative stability and independence of each 

 community system exist and justify the 

 separation of communities and their parts. 

 Many ecological principles must be coor- 

 dinated in order to understand both the 



