684 



ECOLOGY AND EVOLUTION 



These intraspecies populations may be 

 integrated through physiologic or be- 

 havioristic mechanisms that are nongenetic 

 as such. The capacity to differentiate may 

 be genetic, but the different resulting forms 

 may not differ genetically. On the other 

 hand, some of these populations may be 

 dimorphic or polymorphic with a genetic 

 difference for each form. 



Where germinal continuity is broken, in- 

 terspecies populations are coordinated 

 largely through coaction (pp. 437, 698). 

 Terms such as predator, prey, parasite, 

 host, guest, symbiote, biocoenose, society 

 (in the wide sense), sere, biome, com- 

 munity, ecosystem, and so forth, are used 

 for the parts and for the whole units. The 

 terms, both for the various integrated units 

 and their parts, are often inexact and grade 

 into one another. Further classification by 

 addition and division may be carried on 

 indefinitely. 



At this point we shall consider the fact 

 that these levels of individual and group 

 coordination are subject to selection as 

 units and are often under the influence 

 of different selection pressures for dif- 

 ferent arrangements within the same or- 

 ganismic system. The existence of complex 

 internal adaptation between parts of an or- 

 ganism or population, with division of labor 

 and integration within the whole system, 

 is expUcable only through the action of 

 selection upon whole units from the lowest 

 to the highest. Conversely, these integrated 

 levels would not exist as entities unless 

 selection acted upon each whole system." 



At the species level, genes that tended 

 to mutate excessively would be deleterious 

 to the population system, even though some 

 of the characters produced by such genes 

 might be advantageous to the individual. 

 One might, therefore, expect selection to 

 exert a control over the rate of mutation. 

 An optimal rate of mutation and recom- 

 bination is probably adaptive, and the rate 

 is maintained through selection of the sur- 



* This action of selection upon whole popu- 

 lation systems was sensed by Darwin, Spencer, 

 and Weismann, and has been elaborated and 

 analyzed in the light of modern biology by 

 more recent authors ( Marshall, 1936; Wright, 

 1930, 1937, 1945; Sturtevant, 1938; Alice, 1938, 

 1940, 1943; Emerson, 1939, 1939a, 1942, 1943, 

 1947; Mather, 1943; T. Park, 1945; Gerard and 

 Emerson, 1945; Howells, 1947). 



viving species even though many individ- 

 ual variants are eliminated (Sturtevant, 

 1939; Just, 1944; Simpson, 1944, p. 37). 



Wright (1937a) points out: "It is the 

 harmonious development of all characteris- 

 tics that determines the success of an or- 

 ganism, not the absolute grades of the 

 separate characters and still less the com- 

 position with respect to a single series of 

 alleles." This conclusion doubtless applies 

 to all organismic systems and is basic to 

 our understanding of organismic integra- 

 tion and survival at all levels— the individ- 

 ual, the intraspecies population, the in- 

 terspecies population, and the community. 



Density is a phenomenon that has a 

 series of optimal ranges for a given species 

 under environmental conditions that reg- 

 ularly recur, so that there may be under- 

 crowding as well as overcrowding with 

 greater survival at the optimum (p. 395). 

 One would therefore expect that selection 

 would operate on those aspects of natality, 

 mortality, and dispersal that have a genetic 

 basis. Doubtless many environmental fac- 

 tors also directly affect the abundance of 

 any given species and its balance and un- 

 balance in the community (Kendeigh and 

 Baldwin, 1937; Errington, 1934). 



If selection does sort some genetic traits 

 influencing population size, population 

 numbers would often be as characteristic 

 of species and even some higher categories 

 as are other adaptive characters. Popula- 

 tions of individuals should theoretically 

 show growth and maturity as do popula- 

 tions of cells in a multicellular organism 

 (pp. 264, 282; Chap. 21). Although this 

 field of investigation is in its infancy, and 

 many complexities confuse the analysis of 

 any given case (Thompson, 1939; Erring- 

 ton, 1946), there are some definite indica- 

 tions that such intrinsic control of popu- 

 lation numbers has evolved. The more inte- 

 grated the population, the more it takes on 

 supraorganismic aspects, and the greater is 

 the tendency for inherited and adaptive 

 density control (Strandskov and Ondina, 

 1947). 



Numerous authors have questioned the 

 possibility of certain evolutionary tenden- 

 cies because they have concentrated upon 

 a part of the system instead of recognizing 

 the unity of the whole species system. El- 

 ton (1930, p. 47) points out that in large 

 scale emigration most of the migrants 



