ANIMAL AGGREGATIONS 



407 



CONCENTRATION 

 4X 2X X 



12 12 12 



SEEDING 



4.2 5.2 8.0 10.2 10.6 10.4 



Fig. 145. Some effects from varying the 

 number of introduced Oxytricha and bacteria 

 in drop cultures. ( Data from Johnson. ) 



Johnson (1937, p. 13) arrived at the 

 following interpretative summary of this 

 situation: 



"This work with Oxytricha confirms the 

 existence of the phenomenon described by 

 Robertson [see p. 357], but it does not con- 

 finn his theory of allelocatalysis. In cultures 

 where the bacterial concentration is supra- 

 optimal it appears that two protozoans are able 

 to reduce the numbers to the optimal density 

 quicker than one, and as a result exhibit a 

 higher reproductive rate. Tliis indicates a 

 beneficial effect of crowding in protozoans 

 when they are in media very dense wdth 

 bacteria." 



Population Size and the Rate of Evolution 



We can take time to discuss only one 

 more aspect of ecological effects arising 

 from the existence of minimal and optimal 



fjopulations. The relation between popu- 

 ation size and density is mentioned here 

 in part because various ecological aspects 

 of evolution are both neglected and impor- 

 tant, as may be seen from the treatment 

 of that subject in a later section of this 

 book (p. 598). Charles Darv^dn recognized 

 that a relatively large population is an im- 

 portant factor in natural selection. In more 

 recent years, Wright (1931, 1932, 1945) 

 and others have presented good evidence 

 that, at least under certain conditions, evo- 

 lution proceeds most rapidly in popula- 

 tions of random breeding organisms that 



are intermediate in size, as compared with 

 similar populations that are overlarge or 

 oversmall. We are especially interested in 

 the slower rate of evolution in oversmall 

 populations as contrasted with that of those 

 that are somewhat larger. The rate of evo- 

 lution is still more rapid when a large, 

 \\ddely distributed species population is 

 broken into relatively small breeding colon- 

 ies not completely isolated from each other. 



Even in this last situation the more gen- 

 eral rule still holds, and the separation of 

 a species population into small breeding 

 colonies with occasional interemigration is 

 in itself one expression of optimal popula- 

 tion density. Obviously, if the population 

 becomes overdense and is effectively cir- 

 cumscribed, it cannot partially fragment, 

 but becomes rather an overlarge popula- 

 tion. 



Wright (1945), in his review of Simp- 

 son (1945), comments: 



"The reviewer wishes also to make a cor- 

 rection here to a statement . . . similar to ones 

 made by other authors, in which he is credited 

 with the conclusion that the conditions most 

 favorable for rapid evolution are to be found 

 in populations of a certain intermediate size. 

 The actual statement made in several papers 

 was to the effect that conditions are more favor- 

 able in a population of intermediate size than 

 in a very small one or in a very large random 

 breeding one (assuming a constant direction of 

 selection). But such a statement has always 

 been followed by the statement that conditions 

 are enormously more favorable in a population 

 which may be large but which is divided into 

 many small local populations almost but not 

 quite completely isolated from each other . . ." 



Omitting all details, some of which may 

 be found in the section on Evolution (see 

 p. 602)* the important point for general 

 ecology, as well as for the evolutionary 

 processes concerned, is that one of the pri- 

 mary controlling factors in the rate of evo- 

 lution, under many conditions, is the num- 

 ber of animals in the interbreeding group. 

 This gives another illustration of the condi- 

 tions summarized by graph B of Figure 

 139. As with many phases of individual 

 survival, the rate of evolution is highest, 

 other things being equal, in populations of 

 optimal size as contrasted with those that 

 are overlarge or oversmall. Here, the rate 



" A simply expressed statement of many of 

 the fundamental considerations is given by 

 Allee (1938). 



