82 I The Process of Evolution 



flies, some microorganisms, various algae) or less than 100 indi- 

 viduals (whooping cranes or certain rare endemic plants such as 

 Pedicidaris dndleyi and Tetracoccus ilicifolius). In the strictest sense, 

 whenever the number of individuals in a population changes, the dis- 

 tribution of the population changes; often a change in distribution 

 also means a change in numbers. Study of Fig. 5.1, on which the 

 position of first capture of E. editha adults for two consecutive years 

 is plotted, will give some idea of the problems of dealing separately 

 with distribution and abundance. In area C there was little increase 

 in numbers between 1960 and 1961 but some change in the distribu- 

 tion pattern. In area G the numbers decreased, and there was a 

 concomitant shrinkage in the area occupied. In area H the numbers 

 increased greatly, and the population occupied an area that was 

 virtually devoid of individuals in the previous year. It is important 

 to note that the figures, like virtually all representations of distribu- 

 tion, are a stylized, static representation of a dynamic situation. The 

 numbers and distribution of individuals in biological populations 

 are constantly changing, the speed of the changes varying greatly 

 from organism to organism. 



Population dynamics, the study of changes in population size, is a 

 very complex subject which cannot be pursued here. It is of con- 

 siderable interest to the evolutionist, for, as will be seen, changes 

 in population size affect the evolution of a population in diverse 

 ways. This can be understood intuitively, since each individual in a 

 population is part of the environment of every other individual. 

 Therefore any change in population size is automatically a change 

 in the environment of the population, and populations evolve in re- 

 sponse to environmental changes. 



The factors that control population sizes are diverse and in many 

 cases poorly understood. There is little doubt, however, that usually 

 feedback mechanisms operate to regulate the size of populations; 

 that is, the size of the population influences its growth rate. Basic 

 references from the abundant literature on this subject are cited at 

 the end of this chapter. 



ENVIRONMENT 



An individual organism, when such can be recognized, is in a sense 

 the morphological resultant of the physiological processes of which 

 it is composed. Each of these processes is interrelated with the other 

 functions of the organism, and the complex of processes cannot be 

 separated from the environment, except artificially. The function- 

 ing of an individual is determined by the relationship of its constitu- 



