80 I The Process of Evolution 



(98.6 percent) were made in the area of previous capture. Similar 

 behavioral restriction of physical-dispersal ability seems to be the 

 rule rather than the exception in nonmigratory butterflies. 



Few animals seem to be truly nomadic. Most (including most 

 human beings) stay close to their birthplaces, occupying a home 

 range which was "good enough for their parents." Many animals 

 defend all or part of their home ranges from intruders of their kind 

 —the well-known phenomenon of territoriality. This behavior, com- 

 mon in birds, mammals, reptiles, fishes, and some invertebrates, 

 results in a nonrandom distribution of individuals in the population; 

 they are dispersed more evenly than one would expect in a distribu- 

 tion governed solely by chance. Among other things, this often keeps 

 the population size at a level where the supporting resources of the 

 environment (food, nesting space, etc.) are not strained or entirely 

 consumed. Individuals, often young adults, that do not successfully 

 occupy and defend a territory must find greener pastures or starve; 

 thus a dispersing component is added to the population. 



Statistically, the opposite of territoriality is aggregating behavior 

 which results in more "clumping" than if individuals were randomly 

 distributed. Animals showing this behavior may have little or no 

 known social organization, as in the case of prehibernation aggre- 

 gations of ladybird beetles or snakes. At the opposite extreme we 

 have the highly social insects, among which there are morpho- 

 logically differentiated castes and the reproductives in a colony may 

 consist of a single pair of individuals. At this extreme, selection 

 operates largely through differential reproduction of colonies, not 

 individuals. In many vertebrates a social hierarchy (peck-order) is 

 established in which some individuals dominate others, obtaining 

 perquisites ranging from first choice in mate to first place in line 

 going through the barnyard gate. Dominant males may control large 

 harems and, in contrast with their less aggressive brethren, make a 

 large contribution to the pool of genetic information of succeeding 

 generations. Often, social groups (colonies of social insects and 

 bands of howler monkeys, for example) exhibit territoriality. 



NUMBERS OF INDIVIDUALS 



One of the most obvious attributes of any population is the number 

 of items, events, or individuals that it contains at a given time. The 

 number of individuals in biological populations is of great interest, 

 but unhappily it usually is difficult or impossible to ascertain with 

 accuracy the size of any natural population. The most inclusive 

 populations may include billions of individuals (e.g., man, house 



