76 I The Process of Evolution 



two successive years is shown in Fig. 5.1. The distribution in the 

 second year is somewhat different from that in the first. Such colonies 

 of Euphydryas editha occur throughout the San Francisco Bay area; 

 indeed, they are found along the West Coast from Baja California 

 to British Columbia. It is difficult to specify the limits of the most 

 inclusive population in which the Jasper Ridge individuals could 

 be placed. Most biologists would place in this most inclusive group- 

 ing individuals from colonies as far away as Montana. 



Similarly, clusters of individuals in various-sized aggregates are 

 found in plants. Clematis fremontii var. riehlii, which occurs on 

 limestone glades in the midwestern United States, is a perennial 

 plant that has been studied in some detail by Erickson. Individuals 

 are grouped into aggregates of several hundred plants, many such 

 aggregates occupying a single glade. The outcroppings of limestone 

 are clustered and aggregated geographically with respect to the 

 mountain systems and rivers. In the Midwest, the plant has a much 

 wider distribution that represents the most inclusive population 

 (Fig. 5.2). 



In its loosest usage, distribution generally means the smallest geo- 

 graphic area that will enclose all the area normally occupied by the 

 organisms under discussion. On a small world map of the distribution 

 of Homo sapiens, the entire United States would be shaded to indi- 

 cate its occupation by man. (Oceanic areas and most of the Green- 

 land ice cap would be left blank. ) In contrast, if we were mapping 

 the occurrence of man on a large-scale map of Colorado, many high 

 mountain peaks and some other areas would be left blank. The 

 problems of such a mapping are obvious. Organisms are mobile at 

 some stage of their life history, and their distributions are con- 

 stantly changing. Furthermore, no known organisms are uniformly 

 distributed over large areas. Thus the more resolution one strives 

 for in describing a distribution, the more difficult the task becomes. 



ECOLOGICAL DISTRIBUTION 



The nonuniformity of geographic distributions can usually be ex- 

 plained by the relationships of the organisms with their living and 

 nonliving environments. Gross examples of ecological factors con- 

 trolling distribution are easily understood; the factors controlling 

 the fine points of the distribution of a given organism virtually are 

 never fully understood. In the San Francisco Bay region Euphydryas 

 editha larvae feed on Plantago erecta, a small native plantain espe- 

 cially abundant on serpentine outcrops. In this area the butterfly 

 occurs only where both P. erecta and serpentine are found, but the 



