Populations I 75 



tical constitutes an ecologically meaningful assemblage as it forms 

 part of the environment of other organisms. 



An individual is a set of operations (or machine) programmed in 

 advance to do particular things. In organisms, of course, the program 

 is established by the coded genetic information. A group of ge- 

 netically identical individuals is one individual reproductively. Eco- 

 logically they represent a population of individuals with different 

 epigenotypes. If we had, historically, begun to think about biology in 

 ecological terms rather than taxonomic terms, we would probably 

 now deal with biological "facts" very differently. 



It is obvious that the concept of "individual," like other concepts 

 in biology, can be given only operational meaning. To make the 

 definition clear, one must specify whether he is concerned with 

 taxonomy, genetics, or ecology. In what follows, a genetic definition 

 of individual will be employed. Most of the evolutionary work on 

 populations has been done with organisms among which the discrete 

 individuals are the result of sexual reproduction and thus are, usu- 

 ally, genetically diverse. In sexually reproducing organisms the most 

 inclusive populations are generally considered to consist of those 

 individuals sufficiently alike that, given the opportunity, successful 

 reproduction will occur. The criteria for just what sort of assemblage 

 may be labeled a population are hard to establish, and the degree 

 of conformity with these criteria in natural aggregations is usually 

 only guessed. 



In this chapter, examples will, in general, replace definitions. A 

 butterfly and a bison obviously do not belong to the same popula- 

 tion; a pair of robins raising a brood in the garden obviouslv do. 

 Near the center of the continuum, problems arise: Could the Euro- 

 pean brown bear and the American grizzly be part of the same 

 population? Are the eastern and western sycamores part of the same 

 population? They have been geographically separated since the 

 Miocene, but their hybrid is a vigorous and much-used street tree. 

 Since our interest is primarily in the process of evolution, rather than 

 in making arbitrary decisions, no answers will be sought to these 

 questions. 



SPATIAL DISTRIBUTION 



One property possessed by populations, but not ( in the same sense ) 

 by their constituent organisms, is distribution. At any instant in 

 time, checkerspot butterflies (Euphydryas edifha) are distributed 

 along an outcrop of serpentine rock on Stanford University's Jasper 

 Ridge Biological Experimental Area. The distribution of adults in 



