122 Species and Species Change 



are there many known instances of absolute biological differences 

 between closely related species. 



In summary, bisexual species represent cross-sections of phylo- 

 genetic lines moving through time. Each species is usually distinct 

 on the basis of some recognition feature and has distinctive bio- 

 logical and ecological attributes. Above all, each species is geneti- 

 cally distinct to a high degree, and for this reason each one changes 

 genetically independently of other species. Thus each species is 

 independent in an evolutionary sense. 



THE SPECIES AS A PHYSICAL UNIT 



The species is the total of the individuals forming its populations. 

 These populations occupy space and are the tangible expression 

 of the continuity of the genetic basis which controls and directs 

 the growth of the individual. This total population is the result of 

 the interaction between the dynamic genetic composition of the 

 column and the dynamic environment. Certain aspects of this 

 total population are important to an understanding of adaptability 

 in the species as a whole. 



Range 



The range of a species is the sum of all the areas in which the 

 species lives. It is also the total inhabited area of all the local 

 populations. Where the species can live is determined by certain 

 ecological requirements, including food, climatic and physical 

 factors, and a place to live and reproduce. These ecological factors 

 are extremely complex in their action, both singly and in combina- 

 tion. An excellent summary of this topic is given by Alice and 

 Schmidt (1951). Each species can exist only between certain 

 maximum and minimum values for each ecological factor of its 

 environment. This total possible band is the ecological tolerance 

 of the species. A species can occupy only those areas which occur 

 within its ecological tolerance. 



GROSS EXTENT 



Ranges of organisms vary from very small to practically world- 

 wide. The range of the Kankakee mallow Illiamna remota occupies 

 an island of only a few hundred square yards in the Kankakee 

 River near Kankakee, Illinois, and a few hundred square yards on 

 Peter's Mountain, Virginia. This latter colony may be a distinct 



