FIG. 19-1 (A) local plus and minus variations in any charac- 

 teristic of a species occupying a uniform habitat eventually dis- 

 appear because of gene flow throughout the population (pan- 

 mixia). (B) when the habitat characteristics of a geographic 

 range occupied by a species gradually change from one extreme 

 to another, a selection gradient is established that partially 

 counteracts the tendency toward panmixia, producing a dine 

 In the characteristics of the species. (C) when partial barriers 



to otherwise free gene flow occur in an environment or selection 

 gradient, a stepped dine (a) produces distinct races or sub- 

 species. If the barriers become complete, the isolation of popula- 

 tions may result in divergent evolution and complete speciation 

 so that later, after the barriers disappear (b), biological isolat- 

 ing mechanisms prevent the populations from interbreeding 

 (after Womble 1951). 



range should be eliminated, the populations at each 

 of the two extremes of the range would be considered 

 separate species. As it is, however, at least some gene 

 flow takes place throughout the range, and all popula- 

 tions must be considered as belonging in the same 

 species. 



Only through geographic isolation can popula- 

 tions differentiate into distinct species. There is no 

 established case in which any change in habitat, be- 

 havior, structure, or genetics acting singly or in 

 combination has, in the absence of geographic isola- 

 tion, been sufficient to prevent at least some signifi- 

 cant gene exchange with the rest of the species, with 

 resultant preservation of the species. With one ap- 

 parent exception, biotic factors apparently cannot by 

 themselves bring full differentiation of new species. 

 The exception is the simultaneous development of 

 polyploidy in certain individuals, which renders them 

 sterile with normal members of the species, although 

 not with each other. Polyploidy rather commonly 

 gives complete genetic isolation in plants, but it is 

 rare in animals. 



In the presence of geographic isolation, genetic 

 variations and natural selection may bring the af- 

 fected population to a different course of evolution 

 than in the parental species, especially if adaptation 



to a new environment is also involved. Biotic iso- 

 lating mechanisms may develop in the process. If the 

 geographic barrier formerly separating the population 

 should disappear, and the hitherto isolated population 

 again comes into contact with the rest of the species, 

 interbreeding will not then occur. This is the process 

 of speciation, the details of which we will now exam- 

 ine more carefully. 



VARIATIONS IN POPULATION 

 CHARACTERISTICS 



Observable difTerences in structure, func- 

 tion, and behavior between individuals belonging to 

 the same species are common. Actually, no two indi- 

 viduals, except perhaps identical twins, have exactly 

 similar characteristics. Early Mendelian geneticists 

 believed that speciation occurred as the instantaneous 

 result of major mutations (macro-evolution) (Bate- 

 son 1894, Goldschmidt 1940). Modern geneticists 

 are nearly unanimous in the view that it is the gradual 

 accumulation of many small variations over many 

 generations (micro-evolution) which eventually 

 gives a population reproductive isolation and, conse- 

 quently, species identity. 



260 Ecological processes and dynamics 



