606 



ECOLOGY AND EVOLUTION 



therefore, initiating factors and clinching 

 factors that may be separated on occasion, 

 one following the other (Patterson, 1942). 



Various classifications of isolating mech- 

 anisms have recently been proposed (Dob- 

 zhansky, 1941; Mayr, 1942; Muller, 1942; 

 Huxley, 1942; Emerson, 1943; Cain, 1944, 

 p. 357; Mayr, 1948). Some authors empha- 

 size permanent isolation through physio- 

 logic and genetic incompatibihty, as con- 

 trasted with the possibly less permanent 

 geographical and environmental mecha- 

 nisms. On the other hand, initial isolating 

 factors are more often the effect of geo- 

 graphical or ecological barriers to gene flow. 

 Whatever the factor isolating two or more 

 populations, the evolutionary consequences 

 have a certain fundamental similarity. 



The known isolating mechanisms may be 

 arranged in a fairly logical order, although 

 overlapping among categories occurs to 

 some extent. Grouping of categories on the 

 basis of genetics differs somewhat from 

 grouping on the basis of ecology, but both 

 are important. 



Table 52. Isolation Factoid 



Populations 

 geneticalh 

 similar 



3. 



Populations 

 geneticall)^ 

 different 



Spatial 



isolation 



Topographic 



isolation 



Habitat 



isolation 



4. Cyclic isolation 



5. Mechanical 

 isolation 



6. Psychological 

 isolation 



7. Physiological 

 isolation 



8. Genetic isola- 

 tion ( In- 

 fertility) 

 Hybrid in- 

 viability 

 Hybrid 

 sterility 

 Selective 

 hybrid 

 elimination 



Extrinsic 

 bars to 

 gene flow 



Sexual 

 bars to 

 gene flow 



9 



10 



11. 



Hybrid 

 incapacity 



Ecologic factors are important under cer- 

 tain categories of isolation and unimportant 

 under others. Population genetics and se- 

 lection have isolating ejffects that are dis- 

 cussed under these headings. Linear change 

 without divergence has sometimes been in- 

 cluded under the term chronological isola- 

 tion, but is possibly better considered as 



an aspect of selection (see ChronocHne, p. 

 626; Orthoselection, pp. 638, 649). Repro- 

 ductive isolation as exhibited by asexual 

 species is discussed at the end of this 

 chapter (p. 628). The isolating effects of 

 varying population size and structure have 

 been dealt with in the chapter on Ecology 

 and Genetic Variation (p. 602). The 

 genetic and physiologic bases of hybrid 

 inviabiUty (p. 677) and sterility are not 

 discussed further here, since they are ana 

 lyzed in embryological and genetic Utera 

 ture. 



The other mechanisms hsted (Table 52) 

 have ecologic impUcations. Some, such as 

 genetic isolation or infertihty, receive more 

 complete study in works on evolutionary 

 genetics. Spatial and topographic isola- 

 tion may be grouped under geographic iso- 

 lation, but these are certainly ecologic 

 in the broad sense. Timofeeff-Ressovsky 

 (1940a) separates "biological isolation," in- 

 cluding genetic, sexual, physiologic, and 

 ecologic, from "mechanical isolation," in- 

 cluding territorial separation. Various as- 

 pects of sexual isolation are here considered 

 broadly ecologic, one sex belonging to the 

 environment of the other. Habitat and cy- 

 cUc isolation, as well as selective elimina- 

 tion of hybrids, are ecologic in the strict 

 sense. Habitat isolation is referred to as 

 "ecological isolation" by some authors, but 

 our designation seems more distinctive. 

 Habitat isolation might be considered mi- 

 crogeographic, but we prefer to separate the 

 grosser geographic factors from the finer 

 ecologic factors within a region, even 

 though it is obvious that the two categories 

 are not always sharply distinct. 



Patently, more than one isolating mech- 

 anism may divide populations at the same 

 time, and the data may not enable one to 

 evaluate the relative importance of each 

 factor. Each major category also may have 

 innumerable subdivisions. Cases that illus- 

 trate the importance of each major isolating 

 mechanism are discussed in the following 

 pages, but it is seldom possible to study 

 the action of one mechanism to the com- 

 plete exclusion of all others. Hovanitz 

 (1942, 1943) studied racial or species dif- 

 ferences in the butterfly, Colias, and found 

 indications of geographic, cyclic, habitat, 

 and sexual isolation, together with hybrid 

 elimination, all influencing the partial seg- 

 regation of closely related populations. In 



