Increase in Number of Species 177 



(Fig. 76). Probably the original parental species had a somewhat 

 similar range which was broken into a northern and a southern 

 segregate by ecological changes associated with glacial fields in the 

 Cascade and Siskyou Mountain ranges. The two isolated popula- 

 tions presumably evolved distinctive characteristics, but when the 

 two populations rejoined following dissipation of the glaciers, they 

 were still compatible genetically and are now in the process of 

 fusing into a relatively homogeneous intermediate whole. If such a 

 series of events took place, the separation was sufficient to produce 

 diflferences but was not long enough to produce two species in this 

 complex of salamanders. 



Obviously the time required for the evolution of distinctive 

 sister species varies with and within the taxonomic group. In most 

 species 15,000 years would seem to be insufficient. In some species 

 35,000 to 50,000 years would seem to be long enough; but in still 

 others even this length of time is not enough. The rate of diff^erentia- 

 tion of sister species is apparently influenced by two factors : degree 

 of isolation and rate of becoming genetically incompatible. 



Degree of isolation. Theoretically, complete and sustained isola- 

 tion between sister populations would bring about the most rapid 

 genetic divergence. Such absolute isolation undoubtedly occurs in 

 organisms having low vagility and in colonies established on ex- 

 tremely remote areas such as highly isolated oceanic islands. Be- 

 tween other isolated sister populations some dispersal undoubtedly 

 occurs. How much such occasional mixing slows down the process 

 of genetic divergence can only be guessed. 



This probability of dispersal between separated populations leads 

 into a related question: Will certain degrees of partial isolation be 

 sufficient to cause species fission? Certain populations of the Canada 

 goose Branta canadensis suggest a negative answer (Hanson and 

 Smith, 1950). Four of the breeding populations of this species are 

 restricted to discrete areas around Hudson Bay (Fig. 77). Almost 

 all the birds in any one area follow the same flyway, winter in the 

 same southern region, and the following spring return with their 

 young to breed in the same restricted area around Hudson Bay. 

 There is some mixing of birds between flocks, but this, calculated 

 from banding returns, is no more than 5 per cent ( Hanson, per- 

 sonal communication). Presumably this behavior pattern of the 

 Canada goose is of long standing and may antedate Wisconsin 

 time, but to date there appears to be no tendency for the various 

 flyway groups to become distinctive races. 



Although degree of isolation is an important aspect of species 



