172 



Increase in Number of Species 



TIME AND SPECIES FISSION 



The length of time required for isolated sister populations to develop 

 evolutionary independence in a genetic sense appears to vary greatly 

 between different taxonomic groups and even between closely 

 related species. The moss Grimmia patens (Steere, 1938) and the 

 amphibian Rana sylvatica (Smith, 1957) suggest disjunctions of at 

 least 10,000 years without discernible change. The peculiar range 

 of the sword moss Brijoxiphitim norvegiciim (Fig. 73) suggests an 



Fig. 73. The distribution of the sword moss Bnjoxiphium norvegiciim. The 

 wide separation of its populations suggests a disjunction of perhaps several 

 milhon years if not a great deal more. ( After Love and Love. ) 



even longer disjunction, perhaps of several million )'ears, with little 

 discernible change (Steere, 1937; Love and Love, 1953). Judged 

 on the basis of morphological similarity, a similar span of separation 

 has not produced distinctive species in many boreal North American 

 stoneflies and caddisflies each of which has a northeastern popula- 

 tion and a widely disjunct Ozarkian population. An example of the 



