SUBTERRANEAN AMPHIPOD STYGONECTES 171 



terrains and steep stream gi'adients would have been greatly reduced. 

 On the bases of morphological, distributional, and, to some extent, 

 ecological data, three major lines of evolutionary divergence are 

 recognized within the genus. 



Patterns of insular speciation are generally characteristic of species 

 that inhabit caves of the central Appalachians and the Edwards 

 Plateau of central Texas. Geographic isolation of ancestral popu- 

 lations to geologically delimited Imiestone valleys in the Appalachians 

 and to restricted systems of subterranean drainage in the Edwards 

 Plateau is offered as an explanation for these patterns. Extrinsic 

 barriers to dispersal are believed to be ridges of insoluble elastics in 

 the central Appalachians and faults and stratigraphic changes in the 

 Edwards Plateau. 



A number of species in the tenuis group have wide ranges, which 

 extend over parts of several contiguous physiographic provinces. 

 These species are typically found in interstitial habitats and are 

 believed to disperse rather freely through ground waters developed 

 close to the surface. 



In general, large rivers do not appear to impose dispersal barriers 

 or to preclude gene flow between populations of the same species 

 situated on opposite sides. This is true of both cavernicolous and 

 interstitial species. 



The phylogeny of each species in the genus is discussed at some 

 length and theoretical models of speciation are proposed for each 

 species group. Although several closely related species occur sym- 

 patrically, all appear to have originated through processes of allopatric 

 speciation. 



