Molecular phylogenetics at the population/species interface in cave spiders of the southern Appalachians (Araneae:Nesticidae:Nesticus)

This paper focuses on the relationship between population genetic structureand speciation mechanisms in a monophyletic species group of Appalachiancave spiders (Nesticus). Using mtDNA sequence data gathered from 256individuals, I analyzed patterns of genetic variation within and betweenpopulations for three pairs of closely related sister species. Eachsister-pair comparison involves taxa with differing distributional andecological attributes; if these ecological attributes are reflected inbasic demographic differences, then speciation might proceed differentlyacross these sister taxa comparisons. Both frequency-based and gene treeanalyses reveal that the genetic structure of the Nesticus species studiedis characterized by similar and essentially complete populationsubdivision, regardless of differences in general ecology. These findingscontrast with results of prior genetic studies of cave-dwelling arthropodsthat have typically revealed variation in population structurecorresponding to differences in general ecology. Species fragmentationthrough both extrinsic and intrinsic evolutionary forces has resulted indiscrete, perhaps independent, populations within morphologically definedspecies. Large sequence divergence values observed between populationssuggest that this independence may extend well into the past. Thesepatterns of mtDNA genealogical structure and divergence imply that speciesas morphological lineages are currently more inclusive than basalevolutionary or phylogenetic units, a suggestion that has importantimplications for the study of speciation mechanisms.