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 stru cture and speciation mechanisms in a monophyletic species group of App alachian cave spiders (Nesticus). Using mtDNA sequence data gathered f rom 256 individuals, I analyzed patterns of genetic variation within a nd between populations for three pairs of closely related sister speci es. Each sister-pair comparison involves tars with differing distribut ional and ecological attributes; if these ecological attributes are re flected in basic demographic differences, then speciation might procee d differently across these sister taxa comparisons. Both frequency-bas ed and gene tree analyses reveal that the genetic structure of the Nes ticus species studied is characterized by similar and essentially comp lete population subdivision, regardless of differences in general ecol ogy. These findings contrast with results of prior genetic studies of cave-dwelling arthropods that have typically revealed variation in pop ulation structure corresponding to differences in general ecology. Spe cies fragmentation through both extrinsic and intrinsic evolutionary f orces has resulted in discrete, perhaps independent, populations withi n morphologically defined species. Large sequence divergence values ob served between populations suggest that this independence may extend w ell into the past. These patterns of mtDNA genealogical structure and divergence imply that species as morphological lineages are currently more inclusive than basal evolutionary or phylogenetic units, a sugges tion that has important implications for the study of speciation mecha nisms.