Testing Nominal Species Boundaries Using Gene Flow Statistics: The Taxonomy of Two Hybridizing Admiral Butterflies (Limenitis: Nymphalidae)

Gene flow estimates between groups of populations (NmGT) derived from Wright's hierarchical F-statistics can be used to test taxonomic hypotheses about the location of biological species boundaries. The interpretation of NmGT is limited by the assumptions of the island model: balancing selection, time to equilibrium, and the limits to detectability of alleles are biases acting in the same direction as gene flow to cause similarity among populations. However, confidence limits of 0.01 ≤ NmGT ≤ 0.15 between sympatric sibling species suggest that these biases are weak relative to gene flow in causing genetic similarity. NmGT also compares favorably to gene flow estimated from a mark-recapture study, and NmGT values across a well-studied grasshopper hybrid zone suggest that gene flow is limited but not stopped by the barrier. The nominal species Limenitis lorquini and L. weidemeyerii hybridize along their contact areas in western North America. Their hybrid zone was mapped using wing pattern characters, and the same populations were analyzed electrophoretically at 18 enzyme loci. Gene flow is strong within these taxa (1.56 ≤ Nm ≤ 15.23). Across the hybrid zone, 0.57 ≤ NmGT ≤ 1.97, suggesting that although there is a partial barrier to genetic exchange, these taxa share significant portions of their gene pools. Small population sizes and other circumstantial evidence argue that much of the similarity observed among populations is actually due to gene flow, rather than balancing selection, although undetected allelic differences may also be partially responsible. L. weidemeyerii and its “subspecies” are best considered as univoltine subspecies of lorquini, and all should be treated as a single genetic unit when cladistic analyses are performed. The ease with which these results were obtained suggests that systematists should pay more attention to the genetic population structure of taxonomic boundaries.