CHARACTER DISCRIMINATORY POWER, CHARACTER-SET CONGRUENCE, AND THE CLASSIFICATION OF INDIVIDUALS FROM HYBRID ZONES - AN EXAMPLE USING STONE CRABS (MENIPPE)

Many investigators categorize individuals from hybrid zones to facilit ate comparisons among genotypic classes (e.g., parental, F-1, backcros s) for comparative studies in which components of fitness or geographi c variation are being analyzed. Frequently, multiple character sets re presenting genetically independent traits are used to classify these i ndividuals and various methodologies are employed to combine the class ifications obtained from the different character sets. We adapted the principles of total evidence and taxonomic congruence (two formalized approaches used by systematists in formulating phylogenetic hypotheses ) to address the problem of discriminating hybridizing species and cla ssifying individuals from hybrid zones. As our model, we used two morp hological (coloration and morphometric) and two molecular (allozyme an d mitochondrial DNA restriction-fragment-length polymorphism) characte r sets that differentiate two stone crab species (Menippe adina and M. mercenaria). Using principal-components analysis, we determined that combining character sets and eliminating characters or character sets that did not have large eigenvector coefficients for the principal com ponent that best separated the two species yielded the highest level o f discrimination between species and allowed us to classify a broad ra nge of morpho-genotypes as hybrids. For the stone crabs, three diagnos tic allozyme loci and five diagnostic coloration characters best separ ated the species. The two character sets were not completely congruent , but they agreed in their classification of 50% of the individuals fr om the hybrid zone and rarely strongly disagreed in their classificati ons. Classification discrepancies between the two character sets proba bly represent variation between traits in interspecific gene flow rath er than intraspecific, ecologically mediated variation. Our results su pport the assertions of previous investigators who espoused the benefi ts associated with using multiple character sets to classify individua ls from hybrid zones and demonstrate that, if character sets are reaso nably congruent and numerically balanced, combining diagnostic charact ers from multiple character sets (a total-evidence approach) can enhan ce discriminatory power between species and facilitate the assignment of hybrid-zone individuals to genotypic classes. On the contrary, clas sifying hybrid-zone individuals using character sets separately (a tax onomic-congruence approach) provides the opportunity to compare levels of introgression between species and to assess reasons for discordanc e among the data sets.