AN EMPIRICAL-TEST OF PREDICTIONS OF 2 COMPETING MODELS FOR THE MAINTENANCE AND FATE OF HYBRID ZONES - BOTH MODELS ARE SUPPORTED IN A HARD-CLAM HYBRID ZONE

Two models developed to discern the mode of selection in hybrid zones differ in some predictions. The tension-zone model predicts that selec tion acts against hybrids and independently of the environment (endoge nous selection) and that selection is invariant throughout the hybrid zone. The ecological selection-gradient, or ecotone, model maintains t hat fitness of different genotypes varies in response to environmental variation (exogenous selection) and thus, that in a region of the zon e, fitness of hybrids is at least equal to that of the parental specie s. Therefore, to assess the predominant mode of selection operating in a hybrid zone, it is fundamental to evaluate whether selection is act ing specifically against hybrid individuals, that is, whether hybridit y alone is the basis for deficiencies of hybrids, and to evaluate whet her the relative fitness of hybrids versus that of pure species varies across the zone. In a hard-clam (genus Mercenaria) hybrid zone locate d in a polyhaline lagoon in east-central Florida, we used age-specific and location-specific analyses to determine that a hybrid deficit occ urrs, that the deficit seems to be due to selection against hybrids, a nd that selection varies across the zone. Various measures of deviatio n from Hardy-Weinberg equilibrium, linkage disequilibrium analyses, an d shifts in allele frequencies at semidiagnostic loci support the idea that selection is strongest in the northern region of the lagoon, the zone of sympatry and hybridization. Southward, into the range of M. m ercenaria (the numerically predominant species), the percentage of hyb rids remains relatively high and selection against hybrids decreases. For some genetic linkage groups, selection for M. mercenaria alleles s eems to be occurring, but selection seems to be acting principally aga inst alleles characteristic of M. mercenaria and, to a lesser degree, for alleles characteristic of M. campechiensis (the rarer species). Th ese findings and others from previous analyses we have done on this hy brid zone demonstrate that selection in the zone is complex, and that characteristics of both the tension-zone and ecotone models are presen t. Supporting the tension-zone model, selection against hybrids per se clearly occurs, but specific genotypes seem to be at a selective disa dvantage, whereas others have a selective advantage, and selection ope rates differentially on the two parental species within the zone. Supp orting the ecotone model, the strength of overall selection varies thr oughout the zone, and environmentally mediated selection in which each species and hybrids have an advantage in specific habitats occurs, bu t some selection against hybrids is invariant throughout the zone. Thu s, the structure and genetic architecture of this hybrid zone appear t o be products of a complicated interaction between both types of selec tive forces cited in the two competing models.