FIFTY YEARS OF INTERSPECIFIC HYBRIDIZATION: GENETICS AND MORPHOMETRICS OF A CONTROLLED EXPERIMENT ON THE LAND SNAIL...

An unusually well documented case of secondary hybridization and introgression involving two morphologically dissimilar species of land snails is described. In 1915, 55 Bahamian Cerion casablancae were established on Bahia Honda Key, Florida. The introduced snails thrived, bred true to form, and the colony grew until about 1928 when it began to hybridize with C. incanum, the unrelated Florida species. In 1977, morphological hybrids occupied an area of about 3.5 ha. Spatial and temporal aspects of the interaction were characterized morphologically (18 shell characters were studied in source populations, founders, and hybrids sampled in 1933 and 1977) and genetically (variation at 17 allozyme loci in relevant populations sampled in 1977). In addition, special circumstances permitted us to discern the genetics of the founders: C. incanum is isogenic locally, C. casablancae is variable (P_o = 0.29-0.35), and the two species are well-differentiated (Nei's D̄ = 0.27). Hybrid intermediacy of form and continued enhanced variation appeared in characters from three covariance sets, while some unique hybrid morphologies resulted from characters in a fourth set. Comparison of 1933 with 1977 samples showed that the hybrids are gradually approaching C. incanum in phenotype. Regardless of phenotype, the snails on Bahia Honda Key were panmictic and outbreeding. There was no evidence for strong selection against hybrids, and by 1977 introduced alleles had spread over 5 ha. However, no pure C. casablancae remain; low (m = 0.026/generation) but persistent gene flow has brought about a 30% diminution of the introduced genome. These observations are useful in interpreting Cerion's remarkable variability as colonization following hurricane dispersal has undoubtedly played a role in the group's complex evolution. More generally, the results are relevant to the problem of interpreting hybrid zones of unknown origin, and the differences in the generally concordant patterns of morphological and genetic introgression reveal constraints on the way components of different coadapted genomes interact.