EFFECTS OF ISOLATION BY DISTANCE AND GEOGRAPHICAL DISCONTINUITY ON GENETIC SUBDIVISION OF LITTORARIA CINGULATA

Littoraria cingulata (Philippi, 1846) is a Western Australian, mangrov e littorine snail, represented by two morphologically distinct subspec ies, whose distributions are separated by >300 km. The southern subspe cies, L. cingulata prestissini, is distinguished from the northern sub species, L. cingulata cingulata, by having a thinner keelless shell wi th more primary grooves, and lower and much more numerous ribs. In con trast with these striking differences, L. cingulata cingulata is morph ologically very similar to another species. L. sulculosa, with which i t also shares a nearly coincident geographic range. Allozyme compariso ns at 22 presumptive loci confirmed a large genetic distance between L , cingulata and L. sulculosa, and the apparent conspecificity of the m orphologically divergent subspecies of L. cingulata. Based on geologic al evidence, the geographical separation of the morphologically diverg ent forms of L. cingulata has developed within the past 5000 to 10 000 yr. The extensive continuous distribution of the northern subspecies, L. cingulata cingulata, and the large geographic disjunction between the northern and Shark Bay subspecies, L. cingulata pi pristissini, al lowed a test of the generic importance of this relatively recent disju nction. Within the continuous distribution of the two subspecies, a pa ttern of isolation by distance was visible up to distances of 300 km. Beyond 300 km, genetic subdivision, measured by pairwise Gfr (the prop ortion of genetic diversity due to differences between populations), a veraged 0.028, whereas subdivision between Shark Bay and northern popu lations averaged 0.055 over the same range of distances. Although the relative paucity of barriers to gene flow tends to limit genetic subdi vision in marine species with planktotrophic larvae, the results for L , cingulata suggest that subdivision can occur within a continuous dis tribution, but that special events leading to major disjunctions can s ubstantially increase divergence, even over a relatively short period of time.