Incipient speciation in aquatic snails in an arid-zone spring complex

The gastropod genus Caldicochlea is endemic to the Dalhousie Springs in the arid north of South Australia. The genus contains two species, or species complexes, Caldicochlea harrisi and C. globasa that overlap morphologically but can be distinguished genetically, principally by allozymes at the Lap, Sordh-1 and Sordh-2 loci. C. harrisi is found in many of the approximately sixty active springs in the 70 km(2) of the Dalhousie complex, but C. glob osa is confined to the northern half where there are larger, hotter springs . The species co-occur in many springs in this area, generally with apparen tly none, or low levels of hybridization, as judged by the absence of heter ozygotes at the Lap and Sardh-2 loci. Introgression is apparently low and b idirectional. In two springs within the current ranges of both species, Lap genotypes occur in Hardy-Weinberg expected frequencies. Genotypes at other loci suggest that this is unlikely to be due to the independent evolution of Lap polymorphisms in these springs. Reproductive isolation may have brok en down or may not have been established in these springs. For the latter t o be so, reproductive isolation must have evolved through intra-spring inte ractions between the species, and independently in different springs. Isola tion would not be fully established, if at all, until after secondary conta ct of their colonizing populations. (C) 2000 The Linnean Society of London.