Cryptic, genetically extremely divergent, polytypic, convergent, and polymorphic taxa in Madagascan Tropidophora (Gastropoda: Pomatiasidae)

Madagascar's magnificent and environmentally threatened endemic radiation of the land-snail genus Tropidophora Troschel has recently been classified into three subgenera, 95 species and 142 varieties, based on often subtle conchological variation among small samples; it seems best to ignore temporarily this confusing plethora of names until true biological species and their relationships are better understood. The author's field work in 1990 succeeded in obtaining live Tropidophora from 40 populations distributed throughout the island. Allozyme analysis (108 snails, 15 loci, 117 alleles) yielded a cladogram suggesting nine genetically coherent taxa (T. taxa A-I), each represented in the collections by 1–10 populations. Comparisons among shells (total 1634) and penes (total 31 from 20 populations representing eight of the nine taxa) revealed: (1) two conchologically indistinguishable taxa (H and I) fixed for alternative alleles at 13 of 15 loci, with only a very subtle difference in penes, and with mosaic and overlapping geographical distributions in the Northeast; (2) two extremely polytypic taxa (C in the Southwest, F in the Southeast) with parallel trends toward depressed, broadly umbilicate, heavily sculptured shells with apertural lips widely reflected at the umbilicus at inland, more arid sites, resulting in sympatric convergence; (3) one southeastern taxon (G) in which the penis apparently doubles in length but the shell does not change 6.0 km to the northwest, but in which the shell shifts dramatically in sculpture, colour, and lip reflection 0.5 km to the westnorthwest; and (4) generally such extreme intra- and inter-population variation in shell and male-genital characters as to render many of them dubious at best for systematics. Thus the Madagascan Tropidophora present a fascinatingly complex problem in evolutionary morphology/ecology, the solution of which will require even more extensive collecting, followed by molecular comparisons or detailed anatomical comparisons, or both. The total number of biological species is still likely to be quite large, despite irrelevance of much of the current taxonomy, because many smaller species remain to be discovered.