As much as the century-long debate on what species are has created confusion and controversy, not less has speciation been discussed, i.e., the process of how new forms and taxa evolve, ultimately resulting in the multiplication of species. Darwin's often cited “mystery of the mysteries”, i.e., the origin of species and, as a consequence, of biological diversity in general, remains at the forefront of current evolutionary biology. While allopatric speciation by geographical separation is still considered an important mechanism in most cases, recently it became evident (as a slow and quiet revolution) that alternative explanations may account for how new species come into being. Among the most prominent factors, particularly in the context of adaptive radiation, is ecology in concert with specialization, albeit not necessarily in a sympatric setting. Unfortunately, for freshwater gastropods we are far away from really understanding the details of the various speciation processes (allopatric versus sympatric) and the role of natural selection, adaptation and ecology that have lead to the array of radiations described recently for several taxa and cases. Contrasting lacustrine and riverine settings, I will discuss these issues for limnic Cerithioidean gastropods, in particular for (1) paludomids from East Africa (i.e., the thalassoid species from Lake Tanganyika and Potadomoides Leloup, 1953 in the Congo River system), (2) pachychilids Tylomelania Sarasin and Sarasin, 1897 endemic to lakes on Sulawesi in Indonesia, (3) Brotia H. Adams, 1866 in the Kaek River drainage in Thailand, and (4) Madagasikara Köhler and Glaubrecht, 2010 on Madagascar. With an increasing armamentarium of molecular genetic techniques for exploring the genetic structure of populations and species, the one mystery has become many, and possible solutions multiplied, as we uncover further complexities in what species really are and how they multiply.