Variation of shell shape in the clonal snail Melanoides tuberculata and its consequences for the interpretation of fossil series

Interpreting paleontological data is difficult because the genetic nature o f observed morphological variation is generally unknown. Indeed, it is hard ly possible to distinguish among several sources of morphological variation including phenotypic plasticity, sexual dimorphism, within-species genetic variation or differences among species. This can be addressed using fossil organisms with recent representatives. The freshwater snail Melanoides tub erculata ranks in this category. A fossil series of this and other species have been studied in the Turkana Basin (Kenya) and is presented as one of t he best examples illustrating the punctuated pattern of evolution by the te nants of this theory. Melanoides tuberculata today occupies most of the tro pics. We studied variation of shell shape in natural populations of this pa rthenogenetic snail using Raup's model of shell coiling. We considered diff erent sources of variation on estimates of three relevant parameters of Rau p's model: (1) variation in shell shape was detected among clones, and had both genetic and environmental, bases; (2) sexual dimorphism, in those clon es in which males occur, appeared as an additional source of shell variatio n; and (3) ecophenotypic variation was detected by comparing samples from d ifferent sites and years within two clones. We then tested the performance of discriminant function analyses, a classical tool in paleontological stud ies, using several datasets. Although the three sources of variation cited above contributed significantly to the observed morphological variance, the y could not be detected without a priori knowledge of the biological entiti es studied. However, it was possible to distinguish between M, tuberculata and a related thiarid species using these analyses. Overall, this suggests that the tools classically used in paleontological studies are poorly effic ient when distinguishing between important sources of within-species variat ion. Our study also gives some empirical bases to the doubts cast on the in terpretation of the molluscan series of the Turkana Basin.