S. Samadi et al., Variation of shell shape in the clonal snail Melanoides tuberculata and its consequences for the interpretation of fossil series, EVOLUTION, 54(2), 2000, pp. 492-502
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.