Sympatric Convergence and Environmental Correlation between Two Land-Snail Species

Abstract

In the southern Appalachian region of North America, the phylogenetically convergent shells of the polygyrid snails Triodopsinae Neohelix major (Binney) and Polygyrinae Mesodon normalis (Pilsbry) are even more convergent in size and shape in sympatry (7 sites) than in allopatry (23 and 10 sites). Environmental correlations account for 34% and 30% of size and shape variations in N. major (larger, taller, and more loosely coiled at northern, high-altitude, sheltered sites), but for only 14% and 9% in M. normalis (larger, flatter, and more loosely coiled at south-facing, exposed sites). The statistical significance of the sympatric convergence dropped out when these correlations were removed. This phenomenon helps account for the many cases in eastern North America of nearly identical land-snail shells in sympatry and questions the importance of competitive character displacement in the evolution of land-snail shell morphology. This apparently nonmimetic case of sympatric convergence provides an unusually precise and well-delimited, naturally replicated experiment in evolutionary morphology, which is analyzed for controlling factors in a follow-up paper.

Patterns of diversity and habitat relationships in terrestrial mollusc communities of the Pukeamaru Ecological District, northeastern New Zealand

“…The shell is particularly useful in investigations of evolution and ecology, in being species‐specific yet malleable in both form and size by environment (e.g. Goodfriend, 1986; Emberton, 1994; 1995a; Chiba, 1996) within measurable developmental constraints (e.g. Gould, 1992), and with the individuals’ entire ontogeny conserved and on display in the shell of the mature animal.…”

Morphology of the shell of Happiella cf. insularis (Gastropoda: Heterobranchia: Systrophiidae) from three forest areas on Ilha Grande, Southeast Brazil

“…This translates into a deeper leaf litter layer, a more closed canopy, and lower temperature ( Table I). As in other instances (SHIMEK 1930, BOYCOTT 1934, CAIN 1977, CLARKE et al 1978, TILLIER 1981, EMBERTON 1995b, COOK 1997, WELTER-SCHULTES 2000, TESHIMA et al 2003, better conservation certainly influences the environmental conditions overall, including leaf litter quality, structure, humidity, and depth, which in turn influence mollusk morphology (GOULD 1968, PEAKE 1978, CIPRIANI 2007 In the present study, the smallest mean values were obtained for specimens collected from the Parnaioca Trail area (Table II), the most disturbed of the three areas (shallower leaf litter, more open canopy, higher temperature). The lower capacity of the local leaf litter to retain water is likely responsible for the smaller size of the snails.…”

Assessing the conservation status of the land snail Oreohelix peripherica wasatchensis (Family Oreohelicidae)

“…It is not surprising that we find two snail lineages living in the same drainage system under the same environmental conditions converging on a similar shell morphological ''type.'' Convergence in shell morphology has been found in a number of snail genera (Boulding 1990;Emberton 1995b) and has been a repeated observation within Oreohelix (Pilsbry 1916;Henderson 1924). Pilsbry (1916) noted that Oreohelix is a difficult genus with which to work because of the multiplicity of forms and the parallelism of shell characters between species or races not directly related to one another.…”