Patterns of Morphological Covariation in Partula

Abstract

?Patterns of morphological covariation were quantified with factor analysis for 32 populations representing the seven closely related species of Partula snails from the island of Moorea, French Polynesia. Variation in the patterns of covariation shows little connection with the taxonomic hierarchy in this group and does not reflect the reproductive relationships of these species. Nevertheless, there are associations of patterns of covariation with morphologically and geographically recognized subspecies in the two most diverse species, P. taeniata and P. suturalis. Among species, there is an association between the similarity of morphological co? variation and genetic similarity, as measured by allozymes. Morphological covariation in Partula on Moorea provides no special information for distinguishing taxa and appears to have evolved gradually, without sudden or discontinuous change, and independently of speciation. [Covari? ation; morphometrics; gastropods; Partula.] Controversy over possible mechanisms underlying evolutionary stasis has fueled recent interest in morphological covaria? tion (e.g., Lande, 1979, 1986; Maynard Smith et al., 1985). Insofar as they are man? ifestations of basic developmental rules of form, patterns of covariation within pop? ulations might represent constraints on the evolution of individual traits because of restriction of the possible range of varia? tion available for natural selection and the provision of likely directions of evolution? ary change (Gould, 1989). The implication is that patterns of covariation are more fun? damentally important than variation of the component traits because the integration of separate characters may guide the course of evolution of those traits (Blackith, 1960; Sokal, 1962, 1978), Gould and his col? leagues have dealt extensively with this topic in studies of the systematics of the morphologically diverse land snails of the genus Cerion (e.g., Gould et al.,1974; Wood? ruff and Gould, 1980; Gould and Woodruff, 1987). This work has shown the value of studies of covaration for understanding the rules of form underlying morphological diversity (e.g., Gould, 1984a, 1989; Gould et al., 1985) and has resulted in substantial reduction in the number of taxa in Cerion. The basic argument for this reduction is that similar patterns of covariation reveal common patterns of growth that through consistent allometries may underlie large differences in individual traits; thus, su? perficial differences in morphology are no basis for the description of new species (Gould et al., 1974). The possibly fundamental role of co? variation raises important questions about the evolution of patterns of covariation. Do these patterns represent something fun? damental that unifies a species? Do they reflect patterns of reproductive isolation? Are evolutionary changes in the patterns of covariation discontinuous or gradual? Are such changes associated with other in? dicators of divergence? From the perspec? tive of systematics, are these changes more informative than the component morpho? logical traits? Land snails of the genus Partula offer ex? cellent opportunities to examine these questions in the context of genetic diver? gence and speciation. This morphological? ly diverse genus is endemic to islands in the Pacific Ocean (Crampton, 1916, 1925, 1932; Kondo, 1968; Cowie, 1992). The ge? nus possesses characteristics that make it, like Cerion, a biometrician's dream (Gould and Woodruff, 1986): (1) the shell pre1 Present address: Environmental Protection Agen? cy, 401 M Street, SW, Washington, D.C. 20460, USA. 2 To whom correspondence should be addressed.