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The Consequences of Being Different: Sinistral Coiling in Cerion

Stephen Jay Gould and Nelson D. Young
Evolution
Vol. 39, No. 6 (Nov., 1985), pp. 1364-1379
DOI: 10.2307/2408792
Stable URL: http://www.jstor.org/stable/2408792
Page Count: 16
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The Consequences of Being Different: Sinistral Coiling in Cerion
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Abstract

The overwhelming predominance of dextral coiling in gastropods is an outstanding and puzzling phenomenon. A few sinistral specimens (left coiling individuals) have been found in many dextral species. Only six sinistral shells have ever been found in Cerion; we base this analysis on the five available shells. We ask whether reversed symmetry is a simple either-or switch without further consequences for shell form, or whether sinistrality engenders associated effects, making left-coiling shells unlike their dextral deme-mates in other ways. All five sinistral shells differ in features of size and coiling late in growth, leading to relatively small apertures and a slight twist in the axis of coiling. We detect and measure this effect as follows: in multivariate morphospace, sinistrals occupy peripheral positions among their dextral deme-mates; in univariate analysis, sinistrals are consistently different for a set of characters involving covariance patterns never before seen in a decade of studies on ontogenetic and age-standardized variation in dextrals; a bootstrap procedure does not recover similar patterns in randomly constituted samples of dextrals matching the true smistral distribution; direct x-ray measures of the coiling axis detect its slight twist in sinistrals. We discuss the implications of these unsuspected associations for the issues of developmental constraint upon the evolution of morphology.

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