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DEVELOPMENTAL BUFFERING AS A MECHANISM FOR STASIS: EVIDENCE FROM THE PULMONATE THEBA PISANA

Morphological stasis, the absence of significant directional evolution within a species over geologic time, is a phenomenon that now seems generally accepted and in need of an explanation (Lande, 1986; Williamson, 1987). Many proposed mechanisms for stasis and the related phenomenon of geographic species integration (Van Valen, 1982) have been categorized as either selective factors or constraints (Williamson, 1987). As Williamson (1987) points out, some confusion can be avoided in the selection/constraint discussion if one considers that these two types of mechanism are not mutually exclusive and need not oppose each other. Stasis need not result from selection or constraint, but rather from some combination of these factors. Any process that causes populations to shift toward phenotypic modality through a contraction in variability during a generation may contribute to stasis by repeated action over many generations. Stabilizing selection, one such process that acts by truncating the tails of the phenotypic distribution, has been demonstrated in a variety of organisms (Mayr, 1963 pp. 282283). Stabilizing selection is typically inferred when the variability of surviving adults is less than the variability of all juveniles, including those that die before adulthood. However, if reduced variability is observed when adults are traced back through ontogeny so that they are compared to what they themselves were like as juveniles, then developmental buffering can be inferred. Here, we show that a contraction in variability, and therefore a shift toward phenotypic modality, occurs within successful subsets of populations of the pulmonate land snail Theba pisana (Muller). It is stressed that this change in variability is observed when only the individuals that survive to adulthood are considered. All adult snails in this study were collected live, and therefore, no individuals that died before adulthood were considered. Because the observed changes in phenotypic distribution do not involve any mortality, the changes are, by definition, nonselective. It is not argued that selection in general is unimportant in maintaining phenotypic modality in