Phenotypic plasticity and the selection of taxonomic characters in Puccinellia (Poaceae)

Phenotypic plasticity diminishes a character's taxonomic utility because the environmental contribution to phenotypic variation obscures genetically based relationships. To evaluate potential taxonomic characters of Puccinellia, divided clones representing several putative taxa were grown under a series of saltand drought-stress treatments representing naturally encountered environmental conditions. The resulting variation in 41 characters was partitioned by analysis of variance (ANOVA) into genetic and environmental components. Logarithmic transformation facilitates comparisons of variance levels among characters. Analogous nonparametric tests were used to analyze 40 characters not amenable to ANOVA. Levels of variance among genotypes and among treatments (phenotypic plasticity) are continuous in distribution and highly variable in magnitude. Attributes of the spikelet dominate the list of characters best meeting the criteria of high genotypic variability and low plasticity. Ratios representing shapes and relative sizes tend to be more reliable than their component simple measurements. This analysis empirically confirms the importance of many characters emphasized in grass systematics and suggests characters for Puccinellia that previously have not been recognized as significant. While much recent effort has been directed at the theory and methodology of taxonomic and phylogenetic analysis, the choice of characters to be used in such analyses has received less attention. Yet the suite of characters underlying any systematic study profoundly influences the outcome. Different character sets derived from the same plants, and analyzed identically, may support different conclusions (Denton 1978; Hauser and Crovello 1982). The true relationships of natural groups are never known with certainty. Faced with conflicting evidence, systematists must decide which characters reliably reflect past evolutionary events. This decision process is weighting. Two broad strategies of character weighting can be discerned, generalized correlation and a priori weighting (Cain 1959). Correlational methods weight individual characters by their tendencies to agree with others; weighting proceeds simultaneously with taxonomic analysis. These methods range from informal comparison of vxariation patterns to strictly defined relational criteria such as parsimony and character compatibility (cf. Felsenstein 1981). Systematic schemes based upon correlational methods are often considered unweighted, because results are determined by the aggregate number of equally weighted characters supporting a particular scheme. However, characters identified as reliable in one analysis are often accorded greater weight in related taxa whose character combinations have not been analyzed. In contrast with correlational weighting, a priori methods assign weight to characters by their degrees of conformance to intrinsic criteria of good and bad characters. Some a priori criteria refer to general classes of characters, e.g., high weight to structurally complex organs (Mayr 1969), low weight to monogenic characters (Rollins 1958). Other a priori methods involve expectations of variation patterns to be exhibited by good characters. For example, Farris (1966) argued that good characters will vary little within biological populations. Davis and Heywood (1963, p. 119) stated the a priori criterion that good characters "are not easily susceptible to environmental modification. " In a general sense this principle is widely recognized. Leaf shape in Sagittaria, varying with the depth of water in which a plant grows, is well known to be taxonomically unreliable because it is environmentally modified. In less obvious cases, the environmental contribution to variation has been investigated for small numbers of characters (Jones and Newton 1970; Chapman 1975). In this paper, the environmental component of variation is analyzed for numerous characters of Puccinellia Parl. (Poaceae), in order to identify stable characters of potential taxonomic value.