SEED MORPHOMETRICS AND ADAPTIVE GEOGRAPHIC DIFFERENTIATION

Adaptive geographic differentiation is documented for seed morphology of 36 populations of Heterosperma pinnatum Cav. (Asteraceae), a seed h eteromorphic annual plant in the central highlands of Mexico. Achenes (single-seeded fruits) vary continuously within heads but are classifi ed by shape and position as central, intermediate, or peripheral morph s and as having adhesive awns or not. Here we quantify shape as a prin cipal component score contrasting log length and width of achenes. Hea ds and their variation among populations are described in terms of max imum, minimum, and range of shape scores; the number of achenes per he ad; quantitative indices of the abruptness of shape shift; where in th e head the most abrupt change in shape occurs; and what achene shapes have awns. First and second principal components of these descriptors summarize 86% of among-population variation in achene and head morphol ogy and correlate strongly with percent central achenes per head (%C) and percent of achenes with awns (%A), respectively. Awns are associat ed with greater dispersibility and achene shape is correlated with spe ed of dormancy loss. We hypothesized that dispersal morphology would b e associated with vegetation attributes indicative of population ephem erality and that dormancy morphology would be associated with precipit ation patterns during the early germination season. Morphological dist ance matrices were calculated using Euclidean distances among populati ons in %A and %C. Geographic distances among populations were calculat ed, as were genetic distances based on isozyme frequencies from 29 ban ds of six enzymes. Vegetation was classified as open or closed and ear ly spring (germination season) and summer precipitation means were det ermined for each site. Closed vegetation was assumed to provide only e phemeral habitats for H. pinnatum. Partial matrix correlations between morphology and environment controlled for geographic but not genetic distance among sites, since the latter was not significantly correlate d with either morphology or geography. A significant relationship was found between %A and closed vegetation, lower spring, and higher summe r precipitation. %C was only correlated with lower spring precipitatio n. Independence of isozyme and morphological traits is interpreted in terms of selection on the latter but not the former.