This research examines the genetic architecture of several demographic traits and underlying physiological mechanisms in the model systems Eriogonum abertiamum, a desert annual. The work is designed to further understanding of the genetic basis for life history adaptation and of apparent evolutionary "constraints." More Sonoran than Chihuahuan Desert Eriogonums begin flowering in spring, but populations in both regions retain the ancestral character of biseasonal reproduction (spring and summer). Demographic and genetic studies suggest that natural selection may explain earlier Sonoran flowering. However, retention of Biseasonality is not consistent with adaptive explanations in Sonoran populations: Expected summer fecundity does not outweigh the risk of between-season mortality. Fossil, distributional, and biogeographic evidence suggest the same conclusion. Correlations among life history traits may block selection for increased spring fecundity, as suggested by preliminary field and greenhouse studies. Selection experiments are proposed to determine a) whether heritabilities and genetic correlations among demographic traits are significantly different from zero; and b) whether selection on the correlations themselves can change their signs, i.e., whether the correlations can act as constraints. Sib analyses of growth and allocation traits--which underlie demographic traits--will examine genetic variation for these traits both within and between populations.
