Premise: The study of phenotypic divergence of, and selection on, functional traits in

closely related taxa provides the opportunity to detect the role of natural selection in

driving diversification. If the strength or direction of selection in field populations

differs between taxa in a pattern that is consistent with the phenotypic difference

between them, then natural selection reinforces the divergence. Few studies have

sought evidence for such concordance for physiological traits.

Methods: Herbarium specimen records were used to detect phenological differences

between sister taxa independent of the effects on flowering time of long‐term

variation in the climate across collection sites. In the field, physiological divergence in

photosynthetic rate, transpiration rate, and instantaneous water‐use efficiency were

recorded during vegetative growth and flowering in 13 field populations of two taxon

pairs of Clarkia, each comprising a self‐pollinating and a outcrossing taxon.

Results: Historically, each selfing taxon flowered earlier than its outcrossing sister

taxon, independent of the effects of local long‐term climatic conditions. Sister taxa

differed in all focal traits, but the degree and (in one case) the direction of divergence

depended on life stage. In general, self‐pollinating taxa had higher gas exchange rates,

consistent with their earlier maturation. In 6 of 18 comparisons, patterns of selection

were concordant with the phenotypic divergence (or lack thereof) between sister taxa.

Conclusions: Patterns of selection on physiological traits measured in heterogeneous

conditions do not reliably reflect divergence between sister taxa, underscoring the

need for replicated studies of the direction of selection within and among taxa.