To date, most herbarium-based studies of phenological sensitivity to climate and of climate-driven

phenological shifts fall into two categories: detailed species-specific studies vs. multi-species investigations

designed to explain inter-specific variation in sensitivity to climate and/or the magnitude and direction of their

long-term phenological shifts. Few herbarium-based studies, however, have compared the phenological

responses of closely related taxa to detect: (1) phenological divergence, which may result from selection for the

avoidance of heterospecific pollen transfer or competition for pollinators, or (2) phenological similarity, which

may result from phylogenetic niche conservatism, parallel or convergent adaptive evolution, or genetic

constraints that prevent divergence. Here, we compare two widespread Clarkia species in California with

respect to: the climates that they occupy; mean flowering date, controlling for local climate; the degree and

direction of climate change to which they have been exposed over the last ~115 yr; the sensitivity of flowering

date to inter-annual and to long-term mean maximum spring temperature and annual precipitation across

their ranges; and their phenological change over time. Specimens of C. cylindrica were sampled from sites that

were chronically cooler and drier than those of C. unguiculata, although their climate envelopes broadly

overlapped. Clarkia cylindrica flowers ~ 3.5 d earlier than C. unguiculata when controlling for the effects of

local climatic conditions and for quantitative variation in the phenological status of specimens. However, the

congeners did not differ in their sensitivities to the climatic variables examined here; cumulative annual

precipitation delayed flowering and higher spring temperatures advanced flowering. In spite of significant

spring warming over the sampling period, neither species exhibited a long-term phenological shift.

Precipitation and spring temperature interacted to influence flowering date: the advancing effect on

flowering date of high spring temperatures was greater in dry than in mesic regions, and the delaying effect of

high precipitation was greater in warm than in cool regions. The similarities between these species in their

phenological sensitivity and behavior are consistent with the interpretation that facilitation by pollinators

and/or shared environmental conditions generate similar patterns of selection, or that limited genetic variation

in flowering time prevents evolutionary divergence between these species.