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

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

42 diversification. When selection in field populations differs between taxa in a pattern that is

43 consistent with the phenotypic difference between them, this provides evidence that natural

44 selection reinforces the divergence. Few studies have sought evidence for such concordance for

45 physiological traits.

46

47 METHODS: Herbarium specimen records were used to detect phenological differences between

48 sister taxa independent of the effects on flowering time of long-term variation in the climate

49 across collection sites. In the field, physiological divergence in photosynthetic rate, transpiration

50 rate, and instantaneous water use efficiency were recorded during vegetative growth and

51 flowering in 13 field populations of two taxon pairs of Clarkia, each comprised of a self52

pollinating and a outcrossing taxon.

53

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

55 independent of the effects of local long-term climatic conditions. Sister taxa differed in all focal

56 traits, but the degree and (in one case) the direction of divergence depended on life stage. In

57 general, self-pollinating taxa exhibited higher gas exchange rates, consistent with their earlier

58 maturation. In 6 of 18 comparisons, patterns of selection were concordant with the phenotypic

59 divergence (or lack thereof) between sister taxa.

60

61 CONCLUSIONS: Patterns of selection on physiological traits measured in heterogeneous

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

63 replicated studies of the direction of selection within and among taxa.

64

65 Keywords: adaptation, Clarkia, divergence, gas exchange, mating system evolution, phenology,

66 phenotypic selection, water use efficiency, climate.