Competition for pollination influences selection on floral traits of Ipomopsis aggregata

Although rarely tested, it is often assumed that interspecific competition results in the divergence of traits related to resource use. Using a plant-pollinator system as a model, I tested the prediction the presence of a competitor for pollination influences the strength and/or direction of pollinator-mediated selection on floral traits. I measured phenotypic selection via female fitness on five floral traits of Ipomopsis aggregata in seven populations. Four contained only conspecifics (I only) and three also contained the competitor Castilleja linariaefolia (C + I). Directional selection via fruits/plant and conspecific pollen deposited/flower on corolla length was positive and significantly stronger in C + I populations. This difference in selection was apparently driven by interpopulation variation in the degree to which reproduction of I. aggregata was pollen limited. Consistent with expectations of interspecific competition, I. aggregata plants in C + I populations received less conspecific pollen per flower and set fewer seeds per fruit and fruits per plant than those in I only populations. Ipomopsis aggregata's corollas were also significantly longer in C + I populations, suggesting that there had been a response to a similar selective regime in past generations. Phenotypic correlations between corolla length and width, which determine the variation in I. aggregata's flower shape, were significantly weaker in C + I populations. These data suggest that competition for pollination can influence the strength of selection on and patterns of correlations among floral traits of I. aggregata. If I. aggregata populations with and without competitors for pollination are linked by gene flow, then measuring selection in competitive and noncompetitive environments maybe necessary to accurately predict how floral traits will evolve.     

The plot thickens: does low density affect visitation and reproductive success in a perennial herb, and are these effects altered in the presence of a co-flowering species?

Plants may experience reduced reproductive success at low densities, due to lower numbers of pollinator visits or reduced visit quality. Co-occurring plant species that share pollinators have the potential to facilitate pollination by either increasing numbers of pollinator visits or increasing the quality of visits, but also have the potential to reduce plant reproductive success through competition for pollination. I used a field experiment with a common distylous perennial (Piriqueta caroliniana) in the presence and absence of a co-flowering species (Coreopsis leavenworthii) in plots with one of four different distances between conspecific plants. I found strong negative effects of increasing interplant distance (related to conspecific density) on several components of P. caroliniana reproductive success: pollinator visits to plants per plot visit, visits received by individual plants, conspecific pollen grains on stigmas, outcross pollen grains on stigmas, and probability of fruit production. Although P. caroliniana and C. leavenworthii share pollinators, the co-flowering species did not affect visitation, pollen receipt or reproductive effort in P. caroliniana. Pollinators moved very infrequently between species in this experiment, so floral constancy might explain the lack of effect of the co-flowering species on P. caroliniana reproductive success at low densities. In co-occurring self-incompatible plants with floral rewards, reproductive success at low density may depend more on conspecific densities than on the presence of other species.     

Self‐ and intra‐morph incompatibility and selection analysis of an inconspicuous distylous herb growing on the Tibetan plateau (Primula tibetica)

There is discussion over whether pollen limitation exerts selection on floral traits to increase floral display or selects for traits that promote autonomous self‐fertilization. Some studies have indicated that pollen limitation does not mediate selection on traits associated with either pollinator attraction or self‐fertilization. Primula tibetica is an inconspicuous cross‐fertilized plant that may suffer from pollen limitation. We conducted a selection analysis on P. tibetica to investigate whether pollen limitation results in selection for an increased floral display in case the evolution of autonomous self‐fertilization has been difficult for this plant. The self‐ and intra‐morph incompatibility features, the capacity for autonomous self‐fertilization, and the magnitude of pollen limitation were examined through hand‐pollination experiments. In 2016, we applied selection analysis on the flowering time, corolla width, stalk height, flower tube length, and flower number in P. tibetica by tagging 76 open‐pollinated plants and 37 hand‐pollinated plants in the field. Our results demonstrated that P. tibetica was strictly self‐ and intra‐morph incompatible. Moreover, the study population underwent severe pollen limitation during the 2016 flowering season. The selection gradients were found to be significantly positive for flowering time, flower number, and corolla width, and marginally significant for the stalk height. Pollinator‐mediated selection was found to be significant on the flower number and corolla width, and marginally significant on stalk height. Our results indicate that the increased floral display may be a vital strategy for small distylous species that have faced difficulty in evolving autonomous self‐fertilization.     

Pollinator‐mediated selection in a specialized pollination system: matches and mismatches across populations

Most studies on pollinator‐mediated selection have been performed in generalized rather than specialized pollination systems. This situation has impeded evaluation of the extent to which selection acts on attraction or specialized key floral traits involved in the plant‐pollinator phenotypic interphase. We studied pollinator‐mediated selection in four populations of Nierembergia linariifolia, a self‐incompatible and oil‐secreting plant pollinated exclusively by oil‐collecting bees. We evaluated whether floral traits experience variable selection among populations and whether attraction and fit traits are heterogeneously selected across populations. Populations differed in every flower trait and selection was consistently observed for corolla size and flower shape, two traits involved in the first steps of the pollination process. However, we found no selection acting on mechanical‐fit traits. The observation that selection occurred upon attraction rather than mechanical‐fit traits, suggests that plants are not currently evolving fine‐tuned morphological adaptations to local pollinators and that phenotypic matching is not necessarily an expected outcome in this specialized pollination system.     

Linking pollinator visitation rate and pollen receipt

The majority of flowering plants require animals for pollination, a critical ecosystem service in natural and agricultural systems. However, quantifying useful estimates of pollinator visitation rates can be nearly impossible when pollinator visitation is infrequent. We examined the utility of an indirect measure of pollinator visitation, namely pollen receipt by flowers, using the hummingbird-pollinated plant, Ipomopsis aggregata (Polemoniaceae). Our a priori hypothesis was that increased pollinator visitation should result in increased pollen receipt by stigmas. However, the relationship between pollinator visitation rate and pollen receipt may be misleading if pollen receipt is a function of both the number of pollinator visits and variation in pollinator efficiency at depositing pollen, especially in the context of variable floral morphology. Therefore, we measured floral and plant characters known to be important to pollinator visitation and/or pollen receipt in I. aggregata (corolla length and width and plant height) and used path analysis to dissect and compare the effect of pollinator visitation rate vs. pollinator efficiency on pollen receipt. Of the characters we measured, pollinator visitation rate (number of times plants were visited multiplied by the mean percentage of flowers probed per visit) had the strongest direct positive effect on pollen receipt, explaining 36% of the variation in pollen receipt. Plant height had a direct positive effect on pollinator visitation rate and an indirect positive effect on pollen receipt. Despite the supposition that floral characters would directly affect pollen receipt as a result of changes in pollinator efficiency, corolla length and width only weakly affected pollen receipt. These results suggest a direct positive link between pollinator visitation rate and pollen receipt across naturally varying floral morphology in I. aggregata. Understanding the relationship between pollinator visitation rate and pollen receipt may be of critical importance in systems where pollinator visitation is difficult to quantify.     

Spatially inconsistent direct and indirect effects of herbivory on floral traits and pollination success in a tropical shrub

Investigations on plant–animal interactions have traditionally focused on single interactions at a time (e.g. herbivory, pollination), yet plant fitness is generally influenced in complex ways by several interactions operating concurrently, and very little is known on the degree of spatial consistency of the direct and indirect effects that link different interactions. This paper evaluates experimentally whether direct and indirect effects of herbivory on male and female flower size and pollination success of the monoecious tropical shrub Cnidoscolus aconitifolius (Euphorbiaceae) remain consistent at three distant regions in Yucatan (southeastern Mexico). Plants were subjected to different levels of leaf defoliation, and treatment effects on floral traits (corolla area, corolla tube length, pollen production), and male and female components of pollination success (percent pollen removal, number of pollen tubes) were subsequently measured to evaluate the indirect effect of herbivory on plant reproductive success via pollination. Defoliation had significant direct effects on floral traits, but its indirect effects differed between the male and female components of pollination success. Moreover, the relationships between defoliation, floral traits and male and female pollination success varied spatially (i.e. between regions), although they were frequently more spatially‐consistent for male success than for female success. Results from this study stress the importance of explicitly testing for spatial variation in direct and indirect effects arising from plant‐animal interactions.     

Effects of conspecific and heterospecific floral density on the pollination of two related rewarding orchids

Variation in within-population floral density can affect interactions between plants and pollinators, resulting in variable pollen export for plants. We investigated the effects of conspecific and heterospecific floral densities on pollination success both of two related, self-compatible, nectar-rewarding orchid species in Ireland, Spiranthes romanzoffiana (rare and listed as endangered) and its congener, S. spiralis (more abundant and not of conservation concern). Floral densities, insect visitation rates, and orchid pollen transport were recorded in multiple quadrats in four populations of both orchid species over their flowering season. We found that conspecific and heterospecific co-flowering plant density affected pollination in both orchid species. For S. romanzoffiana, higher heterospecific density increased pollen removal. For S. spiralis, higher conspecific visitation increased pollen removal and increased heterospecific density decreased pollen deposition. In addition, increased conspecific density increased pollen deposition in both species. This study shows that plants may interact to facilitate or compete for different components of the pollination process, namely; pollinator attraction, pollen removal and deposition. Such interactions have immediate consequences for endangered plant species, as increases in both conspecific and heterospecific coflowering density may ameliorate the negative effects of rarity on pollination, hence overall reproductive success.     

'Anti-bee' and 'pro-bird' changes during the evolution of hummingbird pollination in Penstemon flowers

Floral phenotypes may be as much the result of selection for avoidance of some animal visitors as selection for improving the interaction with better pollinators. When specializing on hummingbird-pollination, Penstemon flowers may have evolved to improve the morphological fit between bird and flower, or to exclude less-efficient bees, or both. We hypothesized how such selection might work on four floral characters that affect the mechanics of pollen transfer: anther/stigma exsertion, presence of a lower corolla lip, width of the corolla tube, and angle of flower inclination. We surgically modified bee-pollinated P. strictus flowers changing one trait at a time to make them resemble hummingbird-pollinated P. barbatus flowers, and measured pollen transfer by bumblebees and hummingbirds. Results suggest that, apart from 'pro-bird' adaptations, specific 'anti-bee' adaptations have been important in shaping hummingbird-flowers. Moreover, some trait changes may have been selected for only if changing in concert with other traits.     

Phenotypic selection on flowering phenology and pollination efficiency traits between Primula populations with different pollinator assemblages

Floral traits have largely been attributed to phenotypic selection in plant–pollinator interactions. However, the strength of this link has rarely been ascertained with real pollinators. We conducted pollinator observations and estimated selection through female fitness on flowering phenology and floral traits between two Primula secundiflora populations. We quantified pollinator‐mediated selection by subtracting estimates of selection gradients of plants receiving supplemental hand pollination from those of plants receiving open pollination. There was net directional selection for an earlier flowering start date at populations where the dominant pollinators were syrphid flies, and flowering phenology was also subjected to stabilized quadratic selection. However, a later flowering start date was significantly selected at populations where the dominant pollinators were legitimate (normal pollination through the corolla tube entrance) and illegitimate bumblebees (abnormal pollination through nectar robbing hole which located at the corolla tube), and flowering phenology was subjected to disruptive quadratic selection. Wider corolla tube entrance diameter was selected at both populations. Furthermore, the strength of net directional selection on flowering start date and corolla tube entrance diameter was stronger at the population where the dominant pollinators were syrphid flies. Pollinator‐mediated selection explained most of the between‐population variations in the net directional selection on flowering phenology and corolla tube entrance diameter. Our results suggested the important influence of pollinator‐mediated selection on floral evolution. Variations in pollinator assemblages not only resulted in variation in the direction of selection but also the strength of selection on floral traits.     

Sex‐specific phenotypic selection and geographic variation in gender divergence in a gynodioecious shrub

In sexually polymorphic plant species the extent of gender divergence in floral morphology and phenology may be influenced by gender‐specific selection patterns imposed by pollinators, which may change geographically. Distribution margins are areas where changes in the pollinator fauna, and thus variation in gender divergence of floral traits, are expected. We tested for pollination‐driven geographic variation in the gender divergence in floral and phenological traits in the gynodioecious shrub Daphne laureola, in core and marginal areas differing in the identity of the main pollinator. Pollinators selected for longer corolla tubes in hermaphrodite individuals only in core populations, which in turn recorded higher fruit set. Consistent with these phenotypic selection patterns, gender divergence in flower corolla length was higher in core populations. Moreover, pollinators selected towards delayed flowering on hermaphrodite individuals only in marginal populations, where the two sexes differed more in flowering time. Our results support that a shift in main pollinators is able to contribute to geographic variation in the gender divergence of sexually polymorphic plant species.     

Pollen limitation,species’ floral traits and pollinator visitation: different relationships in contrasting communities

Failures in the process of pollen transfer among conspecific plants can severely impact female reproductive success. Thus, pollen limitation can cause selection on plant mating systems and floral traits. The relationships between pollen limitation and floral traits might be partly mediated by the quantity and identity of pollinator visits. However, very little is known about the relationship between pollinator visits and pollen limitation. We examined the relationships between pollen limitation and floral traits at the community level to connect them to community ecology processes. We used 48 plant species from two contrasting communities: one species‐rich lowland community and one species‐poor alpine community. In addition, we calculated visitation rates and ecological pollination generalization for 38 of the species to examine the relationship between pollinator visitation and pollen limitation at the community level. We found low overall levels of pollen limitation that did not differ significantly between the alpine and the lowland community. In both communities, species with evolutionary specialized flowers were more pollen limited than species with unspecialized flowers. Species’ visitation rates and selfing capability were negatively related to pollen limitation in the alpine community, where pollinators are scarcer. However, flower size/number, ecological generalization of plants and flowering onset had greater effects on pollen limitation levels at the lowland community, indicating that the identity of the visitors and plant‐plant competitive interactions are more decisive for plant reproduction in this species‐rich community. There, pollen limitation increased with flower size and flowering onset, and decreased with ecological generalization, but only in species with evolutionary specialized flowers. Our study suggests that selection on plant mating system and floral traits may be idiosyncratic to each particular community and highlights the benefits of conducting community‐level studies for a better understanding of the processes underlying evolutionary responses to pollen limitation.     

Patterns of contemporary phenotypic selection and flower integration in the hummingbird‐pollinated Nicotiana glauca between populations with different flower–pollinator combinations

We studied six populations of the hummingbird‐pollinated Nicotiana glauca to determine if the marked differences in the degree of floral‐pollinator mismatch between populations promote divergences in the pattern of pollinator‐mediated phenotypic selection on single traits and on the evolution of complexes of many interacting floral traits. We found evidence that flower phenotype is being shaped by pollinator‐mediated phenotypic selection, since corolla length was consistently under contemporary directional or stabilizing selection. Weak directional selection for longer corollas was found in two populations with low flower–pollinator mismatch; much stronger directional selection was detected for shorter corollas in two populations with high flower–pollinator mismatch; finally, the remaining two populations with intermediate flower–pollinator mismatch showed stabilizing selection for corolla length. N. glauca populations differed in every flower character measured but variations in pollinator‐mediated selection among populations were only observed for corolla length. Multiple covariation among traits was favoured, as suggested by the predominately functional patterns of integration and selection of complexes of many interacting floral traits. This was consistent with the patterns of correlational selection exhibited by four of the six populations, where corolla length was under significant selection in combination with corolla width, style length or stamen length. Overall floral integration was relatively high in all populations but phenotypic integration patterns were not clearly accounted by the degree of flower–pollinator mismatch or type of phenotypic selection, suggesting that trait covariation at the entire flower level is not explained by the current scenario of pollinator‐mediated selection.     

The relative strength of different floral visitors driving floral evolution within a <Emphasis Type="Italic">Primula secundiflora</Emphasis> population

Floral visitor assemblages within plant populations are usually composed of different visitors, and the relative abundance of these visitors also varies. Therefore, identifying the relative strength of these floral visitors driving floral evolution within the population is an important step in predicting the evolutionary trajectory of floral traits. Using supplemental hand pollination and nectar-robbing exclusion treatments, we experimentally identified the relative strengths of legitimate pollinators (that visit flowers through the corolla tube entrance) and nectar robbers (that visit flowers by biting a hole in the corolla tube or using an existing hole) driving floral evolution within the Primula secundiflora population. We also estimated legitimate pollinator- and nectar robber-mediated selection separately for pin and thrum flowers. Both legitimate pollinators and nectar robbers mediated selection on pollination efficiency traits in P. secundiflora population. Legitimate pollinators mediated selection for wider corolla tubes, whereas nectar robbers mediated selection for longer corolla tubes. In addition, nectar robber-mediated selection on corolla tube length marginally varied between the pin and thrum flowers. Nectar robber mediated selection for longer corolla tube length in the pin flowers not in the thrum flowers. These results indicate that legitimate pollinators and nectar robbers within a population can drive differential evolutionary trajectories of floral traits.     

Direct and indirect effects of pollinators and seed predators to selection on plant and floral traits

Although flowering traits are often assumed to be under strong selection by pollinators, significant variation in such traits remains the norm for most plant species. Thus, it is likely that the interactions among plants, mutualists, and other selective agents, such as antagonists, ultimately shape the evolution of floral and flowering traits. We examined the importance of pollination vs pre-dispersal seed predation to selection on plant and floral characters via female plant-reproductive success in Castilleja linariaefolia (Scrophulariaceae). C. linariaefolia is pollinated by hummingbirds and experiences high levels of pre-dispersal seed predation by plume moth and fly larvae in the Rocky Mountains of Colorado, USA, where this work was conducted. We first examined whether female reproduction in C. linariaefolia was limited by pollination. Supplemental pollination only marginally increased components of female reproduction, likely because seed predation masked, in part, the beneficial effects of pollen addition. In unmanipulated populations, we measured calyx length, flower production, and plant height and used path analysis combined with structural equation modeling to quantify their importance to relative seed set through pathways involving pollination vs seed predation. We found that the strength of selection on calyx length, flower production, and plant height was greater for seed predation pathways than for pollination pathways, and one character, calyx length, experienced opposing selection via pollination vs seed predation. These results suggest that the remarkable intraspecific variation in plant and floral characters exhibited by some flowering plants is likely the result of selection driven, at least in part, by pollinators in concert with antagonists, such as pre-dispersal seed predators. This work highlights the subtle but complex interactions that shape floral and vegetative design in natural ecosystems.     

Does the surrounding matrix influence corridor effectiveness for pollen dispersal in farmland?

Recent decades have seen a shift in agricultural land use from pasture to arable combined with increased use of fertilisers and pesticides. In this quite hostile landscape matrix, pollinator movements between native vegetation remnants may be impeded. Linear landscape elements (LLEs) in farmland can function as biological corridors by facilitating pollinator movements and pollen flow between fragmented plant populations. The type of landscape matrix surrounding LLEs and plant populations, and LLE habitat quality may influence the effectiveness of LLEs as corridors for pollen dispersal through the availability of floral resources and nesting opportunities for pollinators. Using fluorescent dyes as pollen analogues, we investigated dye dispersal patterns between fragmented populations of the rare insect-pollinated Primula vulgaris connected by existing LLEs. We examined how dye deposition on P. vulgaris and within LLEs (on other co-flowering insect-pollinated species) could be influenced by the surrounding matrix type (pasture, arable field), the recipient population traits of P. vulgaris (flowering population size, flower display, flowering plant density and co-flowering floral resources) and by LLE traits (LLE length and co-flowering floral resources). Dye dispersal through corridors was significantly higher when the landscape matrix surrounding P. vulgaris recipient populations and LLEs consisted of pastures rather than arable fields (or a mix of both), even after accounting for differences in floral resources. A higher cover and diversity of insect-pollinated plants increased dye deposition when co-flowering within small P. vulgaris populations, but led to dye loss within LLEs. Large P. vulgaris populations appeared more attractive thereby increasing heterospecific dye deposition in the LLEs. Our study shows that farming practices shifting from pastures to maize arable fields have a negative impact on dye dispersal patterns, and so possibly affect pollen dispersal of P. vulgaris, likely through a reduced pollinator service. Corridor effectiveness depends on the type of surrounding matrix, and the abundance of floral resources directly influences dye dispersal patterns. Preserving the remaining pastureland is essential, but restoring rich flowering vegetation at field boundaries and along LLEs may also promote corridor effectiveness for pollen dispersal.     

Floral stability,pollination efficiency,and experimental manipulation of the corolla phenotype in Nemophila menziesii (Hydrophyllaceae)

The present study examines the degree of phenotypic stability in vegetative and floral traits in a population of the annual, self-compatible, and protandrous Nemophila menziesii (Hydrophyllaceae) and examines whether local pollinators select for a uniform floral phenotype in this population. I found a strong positive association between flower number, leaf size, shoot length, and pedicel length, implying a general effect of overall plant vigor on this suite of traits, while characters reflecting corolla shape were phenotypically stable across a broad range of plant sizes. However, results of a manipulative experiment indicate that individual flowers maintain high pollination success despite drastic changes in the size and shape of the corolla. This suggests that pollinator-mediated selection does not directly favor floral stability in N. menziesii or that selection was weak during the course of this study. Seed number per flower was positively correlated with plant size, which varied greatly from one patch to another.     

Pollen:ovule ratio and its relationship with other floral traits in Papilionoideae (Leguminosae): an evaluation with Argentine species

The pollen:ovule ratio (P/O) has traditionally been used as a rough estimator of plant breeding systems. It has been shown that plant breeding systems are associated with particular floral traits. In this study, we determined the P/O in 21 Leguminosae species from Argentina and explored relationships between P/O and taxonomic position, flower size, floral rewards, pollen presentation and pollination mechanisms. According to the results, 15 out of the 21 species classified were obligate xenogamous, although some of them have been recorded as facultative xenogamous in previous studies. There was a significant effect of taxonomic position (genus), reward type and pollination mechanism on P/O. Species offering only nectar as a floral reward (which were species with a brush mechanism) had a significantly lower P/O than species offering pollen or pollen and nectar. Species with the brush pollination mechanism had the lowest P/O, while species with valvular and pump mechanism had the highest P/O. However, pollen presentation (primary and secondary) and flower size did not have a significant effect on P/O. Our results demonstrate that P/O variability is determined by taxonomic position and pollination mechanism in this plant group.     

Selection of trait combinations through bee and fly visitation to flowers of Polemonium foliosissimum

Pollinators are known to exert natural selection on floral traits, but the extent to which combinations of floral traits are subject to correlational selection (nonadditive effects of two traits on fitness) is not well understood. Over two years, we used phenotypic manipulations of plant traits to test for effects of flower colour, flower shape and their interaction on rates of pollinator visitation to Polemonium foliosissimum. We also tested for correlational selection based on weighting visitation by the amount of conspecific pollen delivered per visit by each category of insect visitor. Although bumblebees were the presumed pollinators, solitary bees and flies contributed substantially (42%) to pollination. In manipulations of one trait at a time, insects visited flowers presenting the natural colour and shape over flowers manipulated to present artificial mutants with either paler colour or a more open or more tubular flower. When both colour and shape were manipulated in combination, selection on both traits arose, with bumblebees responding mainly to colour and flies responding mainly to shape. Despite selection on both floral traits, in a year with many bumblebees, we saw no evidence for correlational selection of these traits. In a year when flies predominated, fly visitation showed a pattern of correlational selection, but not favouring the natural phenotype, and correlational selection was still not detected for expected pollen receipt. These results show that flower colour and shape are subject to pollinator‐mediated selection and that correlational selection can be generated based on pollinator visitation alone, but provide no evidence for correlational selection specifically for the current phenotype.     

Morph‐specific selection on floral traits in a polymorphic plant

Correlations between phenotypic traits are common in many organisms, but the relative importance of nonadaptive mechanisms and selection for the evolution and maintenance of such correlations are poorly understood. In polymorphic species, morphs may evolve quantitative differences in additional characters as a result of morph‐specific selection. The perennial rosette herb Primula farinosa is polymorphic for scape length. The short‐scaped morph is less damaged by grazers and seed predators but is more strongly pollen limited than the long‐scaped morph. We examined whether morph‐specific differences in biotic interactions are associated with differences in selection on two other traits affecting floral display (number of flowers and petal size) and on one trait likely to affect pollination efficiency (corolla tube width) in three P. farinosa populations. Differences in selection between morphs were detected in one population. In this population, selection for more flowers and larger petals was stronger in the short‐scaped than in the long‐scaped morph, and although there was selection for narrower corolla tubes in the short‐scaped morph, no statistically significant selection on corolla tube width could be detected in the long‐scaped morph. In the study populations, the short‐scaped morph produced more and larger flowers and wider corolla tubes. Current morph‐specific selection was thus only partly consistent with trait differences between morphs. The results provide evidence of morph‐specific selection on traits associated with floral display and pollination efficiency, respectively.     

Phenotypic selection on floral traits in the arctic plant Parrya nudicaulis (Brassicaceae)

The evolution of floral traits is often attributed to pollinator‐mediated selection; however, the importance of pollinators as selective agents in arctic environments is poorly resolved. In arctic and subarctic regions that are thought to be pollen limited, selection is expected to either favor floral traits that increase pollinator attraction or promote reproductive assurance through selfing. We quantified phenotypic selection on floral traits in two arctic and two subarctic populations of the self‐compatible, but largely pollinator‐dependent, Parrya nudicaulis. Additionally, we measured selection in plants in both open pollination and pollen augmentation treatments to estimate selection imposed by pollinators in one population. Seed production was found to be limited by pollen availability and strong directional selection on flower number was observed. We did not detect consistently greater magnitudes of selection on floral traits in the arctic relative to the subarctic populations. Directional selection for more pigmented flowers in one arctic population was observed, however. In some populations, selection on flower color was found to interact with other traits. We did not detect consistently stronger selection gradients across all traits for plants exposed to pollinator selection relative to those in the pollen augmentation treatment; however, directional selection tended to be higher for some floral traits in open‐pollinated plants.