Covariation among floral traits in Spergularia marina (Caryophyllaceae): geographic and temporal variation in phenotypic and among-family correlations

Strong covariation among traits suggests the presence of constraints on their independent evolution due to pleiotropy, to linkage, or to selective forces that maintain particular trait combinations. We examined floral trait covariation among individuals, among maternal families within and across populations, and over time, in greenhouse-raised plants of the autogamous Spergularia marina. We had three aims. First, since the phenotype of traits expressed by modular organs often changes as individuals age, estimates of the degree of genetic covariation between such traits may also change over time. To seek evidence for this, we measured weekly (for five weeks) an array of floral traits among plants representing ～ 10 maternal families from each of four populations. The statistical significance of the phenotypic and among-family correlations among traits changed over time. Second, we compared populations with respect to trait covariation to determine whether populations or traits appear to be evolving independently of one another. Differences observed among populations suggest that they have diverged genetically. Third, we sought correlations that might reflect constraints on the independent evolution of floral traits. Investment in another and ovule production per flower vary independently among maternal families; there was no evidence for a “trade-off” between male and female investment. We propose that in autogamous taxa one should not find a negative correlation between pollen and ovule production per flower, as such taxa cannot evolve sexual specialization and should be under strong selection to maintain an efficient pollen:ovule ratio, preventing the evolution of male-biased or female-biased genotypes. We found that other pairs of floral traits, however, expressed highly signficant correlation coefficients, suggesting the presence of some evolutionary constraints, at least within some populations, although their strength depended on exactly when flowers were sampled.     

Riding across the selection landscape: fitness consequences of annual variation in reproductive characteristics

Evolutionary models estimating phenotypic selection in character size usually assume that the character is invariant across reproductive bouts. We show that variation in the size of reproductive traits may be large over multiple events and can influence fitness in organisms where these traits are produced anew each season. With data from populations of two orchid species, Caladenia valida and Tolumnia variegata, we used Bayesian statistics to investigate the effect on the distribution in fitness of individuals when the fitness landscape is not flat and when characters vary across reproductive bouts. Inconsistency in character size across reproductive periods within an individual increases the uncertainty of mean fitness and, consequently, the uncertainty in individual fitness. The trajectory of selection is likely to be muddled as a consequence of variation in morphology of individuals across reproductive bouts. The frequency and amplitude of such changes will certainly affect the dynamics between selection and genetic drift.     

POLLINATOR-MEDIATED SELECTION ON FLORAL DISPLAY AND FLOWERING TIME IN THE PERENNIAL HERB ARABIDOPSIS LYRATA

The evolution of floral display and flowering time in animal-pollinated plants is commonly attributed to pollinator-mediated selection. Yet, the causes of selection on flowering phenology and traits contributing to floral display have rarely been tested experimentally in natural populations. We quantified phenotypic selection on morphological and phenological characters in the perennial, outcrossing herb Arabidopsis lyrata in two years using female reproductive success as a proxy of fitness. To determine whether selection on floral display and flowering phenology can be attributed to interactions with pollinators, selection was quantified both for open-pollinated controls and for plants receiving supplemental hand-pollination. We documented directional selection for many flowers, large petals, late start of flowering, and early end of flowering. Seed output was pollen-limited in both years and supplemental hand-pollination reduced the magnitude of selection on number of flowers, and reversed the direction of selection on end of flowering. The results demonstrate that interactions with pollinators may affect the strength of selection on floral display and the direction of selection on phenology of flowering in natural plant populations. They thus support the contention that pollinators can drive the evolution of both floral display and flowering time.     

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.     

Phenotypic selection in a natural population of Impatiens pallida Nutt. (Balsaminaceae)

In the annual plant Impatiens pallida, individuals exhibit a floral heteromorphism consisting of autogamously selfing, cleistogamous (CL) flowers and partially outcrossing, chasmogamous (CH) flowers. As part of an investigation into natural selection and mating system evolution in I. pallida, we measured the magnitude and direction of phenotypic selection on nine life history characters (two traits measured on three dates, one measured on two dates and one measured once). Three of these characters were positively correlated with the ratio of CH/CL flowers produced per plant, which is an important determinant of the mating system. Values for the nine characters and three different measures of fitness (viability, fecundity, lifetime) were estimated for 500 plants in five locations over a single growing season. Based on lifetime fitness, linear selection differentials were significant for all nine characters, indicating a selective advantage to tall, leafy, highly branched plants that flowered early. However, only two of these characters had a direct effect on fitness. Selection was significant on all nine characters when based on fecundity as well as lifetime fitness; however, only three of five characters examined had significant selection based on viability fitness. For all fitness components, the frequency of significant linear and nonlinear selection coefficients was comparable (23% vs 17% of all cases, respectively), but nonlinear coefficients were generally larger. Finally, the magnitude and direction of direct linear selection was heterogeneous among locations, for all characters and all fitness components. Collectively these results suggest that selection is strong, favouring large size, high allocation to reproduction and high CH/CL flower ratios. However, any directional evolutionary changes in vegetative or reproductive characters may be constrained by strong non-linear and correlational selection.     

Selection by Pollinators on Floral Traits in Generalized Trollius ranunculoides (Ranunculaceae) along Altitudinal Gradients

Abundance and visitation of pollinator assemblages tend to decrease with altitude, leading to an increase in pollen limitation. Thus increased competition for pollinators may generate stronger selection on attractive traits of flowers at high elevations and cause floral adaptive evolution. Few studies have related geographically variable selection from pollinators and intraspecific floral differentiation. We investigated the variation of Trollius ranunculoides flowers and its pollinators along an altitudinal gradient on the eastern Qinghai-Tibet Plateau, and measured phenotypic selection by pollinators on floral traits across populations. The results showed significant decline of visitation rate of bees along altitudinal gradients, while flies was unchanged. When fitness is estimated by the visitation rate rather than the seed number per plant, phenotypic selection on the sepal length and width shows a significant correlation between the selection strength and the altitude, with stronger selection at higher altitudes. However, significant decreases in the sepal length and width of T. ranunculoides along the altitudinal gradient did not correspond to stronger selection of pollinators. In contrast to the pollinator visitation, mean annual precipitation negatively affected the sepal length and width, and contributed more to geographical variation in measured floral traits than the visitation rate of pollinators. Therefore, the sepal size may have been influenced by conflicting selection pressures from biotic and abiotic selective agents. This study supports the hypothesis that lower pollinator availability at high altitude can intensify selection on flower attractive traits, but abiotic selection is preventing a response to selection from pollinators.     

An analysis of continent-wide patterns of sexual selection in a passerine bird

Patterns of selection are widely believed to differ geographically, causing adaptation to local environmental conditions. However, few studies have investigated patterns of phenotypic selection across large spatial scales. We quantified the intensity of selection on morphology in a monogamous passerine bird, the barn swallow Hirundo rustica, using 6495 adults from 22 populations distributed across Europe and North Africa. According to the classical Darwin-Fisher mechanism of sexual selection in monogamous species, two important components of fitness due to sexual selection are the advantages that the most attractive males acquire by starting to breed early and their high annual fecundity. We estimated directional selection differentials on tail length (a secondary sexual character) and directional selection gradients after controlling for correlated selection on wing length and tarsus length with respect to these two fitness components. Phenotype and fitness components differed significantly among populations for which estimates were available for more than a single year. Likewise, selection differentials and selection gradients differed significantly among populations for tail length, but not for the other two characters. Sexual selection differentials differed significantly from zero across populations for tail length, particularly in males. Controlling statistically for the effects of age reduced the intensity of selection by 60 to 81%, although corrected and uncorrected estimates were strongly positively correlated. Selection differentials and gradients for tail length were positively correlated between the sexes among populations for selection acting on breeding date, but not for fecundity selection. The intensity of selection with respect to breeding date and fecundity were significantly correlated for tail length across populations. Sexual size dimorphism in tail length was significantly correlated with selection differentials with respect to breeding date for tail length in male barn swallows across populations. These findings suggest that patterns of sexual selection are consistent across large geographical scales, but also that they vary among populations. In addition, geographical patterns of phenotypic selection predict current patterns of phenotypic variation among populations, suggesting that consistent patterns of selection have been present for considerable amounts of time.     

Nectar robbers influence the trait–fitness relationship of Primula secundiflora

• The trait–fitness relationship influences the strength and direction of floral evolution. To fully understand and predict the evolutionary trajectories of floral traits, it is critical to disentangle the direct and indirect effects of floral traits on plant fitness in natural populations.
• We experimentally quantified phenotypic selection on floral traits through female fitness and estimated the casual effects of nectar robbing with different nectar robbing intensities on trait–fitness relationships in both the L‐ (long‐style and short‐anther phenotype) and S‐morph (short‐style and long‐anther phenotype) flowers among Primula secundiflora populations.
• A larger number of flowers and wider corolla tubes had both direct and indirect positive effects on female fitness in the P. secundiflora populations. The indirect effects of these two traits on female fitness were mediated by nectar robbers. The indirect effect of the number of flowers on female fitness increased with increasing nectar robbing intensity. In most populations, the direct and/or indirect effects of floral traits on female fitness were stronger in the S‐morph flowers than in the L‐morph flowers. In addition, nectar robbers had a direct positive effect on female fitness, but this effect varied between the L‐ and S‐morph flowers.
• These results show the potential role of nectar robbers in influencing the trait–fitness relationships in this primrose species.

     

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.     

毛茛状金莲花不同花期的花特征和访花昆虫的变化及表型选择

为了研究植物生长季内开花时间对花特征表型选择的影响,我们以青藏高原东缘高寒草地的毛茛状金莲花Trollius ranunculoides)为实验材料,在生长季内不同开花时间(花前期、花末期)测定花特征,观察访花昆虫的类群和访花频率,生长季结束后收集种子.根据昆虫访花的喜好和季节内类群与访花频率的变化,分析了不同开花时间毛茛状金莲花的花特征与昆虫的选择;并用种子产量表示雌性适合度,估计了毛茛状金莲花的花特征在不同开花时间所受的表型选择.结果表明:不同花期植物的花特征有显著差异,相应的访花昆虫的类群和频率也存在差异,不同类群昆虫访花喜好也不一样.蜂喜好花瓣和花萼较宽、花茎短和花茎数少的个体,这正符合花前期的特征,因而蜂的访花频率在花前期较高;蝇对花特征没有明显的偏好.而通过雌性适合度估计毛茛状金莲花花特征所受的表型选择则是:花前期,花茎较长和花茎数多的植株适合度大;花末期,花茎数多的植株适合度大.我们的研究表明:在植物生长季,花期的分化伴随着传粉昆虫活动的变化.不同花期,访花昆虫的变化可能对植物花特征的分化起了至关重要的作用.但是访花昆虫对花特征的选择与通过雌性适合度估计植物受到的选择不尽相同,这可能是由于其他因素造成的.     

Quantitative evaluation of reciprocal herkogamy in the distylous species, Hedyotis caerulea (Rubiaceae)

Hedyotis caerulea possesses two distinct floral morphs that are generally found in equal numbers in naturally occurring populations. Flowers either possess a relatively long style and short anthers, called a “pin,” or a short style and long anthers, called a “thrum.” This placement of reproductive organs is considered herkogamous and distylous, as it encourages outcrossing by restricting pollination to individuals of the alternate morph. Numerous species have been described as distylous without quantitative data establishing stigma-anther reciprocity. Here we assess those assumptions in H. caerulea by measuring stigma height, anther height and a suite of additional floral traits across multiple localities. All populations surveyed were isoplethic, although variation among them was present in all floral traits measured as well as for pollen diameter, pollen count, flower dry weight, and seed set. Pins produced smaller pollen than thrums, but made more of them. Thus, the total volume of pollen was similar for pins and thrums, and seed set was similar, suggesting that each morph has equal male and female fitness with no movement towards dioecy. Given a significant degree of variation found in the morphometric analysis, and that two of the three measures used to assess reciprocity were not consistent with predictions of precise symmetry, extensive change is possible where selection is acting on these traits. Even so, the distylous mating system in H. caerulea appears to be stable.     

Population size,pollination and phenotypic trait selection in Phyteuma spicatum

Plants in small populations may receive fewer visits, smaller pollen loads or pollen of poorer quality and suffer from reduced reproductive success compared to plants in larger populations. Consequently, pollen limitation of plants in small populations has been suggested to result in the evolution of reduced reliance on pollinators or the enhancement of traits that attract pollinators. The main aim of this study was to experimentally quantify the strength of pollinator-mediated selection on floral display size and flowering phenology in populations of varying size, using the self-incompatible, perennial herb Phyteuma spicatum as study species. We conducted supplementary hand pollinations in six populations (ranging in size between ca. 20–3000 flowering individuals) over two consecutive years and assessed selection gradients (i.e., trait–fitness relationships) in open- and hand-pollinated plants. Our results show that some populations are pollen limited in some years, but, contrary to our expectation, the degree of pollen limitation was not significantly related to population size. We found phenotypic selection for increased inflorescence size (in most populations and in both years), but we obtained no or no strong evidence that selection was pollinator-mediated or that the strength of selection was related to population size. This may have been the result of low statistical power, an inherent problem of studies examining effects of population size that require the inclusion of populations with only few individuals. In addition, given that selection appeared to be spatially and temporally variable, abiotic or biotic factors other than pollinators may have contributed to selection on inflorescence size.     

A PATH-ANALYTIC MODEL FOR THE MEASUREMENT OF SELECTION ON MORPHOLOGY

This paper describes a path model for the analysis of phenotypic selection upon continuous morphological characters. The path-analysis model assumes that selection occurs on unmeasured general size and shape allometry factors that summarize linear relations among sets of ontogenetically, phylogenetically, or functionally related traits. An unmeasured factor for general size is considered the only aspect of morphometric covariance matrices for which there is an a priori biological explanation. Consequently, selection coefficients are derived for each measured character by holding constant only a general size factor, rather than by using multiple regression to adjust for the full covariance matrix. Fitness is treated as an unmeasured factor with loadings, representing directional selection coefficients, computed as the covariances of the size-adjusted characters with the measured fitness indicator. The magnitudes and signs of the selection coefficients, combined with biological insight, may suggest hypotheses of selection on one or more shape allometry factors. Hypotheses of selection on general size and shape allometry factors are evaluated through cycles of measurement, analysis, and experimentation, designed to refine the path diagram depicting the covariances among the measured characters, the measured indicator of fitness, and unmeasured factors for morphology and fitness. The path-analysis and multiple-regression models were applied to data from remeasurement of Lande and Arnold's (1983) pentatomid bugs and to Bumpus's (1899) data on house sparrows. The path analysis suggested the hypothesis that variation in bug survivorship was an expression of directional selection on wing loading. Bumpus's data are consistent with a hypothesis of stabilizing selection on general size in females and directional selection for small wing size relative to body size in males.     

A comparison of methods to partition selection acting via components of fitness: Do larger male bullfrogs have greater hatching success?

We contrast three methods for measuring selection at sequential fitness components (here called the additive, changing variance, and independent methods). The independent method (Koenig and Albano, 1987; Conner, 1988) describes the relationship between a phenotypic character and one fitness component independent of other components. This method is appropriate when the question is whether or not a character has fitness consequences independent of selection at other stages. The additive (Arnold and Wade, 1984a) and changing variance (Kalisz, 1986; Koenig and Albano, 1987) methods measure selection via one component of fitness, taking into consideration constraints imposed by selection via earlier components in the sequence. These methods therefore more accurately track selection over a sequence of fitness components. Of these latter two methods, the changing variance method yields erratic results in simulation studies and is not recommended in its unmodified form. The additive method (equivalent to the changing variance method weighted as described in Wade and Kalisz [1989]) explicitly partitions selection into additive components and is useful for measuring selection taking into account the constraints imposed by selection acting via prior fitness components. The methods often yield very different estimates of the relative degree to which the mean of a character is changed by selection acting via a particular component of fitness (the “strength” of selection). However, neither the additive nor independent method is inherently superior to the other; rather, these measures are complementary.     

Differences in the temporal dynamics of phenotypic selection among fitness components in the wild

The balance of selection acting through different fitness components (e.g. fecundity, mating success, survival) determines the potential tempo and trajectory of adaptive evolution. Yet the extent to which the temporal dynamics of phenotypic selection may vary among fitness components is poorly understood. Here, we compiled a database of 3978 linear selection coefficients from temporally replicated studies of selection in wild populations to address this question. Across studies, we find that multi-year selection through mating success and fecundity is stronger than selection through survival, but varies less in direction. We also report that selection through mating success varies more in long-term average strength than selection through either survival or fecundity. The consistency in direction and stronger long-term average strength of selection through mating success and fecundity suggests that selection through these fitness components should cause more persistent directional evolution relative to selection through survival. Similar patterns were apparent for the subset of studies that evaluated the temporal dynamics of selection on traits simultaneously using several different fitness components, but few such studies exist. Taken together, these results reveal key differences in the temporal dynamics of selection acting through different fitness components, but they also reveal important limitations in our understanding of how selection drives adaptive evolution.     

Formal Darwinism

Two questions are raised for Grafen’s formal darwinism project of aligning evolutionary dynamics under natural selection with the optimization of phenotypes for individuals of a population. The first question concerns mean fitness maximization during frequency-dependent selection; in such selection regimes, not only is mean fitness typically not maximized but it is implausible that any parameter closely related to fitness is being maximized. The second question concerns whether natural selection on inclusive fitness differences can be regarded as individual selection or whether it leads to a departure from the central motivation that led to the formal darwinism project, viz., to show that “Darwinian” evolution through individual selection leads to “good design” or phenotypic adaptation through trait optimization.     

Evolvability and genetic constraint in Dalechampia blossoms: components of variance and measures of evolvability

Many evolutionary arguments are based on the assumption that quantitative characters are highly evolvable entities that can be rapidly moulded by changing selection pressures. The empirical evaluation of this assumption depends on having an operational measure of evolvability that reflects the ability of a trait to respond to a given external selection pressure. We suggest short-term evolvability be measured as expected proportional response in a trait to a unit strength of directional selection, where strength of selection is defined independently of character variation and in units of the strength of selection on fitness itself. We show that the additive genetic variance scaled by the square of the trait mean, IA, is such a measure. The heritability, h2, does not measure evolvability in this sense. Based on a diallel analysis, we use IA to assess the evolvability of floral characters in a population of the neotropical vine Dalechampia scandens (Euphorbiaceae). Although we are able to demonstrate that there is additive genetic variation in a number of floral traits, we also find that most of the traits are not expected to change by more than a fraction of a percent per generation. We provide evidence that the degree of among-population divergence of traits is related to their predicted evolvabilities, but not to their heritabilities.     

Floral trait differentiation in Anacamptis coriophora: Phenotypic selection on scents,but not on colour

Current divergent selection may promote floral trait differentiation among conspecific populations in flowering plants. However, whether this applies to complex traits such as colour or scents has been little studied, even though these traits often vary within species. In this study, we compared floral colour and odour as well as selective pressures imposed upon these traits among seven populations belonging to three subspecies of the widespread, generalist orchid Anacamptis coriophora. Colour was characterized using calibrated photographs, and scents were sampled using dynamic headspace extraction and analysed using gas chromatography–mass spectrometry. We then quantified phenotypic selection exerted on these traits by regressing fruit set values on floral trait values. We showed that the three studied subspecies were characterized by different floral colour and odour, with one of the two predominant floral volatiles emitted by each subspecies being taxon‐specific. Plant size was positively correlated with fruit set in most populations, whereas we found no apparent link between floral colour and female reproductive success. We detected positive selection on several taxon‐specific compounds in A. coriophora subsp. fragrans, whereas no selection was found on floral volatiles of A. coriophora subsp. coriophora and A. coriophora subsp. martrinii. This study is one of the first to document variation in phenotypic selection exerted on floral scents among conspecific populations. Our results suggest that selection could contribute to ongoing chemical divergence among A. coriophora subspecies.     

浙江省野生蜡梅花部形态变异及其与环境因子的相关性

以浙江省蜡梅自然分布区的5个居群为研究对象,采用巢式方差分析、主成分分析、相关分析、聚类分析等多种分析方法,探究蜡梅花部的形态变异及其与环境因子相关性以及居群间和居群内的表型多样性。结果表明:蜡梅花部性状中除雄蕊长度、雌蕊数、雌蕊长度、内被片数以外,其他12个性状在居群间均有极显著差异。胚珠数的平均变异系数最大,花冠内径的平均变异系数最小,各性状的平均变异系数为15.03%(10.81%—23.38%)。五尖山的表型多样性最丰富,碧东山居群最小,种群间平均表型分化系数为44.38%(1.57%—89.62%),种群内变异大于种群间变异,种群内变异是花部变异的主要来源。主成分分析显示花冠直径、花冠内径、花筒深度和中被片长对种群变异起主要贡献作用。花部多数性状间存在显著或极显著的相关关系,生态因子中花部性状与土壤酸度相关性最大,土壤中花部性状与大量元素相关性最大,微量元素次之,中量元素最小。通过UPGMA聚类分析可以将5个种群分成两组。浙江省野生蜡梅花部形态存在丰富的变异和多样性,花部部分性状与土壤中钙、铁、铜含量和海拔有显著或极显著相关关系。     

Variation in resource limitation of plant reproduction influences natural selection on floral traits of <Emphasis Type="Italic">Asclepias syriaca</Emphasis>

The availability of both pollen and resources can influence natural selection on floral traits, but their relative importance in shaping floral evolution is unclear. We experimentally manipulated pollinator and resource (fertilizer and water) availability in the perennial wildflower Asclepias syriaca L. Nine floral traits, one male fitness component (number of pollinia removed), and two female fitness components (number of pollinia inserted and number of fruits initiated) were measured for plants in each of three treatments (unmanipulated control, decreased pollinator access, and resource supplementation). Although decreasing pollinators’ access to flowers did result in fewer pollinia inserted and removed, fruit set and phenotypic selection on floral traits via female and male fitness did not differ from the control. In contrast, resource supplementation increased fruit set, and phenotypic selection on seven out of nine floral traits was stronger via female than male fitness, consistent with the prediction that selection via female fitness would be greater when reproduction was less resource-limited. Our results support the hypothesis that abiotic resource availability can influence floral evolution by altering gender-specific selection.