BUMBLE BEE POLLINATION AND FLORAL MORPHOLOGY: FACTORS INFLUENCING POLLEN DISPERSAL IN THE ALPINE SKY PILOT,POLEMONIUM VISCOSUM (POLEMONIACEAE)

Pollen dispersal success in entomophilous plants is influenced by the amount of pollen produced per flower, the fraction of pollen that is exported to other flowers during a pollinator visit, visitation frequency, and the complementarity between pollen donor and recipients. For bumble bee-pollinated Polemonium viscosum the first three determinants of male function are correlated with morphometric floral traits. Pollen production is positively related to corolla and style length, whereas pollen removal per visit by bumble bee pollinators is a positive function of corolla flare. Larger-flowered plants receive more bumble bee visits than small-flowered individuals. We found no evidence of tradeoffs between pollen export efficiency and per visit accumulation of outcross pollen; each was influenced by unique aspects of flower morphology. Individual queen bumble bees of the principal pollinator species, Bombus kirbyellus, were similar in male, female, and absolute measures of pollination effectiveness. An estimated 2.9% of the pollen that bumble bees removed from flowers during a foraging bout was, on average, deposited on stigmas of compatible recipients. Significant plant-to-plant differences in pollen production, pollen export per visit, and outcross pollen receipt were found for co-occurring individuals of P. viscosum indicating that variation in these fitness related traits can be seen by pollinator-mediated selection.     

RATES OF FLORAL EVOLUTION: ADAPTATION TO BUMBLEBEE POLLINATION IN AN ALPINE WILDFLOWER,POLEMONIUM VISCOSUM

Animal pollinators are thought to shape floral evolution, yet the tempo of this process has seldom been measured. I used the prediction equation of quantitative genetics, R = h²S , to predict the rate at which a change in pollinator abundance may have caused divergence in floral morphology of the alpine skypilot, Polemonium viscosum. A selection experiment determined the rate at which such divergence can actually proceed. Corolla flare in this species increases by 12% from populations pollinated by a wide assemblage of insect visitors to those pollinated only by bumblebees. To simulate the evolutionary process giving rise to this change, I used a pollinator selection experiment. Plants with broad flowers set significantly more seeds than plants with narrow flowers under bumblebee pollination but had equivalent fecundity when visited by other insects or hand-pollinated. Bumblebee-mediated selection for broad corolla flare intensified from 0.07 at seed set to 0.17 at progeny establishment. Maternal parent-offspring regression yielded a confidence interval of 0.22–1.00 for trait heritability. Given these parameter estimates, the prediction equation shows that broadly flared flowers of bumblebee-pollinated P. viscosum could have evolved from narrower ones in a single generation. This prediction is matched by an observed 9% increase in offspring corolla flare after a single bout of bumblebee-mediated selection, relative to offspring of unselected controls. Findings show that plant populations can adapt rapidly to abrupt changes in pollinator assemblages.     

POLLINATION IN FLORAL SCENT MORPHS OF POLEMONIUM VISCOSUM: A MECHANISM FOR DISRUPTIVE SELECTION ON FLOWER SIZE

Plants of Polemonium viscosum have flowers that are either sweet or skunky in scent. The two morphs are preferentially pollinated by insects of strongly contrasting body size: bumblebee queens specialize on sweet flowers, flies on skunky ones. In this study 13 characters were examined in plant specimens from five populations to identify major components of intraspecific variation in flower and inflorescence morphology and test their correlation with floral scent. Factor analysis identified four major axes of morphological variation. The first explained 22% of the variance among specimens and correlated strongly with four flower size characters: sepal length, corolla tube length, corolla lobe width, and corolla lobe length. Floral scent morphs differed significantly in the multivariate representation of flower size defined by these characters. Sweet flowers had wider corolla lobes, longer corolla tubes, and longer sepals than skunky ones. Corolla lobe width accounted for the greatest amount of intermorph divergence. Divergence in flower size between morphs was maintained in mixed populations at four locations in alpine Colorado, with corollas of sweet flowers significantly broader or more flared than those of skunky flowers. Patterns of pollen receipt suggest that this difference is adaptive. In the sweet morph, pollination intensity and purity increased significantly with corolla flare. Conversely, in the skunky morph, corolla flare had little influence on pollination intensity and had a strong negative effect on purity. These findings suggest that selection for effective pollination should favor intraspecific divergence in flower size in Polemonium viscosum.     

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.     

DOWN THE TUBE: POLLINATORS, PREDATORS, AND THE EVOLUTION OF FLOWER SHAPE IN THE ALPINE SKYPILOT, POLEMONIUM VISCOSUM

Abstract We address how a conflict between pollinator attraction and avoidance of flower predation influences the evolution of flower shape in Polemonium viscosum. Flower shape in P. viscosum is the product of an isometric relationship between genetically correlated (r_A= 0.70) corolla flare and length. Bumblebee pollinators preferentially visit flowers that are more flared and have longer tubes, selecting for a funnel‐shaped corolla. However, flower shape also influences nectar‐foraging ants that sever the style at its point of attachment to the ovary. Surveys of ant damage show that plants having flowers with flared, short corollas are most vulnerable to ant predation. Consistent with this result, the ratio of corolla length to flare is significantly greater in a krummholz (high predation risk) population than in a tundra (low predation risk) population. To explicitly test whether the evolution of a better defended flower would exact a cost in pollination, we created tubular flowers by constricting the corolla during development. Performance of tubular flowers and natural controls was compared for defensive and attractive functions. In choice trials, ants entered control flowers significantly more often than tubular ones, confirming that the evolution of tubular flowers would reduce the risk of predation. However, in a bumblebee‐pollinated population, tubular flowers received significantly less pollen and set fewer seeds than controls. A fitness model incorporating these data predicts that in the absence of the genetic correlation between corolla length and flare, intermittent selection for defense could allow tubular flowers to spread in the krummholz population. However, in the tundra, where bumblebees account for nearly all pollination, the model predicts that tubular flowers should always confer a fitness disadvantage.     

ECOTYPIC DIVERGENCE IN ALPINE POLEMONIUM VISCOSUM: GENETIC STRUCTURE,QUANTITATIVE VARIATION,AND LOCAL ADAPTATION

Polemonium viscosum has a continuous distribution from 3,500 m in the krummholz to 4,025 m on the summit ridges of Pennsylvania Mountain, Colorado. Seeds produced by plants at opposite ends of this cline, 1.5 km apart, differed significantly at allozyme loci in two consecutive breeding seasons. Mean multilocus F_st values for both years (0.015 and 0.069) were significantly different from zero, indicating restricted gene exchange between subpopulations. Average allele frequencies at two individual loci also differed significantly between families comprising krummholz and summit subpopulations. Progeny of plants growing on the summit had higher leaf production rates, more densely packed leaflets, and lower resistance to aphids than progeny of plants growing in the krummholz site, when tested under greenhouse conditions. These differences probably reflect the restricted opportunities for growth and severe exposure at high elevations, and the increased risk from herbivores near timberline. The two subpopulations did not differ in leaf length (stature), leaf width, or pubescence. Reciprocal transplanting of seedlings between krummholz and summit sites confirmed that the differences were adaptive, since progeny from each subpopulation performed significantly better in their parent's habitat. Coordinated studies of genetic structure, quantitative variation, and local adaptation across the elevational range of P. viscosum provide a comprehensive view of ecotypic divergence in this widespread alpine plant.     

RESPONSES OF FLORAL TRAITS TO SELECTION ON PRIMARY SEXUAL INVESTMENT IN SPERGULARIA MARINA: THE BATTLE BETWEEN THE SEXES

Two widespread assumptions underlie theoretical models of the evolution of sex allocation in hermaphroditic species: (1) resource allocations to male and female function are heritable; and (2) there is an intrinsic, genetically based negative correlation between male and female reproductive function. These assumptions have not been adequately tested in wild species, although a few studies have detected either genetic variation in pollen and ovule production per flower or evidence of trade-offs between male and female investment at the whole plant level. It may also be argued, however, that in highly autogamous, perfect-flowered plant taxa that exhibit genetic variation in gamete production, strong stabilizing selection for an efficient pollen:ovule ratio should result in a positive correlation among genotypes with respect to mean ovule and mean pollen production per flower. Here we report the results of a three-generation artificial selection experiment conducted on a greenhouse population of the autogamous annual plant Spergularia marina. Starting with a base population of 1200 individuals, we conducted intense mass selection for two generations, creating four selected lines (high and low ovule production per flower; high and low anther production per flower) and a control line. By examining the direct and correlated responses of several floral traits to selection on gamete production per flower, we evaluated the expectations that primary sexual investment would exhibit heritable variation and that resource-sharing, variation in resource-garnering ability, or developmental constraints mold the genetic correlations expressed among floral organs. The observed direct and correlated responses to selection on male and female gamete production revealed significant heritabilities of both ovule and anther production per flower and a significant negative genetic correlation between them. When plants were selected for increased ovules per flower over two generations, ovule production increased and anther production declined relative to the control line. Among plants selected for decreased anthers per flower, we observed a decline in anther production and an increase in ovule production relative to the control line. In contrast, the lines selected for low ovules per flower and for high anthers per flower exhibited no evidence for significant genetic correlations between male and female primary investment. Correlated responses to selection also indicate a genetically based negative correlation between the production of normal versus developmentally abnormal anthers (staminoid organs); a positive correlation between the production of ovules versus staminoid organs; and a positive correlation between the production of anthers and petals. The negative relationship between male versus female primary investment supports classical sex allocation theory, although the asymmetrical correlated responses to selection indicate that this relationship is not always expressed.     

MULTIYEAR STUDY OF MULTIVARIATE LINEAR AND NONLINEAR PHENOTYPIC SELECTION ON FLORAL TRAITS OF HUMMINGBIRD-POLLINATED SILENE VIRGINICA

Pollination syndromes suggest that convergent evolution of floral traits and trait combinations reflects similar selection pressures. Accordingly, a pattern of selection on floral traits is expected to be consistent with increasing the attraction and pollen transfer of the important pollinator. We measured individual variation in six floral traits and yearly and lifetime total plant seed and fruit production of 758 plants across nine years of study in natural populations of Ruby-Throated Hummingbird-pollinated Silene virginica. The type, strength, and direction of selection gradients were observed by year, and for two cohorts selection was estimated through lifetime maternal fitness. Positive directional selection was detected on floral display height in all years of study and stigma exsertion in all years but one. Significant quadratic and correlational selection gradients were rare. However, a canonical analysis of the gamma matrix indicated nonlinear selection was common; if significant curvature was detected it was convex with one exception. Our analyses demonstrated selection favored trait combinations and the integration of floral features of attraction and pollen transfer efficiency that were consistent with the hummingbird pollination syndrome.     

ANTHER SMUT DISEASE IN DIANTHUS SILVESTER (CARYOPHYLLACEAE): NATURAL SELECTION ON FLORAL TRAITS

Mating opportunities, pollination intensity, and pollen dispersal ability may vary with variation in floral traits such as color, size, and shape. Where these traits are selected by pollinators for enhanced elaboration, they should evolve toward the equilibrium between selection for further elaboration and selection against this through reduced fecundity or vitality. Here we show that pollinator-borne fungal diseases of plants may be a factor influencing the position of this equilibrium. Populations of the rock pink, Dianthus silvester often contain individuals infected with the anther smut fungus Microbotryum violaceum (= Ustilago violacea). In a naturally infected population in the Alps of eastern Switzerland we investigated how intrapopulation variation in flower size and nectar rewards influenced spore deposition and how floral traits varied with disease status. We found that spore deposition increased with increasing petal size, suggesting that large-flowered plants were at a greater risk of disease. Spore deposition was also higher for plants growing in patches with many or a high proportion of diseased neighbors. Multiple regression analyses showed that petal size or nectar reward influenced spore deposition when the effects of neighborhood disease abundance were controlled statistically. In sequential analyses, after removing the effects of disease density or frequency and plant gender, petal length explained significant variation in spore deposition. Diseased plants had reduced female reproductive organs, but calyx size was intermediate between that of healthy perfect and female flowers of this gynodioecious-gynomonoecious species, and diseased plants bore flowers with the largest petals. This may reflect a symptom of this disease or the cause, if larger-flowered plants are more likely to become infected. We conclude that investment to pollinator attraction may bring an enhanced risk of contracting this sterilizing pollinator-borne disease, so natural selection by the fungus M. violaceum acts to lower attractiveness to pollinators.     

THE ACTION OF STABILIZING SELECTION,MUTATION, AND DRIFT ON EPISTATIC QUANTITATIVE TRAITS

For a quantitative trait under stabilizing selection, the effect of epistasis on its genetic architecture and on the changes of genetic variance caused by bottlenecking were investigated using theory and simulation. Assuming empirical estimates of the rate and effects of mutations and the intensity of selection, we assessed the impact of two‐locus epistasis (synergistic/antagonistic) among linked or unlinked loci on the distribution of effects and frequencies of segregating loci in populations at the mutation‐selection‐drift balance. Strong pervasive epistasis did not modify substantially the genetic properties of the trait and, therefore, the most likely explanation for the low amount of variation usually accounted by the loci detected in genome‐wide association analyses is that many causal loci will pass undetected. We investigated the impact of epistasis on the changes in genetic variance components when large populations were subjected to successive bottlenecks of different sizes, considering the action of genetic drift, operating singly (D), or jointly with mutation (MD) and selection (MSD). An initial increase of the different components of the genetic variance, as well as a dramatic acceleration of the between‐line divergence, were always associated with synergistic epistasis but were strongly constrained by selection.     

THE MEASUREMENT OF SELECTION ON QUANTITATIVE TRAITS: BIASES DUE TO ENVIRONMENTAL COVARIANCES BETWEEN TRAITS AND FITNESS

The use of regression techniques for estimating the direction and magnitude of selection from measurements on phenotypes has become widespread in field studies. A potential problem with these techniques is that environmental correlations between fitness and the traits examined may produce biased estimates of selection gradients. This report demonstrates that the phenotypic covariance between fitness and a trait, used as an estimate of the selection differential in estimating selection gradients, has two components: a component induced by selection itself and a component due to the effect of environmental factors on fitness. The second component is shown to be responsible for biases in estimates of selection gradients. The use of regressions involving genotypic and breeding values instead of phenotypic values can yield estimates of selection gradients that are not biased by environmental covariances. Statistical methods for estimating the coefficients of such regressions, and for testing for biases in regressions involving phenotypic values, are described.     

MEASUREMENTS OF NATURAL SELECTION ON FLORAL TRAITS IN WILD RADISH (RAPHANUS RAPHANISTRUM). II. SELECTION THROUGH LIFETIME MALE AND TOTAL FITNESS

It has often been suggested that selection on floral traits in hermaphroditic plants should occur primarily through differences in male fitness. However, measurements of selection on floral traits through differences in lifetime male fitness have been lacking. We measured selection on a variety of wild radish floral traits using lifetime male fitness measures derived from genetic paternity analysis. These male fitness estimates were then combined with estimates of lifetime female fitness of the same plants to produce measurements of selection based on lifetime total fitness. Contrary to the prediction above, there was no strong evidence for selection on floral morphology through male fitness differences in any of the three years of the study, but there was strong selection for increased flower size through female fitness differences in one year. The main determinant of both male and female fitness in all years was flower number; this lead to moderately positive correlations between male and female fitness in all three years.     

香鼬的栖息地选择、觅食和育幼行为

香鼬（Ｍｕｓｔｅｌａａｌｔａｉｃａ）喜栖于人类居住区附近的高寒草甸草场,主要以高原鼠兔（Ｏｃｈｏｔｏｎａｃｕｒｚｏｎｉａｅ）为食,其数量分布与高原鼠兔的密度分布成正相关。在高原鼠兔的繁殖盛期,香鼬主要捕食鼠兔幼体,每只成体平均每天捕获鼠兔６．２只,相当于每１００克体重日获取食物重量２７３．５克,在高原鼠兔的繁殖后期和非繁殖期,每只成体平均每天捕获鼠兔２．７５只,相当于每１００克体重日获取食物重量２９６．３克。繁殖期育幼任务全部由雌性成鼬承担,幼鼬从７月初开始地面活动,８月初开始扩散。     

COMPOSITE TRAITS,SELECTION RESPONSE,AND EVOLUTION

It is very difficult to relate macroevolutionary patterns to the microevolutionary processes described by quantitative-genetic models. Quantitative-genetic parameters are statistical abstractions. Their long-term significance and evolution might be understood if they can be related to development, physiology, and other biological properties. Most continuous traits are composites of other traits that may contribute differentially to selection response and long-term divergence. The operation of selection on continuous traits can be indirect, with intermediate optima caused by correlated fitness components. Few realistic models are available, and heritable maternal effects can further complicate selection response. Examples involving allometry of brain and body size in mammals suggest that prenatal and postnatal growth have contributed differently to body-size evolution, with different correlated changes in brain size. Several different models could explain these patterns, and interpretation is further complicated by statistical difficulties in comparative biology. Quantitative-genetic models may become more informative and predictive if variation in their parameters can be explained by developmental and other biological processes that have been shaped by the previous history of the population.     

REPRODUCTION IN POLEMONIUM: PATTERNS AND IMPLICATIONS OF FLORAL NECTAR PRODUCTION AND STANDING CROPS

Patterns of floral nectar production and standing crop were measured in four populations of the herbaceous perennial plant species Polemonium foliosissimum. Contrary to prediction (Pleasants, 1983), individual flowers in this mass-flowering species were found to produce equivalent nectar volumes every day of their lives. Alternative methods of increasing the reward variability presented to pollinators are evaluated for P. foliosissimum and the relationship between that variability and risk-aversive foraging by pollinators is discussed. Significant spatial and temporal variability in rate of nectar production was found. Populations separated by approximately 200 m exhibited different rates. Nectar production declined significantly as a function of time of the flowering season in two populations but not in a third. In spite of such variability, individual plants showed consistency in production both within a single blooming season and across successive seasons. Because of the variability found in the present study, care should be taken to design appropriate sampling protocols in future nectar studies. Patterns of standing nectar crop were consistent with those expected if pollinators were using an area-restricted searching pattern.     

SELECTION ON HERBIVORE LIFE-HISTORY TRAITS BY THE FIRST AND THIRD TROPHIC LEVELS: THE DEVIL AND THE DEEP BLUE SEA REVISITED

Abstract The timing of life‐history events in insects can have important consequences for both survival and reproduction. For insect herbivores with complex life histories, selection is predicted to favor those combinations of traits that increase the size at metamorphosis while minimizing the risk of mortality from natural enemies. Studies quantifying selection on life‐history traits in natural insect herbivore populations, however, have been rare. The purpose of this study was to measure phenotypic selection imposed by elements of the first and third trophic levels on variation in two life‐history traits, the timing of egg hatch and pupal mass, in a population of oak‐feeding caterpillars, Psilocorsis quercicella (Lepidoptera: Oecophoridae). Larvae were collected from the field throughout each of two generations per year for three years and reared to determine the effects of the date of egg hatch on both the risk of attack from parasitoids and the pupal mass of the survivors. The direction and strength of phenotypic selection attributed to aspects of the first and third trophic levels, as well as their combined effects, on the date of egg hatch was measured for each of the six generations. Heritabilities of and genetic correlations between pupal mass and the date of adult emergence from diapause (the life‐history trait expected to have the largest influence on the timing of egg hatch, and thus larval development) were estimated from laboratory matings. In four of the six generations examined, significant directional selection attributed to the first trophic level was detected, always favoring early‐hatching cohorts predicted to experience higher leaf quality than late‐hatching cohorts. Directional phenotypic selection by the third trophic level was detected in only one of three years, and in that year the direction of selection was in opposite directions during the two successive generations. The combined effect of selection by both trophic levels indicated that the third trophic level acted to either reduce or enhance the more predictable pattern of selection attributed to the first trophic level. In addition, I found evidence of truncation selection acting to increase the mean and decrease the variance of pupal mass during the pupa‐adult transition in the laboratory. Pupal mass and diapause duration were found to vary significantly among full‐sibling families; upper bounds for heritability estimates were 0.57 and 0.30, respectively. Furthermore, these two traits were found to be positively genetically correlated (families with larger pupae had longer diapause durations), resulting in a fitness trade‐off, because larger pupae enjoy higher survival through metamorphosis and female fecundity but emerge later, when average leaf quality for offspring is generally poorer.     

MEASURING THE EFFECTS OF PAIRING SUCCESS,EXTRA-PAIR COPULATIONS AND MATE QUALITY ON THE OPPORTUNITY FOR SEXUAL SELECTION

Sexual selection can act through variation in the number of social mates obtained, variation in mate quality, or variation in success at obtaining extra-pair fertilizations. Because within-pair fertilizations (WPF) and extra-pair fertilizations (EPF) are alternate routes of reproduction, they are additive, rather than multiplicative, components of fitness. We present a method for partitioning total variance in reproductive success (a measure of the opportunity for selection) when fitness components are both additive and multiplicative and use it to partition the variance into components that correspond to each mechanism of sexual selection. Computer simulations show that extra-pair fertilizations can either increase or decrease total variance, depending on the covariance between within-pair and extra-pair success. Simulations also suggest that for socially monogamous species, extra-pair fertilizations have a greater effect than variation in mate quality or pairing status on the opportunity for selection. Application of our model to data gathered for a population of red-winged blackbirds (Agelaius phoeniceus) indicates that most of the variance in male reproductive success was attributable to within-pair sources of variance. Nevertheless, extra-pair copulations increased the opportunity for selection because males varied both in the proportion of their social young that they sired and in the number of extra-pair mates that they obtained. Furthermore, large and positive covariances existed between the number of extra-pair mates a male obtained and both social pairing success and within-pair paternity, indicating that, in this population, males preferred as social mates also were preferred as extra-pair mates.     

GENETIC CORRELATIONS AMONG MORPHOMETRIC TRAITS AND RATES OF GROWTH AND DIFFERENTIATION IN THE GREEN TREE FROG,HYLA CINEREA

It is often proposed that the morphometric shape of animals often evolves as a correlated response to selection on life-history traits such as whole-body growth and differentiation rates. However, there exists little empirical information on whether selection on rates of growth or differentiation in animals could generate correlated response in morphometric shape beyond that owing to the correlation between these rates and body size. In this study genetic correlations were estimated among growth rate, differentiation rate, and body-size-adjusted head width in the green tree frog, Hyla cinerea. Head width was adjusted for size by using the residuals from log-log regressions of head width on snout-vent length. Size-adjusted head width at metamorphosis was positively genetically correlated with larval period length. Thus, size-independent shape might evolve as a correlated response to selection on a larval life-history trait. Larval growth rate was not significantly genetically correlated with size-adjusted head width. An additional morphometric trait, size-adjusted tibiofibula length, had a nonnormal distribution of breeding values, and so was not included in the analysis of genetic correlations (offspring from one sire had unusually short legs). This result is interesting because, although using genetic covariance matrices to predict long-term multivariate response to selection depends on the assumption that all loci follow a multivariate Gaussian distribution of allelic effects, few data are available on the distribution of breeding values for traits in wild populations. Size at metamorphosis was positively genetically correlated with larval period and larval growth rate. Quickly growing larvae that delay metamorphosis therefore emerge at a large size. The genetic correlation between larval growth rate and juvenile (postmetamorphic) growth rate was near zero. Growth rate may therefore be an example of a fitness-related trait that is free to evolve in one stage of a complex life cycle without pleiotropic constraints on the same trait expressed in the other stage.     

MEASURING SELECTION ON REACTION NORMS: AN EXPLORATION OF THE EUROSTA-SOLIDAGO SYSTEM

The sensitivity of genotypic expression to the environment can be depicted as the reaction norm, which is defined as the array of phenotypes produced by a single genotype over a range of environments. We studied selection on reaction norms of the gall-inducing insect Eurosta solidaginis (Diptera; Tephritidae), which attacks tall goldenrod Solidago altissima (Compositae). Gall size was treated as a component of insect phenotype and attributes of the host plant as environmental influences on gall development. Genetic differences in the response of gall size to plant lag time (the number of days before a plant responds to the gall maker) were examined. Reaction norms for full-sib families of flies were quantified as linear functions; the elevation of the function denoted gall size produced by the family averaged across all plants, and the function's slope denoted family sensitivity to lag time. Expected fitness of each family was regressed over reaction norm elevation and slope to yield selection gradients on these reaction norm parameters. Directional selection on gall size averaged across environments is four times stronger than selection on sensitivity. Yet, genetic variation for sensitivity contributes more than twice as much to gall phenotypic variance as family mean gall size. Our results suggest that selection on environmental sensitivity will be weak for populations restricted to a narrow segment of an environmental gradient, but strong for broadly distributed species.