NATURAL SELECTION ON GENETICALLY CORRELATED PHENOLOGICAL CHARACTERS IN LYTHRUM SALICARIA L. (LYTHRACEAE)

Genetic variation in reproductive timing in a long-lived herbaceous perennial

PREMISE

Reproductive fitness of individual plants depends on the timing of flowering, especially in mate-limited populations, such as those in fragmented habitats. When flowering time traits are associated with differential reproductive success, the narrow-sense heritability (h² ) of traits will determine how rapidly trait means evolve in response to selection. Heritability of flowering time is documented in many annual plants. However, estimating h² of flowering time in perennials presents additional methodological challenges, often including paternity assignment and trait expression over multiple years.

METHODS

We evaluated the h² of onset and duration of flowering using offspring-midparent regressions and restricted maximum likelihood methods in an experimental population of an iterocarpic, perennial, herbaceous plant, Echinacea angustifolia, growing in natural conditions. We assessed the flowering time of the parental cohort in 2005 and 2006; the offspring in 2014 through 2017. We also examined the effects of the paternity assignment from Cervus and MasterBayes on estimates of h² .

RESULTS

We found substantial h² for onset and duration of flowering. We also observed variation in estimates among years. The most reliable estimates for both traits fell in the range of 0.1-0.17. We found evidence of a genotype by year interaction for onset of flowering and strong evidence that genotypes are consistent in their duration of flowering across years.

CONCLUSIONS

Substantial heritabilities in this population imply the capacity for a response to natural selection, while also suggesting the potential for differential contributions to adaptive evolution among seasons.

Polyploidization: Consequences of genome doubling on the evolutionary potential of populations

Whole-genome duplication is common in plants and is considered to have a broad range of effects on individuals' phenotypes and genomes and to be an important driver of plant adaptation and speciation. Despite their increased capacity to cope with challenging environments, polyploid lineages are generally as prone to extinction, and sometimes more prone, than their diploid progenitors. Although several explanations have been proposed to explain the short- and long-term disadvantages of polyploidy on the survival probability of populations, the consequences of whole-genome doubling on the heritable variance remain poorly studied. Whole-genome doubling can have major effects not only on the genetics, but also on the ecology and life history of the populations. Modifications of other properties of populations can reverse the effects of polyploidization per se on heritable variance. In this synthesis, I summarize the empirical and theoretical knowledge about the multifarious consequences of genome doubling on the heritable variance of quantitative traits and on the evolutionary potential of polyploid populations compared to their diploid progenitors. I propose several ways to decipher the consequences of whole-genome doubling on survival probability and to study the further consequences of shifting the ecological niche and life-history traits of a population. I also highlight some practical considerations for comparing the heritable variance of a trait among different cytotypes. Such investigations appear to be timely and necessary to understand more about the paradoxical aspects of polyploidization and to understand the evolutionary potential of polyploid lineages in a global warming context.

The evolution of the additive variance of a trait under stabilizing selection after autopolyploidization

Whole‐genome duplication is a common mutation in eukaryotes with far‐reaching phenotypic effects. The resulting morphological, physiological and fitness consequences and how they affect the survival probability of polyploid lineages are intensively studied, but little is known about the effect of genome doubling on the evolutionary potential of populations. Historically, it has been argued polyploids should be less able to adapt because gene duplication dilutes the effects of alleles, such that polyploids are less likely to evolve new adaptive gene complexes compared with diploids. In this paper, I investigate the short‐ and long‐term consequences of genome doubling on the additive genetic variance of populations. To do so, I extended the classical models of quantitative traits under stabilizing selection to study the evolution of the additive variance of the trait under study after a shift from diploidy to tetraploidy. I found that, for realistic allele‐dosage effects, polyploidization is associated with an initial decrease in adaptive potential. In the long term, the better masking of recessive deleterious mutations associated with polyploidy compensates for the initial decrease in additive variance. The time for the tetraploid populations to reach or exceed the additive variance of their diploid progenitors is generally lower than 200 generations. These results highlight that polyploidization per se has a negligible negative effect on the adaptive potential of populations in the short term, and a substantial positive effect in the long term.

Simultaneous selection on vegetative and reproductive phenology in a perennial herb

The timing of different life-history events is often correlated, and selection might only rarely be exerted independently on the timing of a single event. In plants, phenotypic selection has often been shown to favor earlier flowering. However, little is known about to what extent this selection acts directly versus indirectly via vegetative phenology, and if selection on the two traits is correlational. We estimated direct, indirect, and correlational phenotypic selection on vegetative and reproductive phenology over 3 years for flowering individuals of the perennial herb Lathyrus vernus. Direct selection favored earlier flowering and shorter timespans between leaf-out and flowering in all years. However, early flowering was associated with early leaf-out, and the direction of selection on leaf-out day varied among years. As a result, selection on leaf-out weakened selection for early flowering in one of the study years. We found no evidence of correlational selection. Our results highlight the importance of including temporally correlated traits when exploring selection on the phenology of seasonal events.

Chronic selection for early reproductive phenology in an annual plant across a steep, elevational gradient of growing season length

Colonization along ubiquitous gradients of growing season length should require adaptation of phenological traits, driven by natural selection. Although phenology often varies with season length and genetic differentiation in phenological traits sometimes seems adaptive, few studies test whether natural selection is responsible for these patterns. The annual plant Rhinanthus minor is genetically differentiated for phenology across a 1000-m elevational gradient of growing season length in the Canadian Rocky Mountains. We estimated phenotypic selection on five phenological traits for three generations of naturally occurring individuals at 12 sites (n = 10,112), and two generations of genetically and phenotypically more variable transplanted populations at nine of these sites (n = 24,611). Selection was weak for most traits, but consistently favored early flowering across the gradient rather than only under short seasons. There was no evidence that apparent selection favoring early reproduction arose from failure to consider all components of fitness, or variation in other correlated phenological traits. Instead, selection for earlier flowering may be balanced by selection for strong cogradient phenological plasticity that indirectly favors later flowering. However, this probably does not explain the consistency of selection on flowering time across this steep, elevational gradient of growing season length.

Assortative mating can help adaptation of flowering time to a changing climate: Insights from a polygenic model

Several empirical studies report fast evolutionary changes in flowering time in response to contemporary climate change. Flowering time is a polygenic trait under assortative mating, since flowering time of mates must overlap. Here, we test whether assortative mating, compared with random mating, can help better track a changing climate. For each mating pattern, our individual‐based model simulates a population evolving in a climate characterized by stabilizing selection around an optimal flowering time, which can change directionally and/or fluctuate. We also derive new analytical predictions from a quantitative genetics model for the expected genetic variance at equilibrium, and its components, the lag of the population to the optimum and the population mean fitness. We compare these predictions between assortative and random mating, and to our simulation results. Assortative mating, compared with random mating, has antagonistic effects on genetic variance: it generates positive associations among similar allelic effects, which inflates the genetic variance, but it decreases genetic polymorphism, which depresses the genetic variance. In a stationary environment with substantial stabilizing selection, assortative mating affects little the genetic variance compared with random mating. In a changing climate, assortative mating however increases genetic variance compared to random mating, which diminishes the lag of the population to the optimum, and in most scenarios translates into a fitness advantage relative to random mating. The magnitude of this fitness advantage depends on the extent to which genetic variance limits adaptation, being larger for faster environmental changes and weaker stabilizing selection.

Flowering phenology in a Eucalyptus loxophleba seed orchard, heritability and genetic correlation with biomass production and cineole: breeding strategy implications

Reproductive synchronicity within a seed orchard facilitates gene exchange and reduces self-fertilisation. Here we assessed key flowering traits, biomass and foliar 1,8-cineole concentrations of Eucalyptus loxophleba (subsp. lissophloia and gratiae) in an open-pollinated seed orchard. Monthly flowering observations were made on 1142 trees from 60 families and nine provenances across 2 years. The percentage of trees flowering in both years was similar at 87%. There were differences between provenances and families within provenances for flowering traits, biomass and 1,8-cineole and interactions between provenances and year for flowering traits. Heritability of start and end flowering, and 1,8-cineole were high to moderate (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{h}^{2}$$\end{document}h^2 = 0.75–0.45) and duration of flowering, propensity to flower and biomass estimates were moderate to low (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{h}^{2}$$\end{document}h^2 = 0.31–0.10). Genetic and phenotypic correlations between flowering traits were high (r_g = 0.96–0.63 and r_p = 0.93–0.34) except between duration and end of flowering. The correlations were weaker between flowering traits and biomass or 1,8-cineole. ‘Dual flowering’, when trees underwent two reproductive cycles in a year, was responsible for out-of-phase flowering and those with low biomass and 1,8-cineole concentration should be removed from the breeding programme to hasten selection for desirable traits.

Could seasonally deteriorating environments favour the evolution of autogamous selfing and a drought escape physiology through indirect selection? A test of the time limitation hypothesis using artificial selection in Clarkia

BACKGROUND AND AIMS

The evolution of selfing from outcrossing may be the most common transition in plant reproductive systems and is associated with a variety of ecological circumstances and life history strategies. The most widely discussed explanation for these associations is the reproductive assurance hypothesis - the proposition that selfing is favoured because it increases female fitness when outcross pollen receipt is limited. Here an alternative explanation, the time limitation hypothesis, is addressed, one scenario of which proposes that selfing may evolve as a correlated response to selection for a faster life cycle in seasonally deteriorating environments.

METHODS

Artificial selection for faster maturation (early flowering) or for low herkogamy was performed on Clarkia unguiculata (Onagraceae), a largely outcrossing species whose closest relative, C. exilis, has evolved higher levels of autogamous selfing. Direct responses to selection and correlated evolutionary changes in these traits were measured under greenhouse conditions. Direct responses to selection on early flowering and correlated evolutionary changes in the node of the first flower, herkogamy, dichogamy, gas exchange rates and water use efficiency (WUE) were measured under field conditions.

KEY RESULTS

Lines selected for early flowering and for low herkogamy showed consistent, statistically significant responses to direct selection. However, there was little or no evidence of correlated evolutionary changes in flowering date, floral traits, gas exchange rates or WUE.

CONCLUSIONS

These results suggest that the maturation rate and mating system have evolved independently in Clarkia and that the time limitation hypothesis does not explain the repeated evolution of selfing in this genus, at least through its indirect selection scenario. They also suggest that the life history and physiological components of drought escape are not genetically correlated in Clarkia, and that differences in gas exchange physiology between C. unguiculata and C. exilis have evolved independently of differences in mating system and life history.

Explaining the apparent paradox of persistent selection for early flowering

Decades of observation in natural plant populations have revealed pervasive phenotypic selection for early flowering onset. This consistent pattern seems at odds with life-history theory, which predicts stabilizing selection on age and size at reproduction. Why is selection for later flowering rare? Moreover, extensive evidence demonstrates that flowering time can and does evolve. What maintains ongoing directional selection for early flowering? Several non-mutually exclusive processes can help to reconcile the apparent paradox of selection for early flowering. We outline four: selection through other fitness components may counter observed fecundity selection for early flowering; asymmetry in the flowering-time-fitness function may make selection for later flowering hard to detect; flowering time and fitness may be condition-dependent; and selection on flowering duration is largely unaccounted for. In this Viewpoint, we develop these four mechanisms, and highlight areas where further study will improve our understanding of flowering-time evolution.

Evidence that a herbivore tolerance response affects selection on floral traits and inflorescence architecture in purple loosestrife (Lythrum salicaria)

BACKGROUND AND AIMS

The study of the evolution of floral traits has generally focused on pollination as the primary driver of selection. However, herbivores can also impose selection on floral traits through a variety of mechanisms, including florivory and parasitism. Less well understood is whether floral and inflorescence architecture traits that influence a plant's tolerance to herbivory, such as compensatory regrowth, alter pollinator-mediated selection.

METHODS

Because herbivore damage to Lythrum salicaria meristems typically leads to an increase in the number of inflorescences and the size of the floral display, an experiment was conducted to test whether simulated herbivory (i.e. clipping the developing meristem) could alter the magnitude or direction of pollinator-mediated selection on a suite of floral and inflorescence architecture traits. Using a pollen supplementation protocol, pollen limitation was compared in the presence and absence of meristem damage in order to quantify any interaction between pollinator and herbivore-mediated selection on floral traits.

KEY RESULTS

Surprisingly, in spite of an obvious impact on floral display and architecture, with clipped plants producing more inflorescences and more flowers, there was no difference in pollen limitation between clipped and unclipped plants. Correspondingly, there was no evidence that imposing herbivore damage altered pollinator-mediated selection in this system. Rather, the herbivory treatment alone was found to alter direct selection on floral display, with clipped plants experiencing greater selection for earlier flowering and weaker selection for number of inflorescences when compared with unclipped plants.

CONCLUSIONS

These findings imply that herbivory on its own can drive selection on plant floral traits and inflorescence architecture in this species, even more so than pollinators. Specifically, herbivory can impose selection on floral traits if such traits influence a plant's tolerance to herbivory, such as through the timing of flowering and/or the compensatory regrowth response.

Constraints imposed by pollinator behaviour on the ecology and evolution of plant mating systems

Most flowering plants rely on pollinators for their reproduction. Plant-pollinator interactions, although mutualistic, involve an inherent conflict of interest between both partners and may constrain plant mating systems at multiple levels: the immediate ecological plant selfing rates, their distribution in and contribution to pollination networks, and their evolution. Here, we review experimental evidence that pollinator behaviour influences plant selfing rates in pairs of interacting species, and that plants can modify pollinator behaviour through plastic and evolutionary changes in floral traits. We also examine how theoretical studies include pollinators, implicitly or explicitly, to investigate the role of their foraging behaviour in plant mating system evolution. In doing so, we call for more evolutionary models combining ecological and genetic factors, and additional experimental data, particularly to describe pollinator foraging behaviour. Finally, we show that recent developments in ecological network theory help clarify the impact of community-level interactions on plant selfing rates and their evolution and suggest new research avenues to expand the study of mating systems of animal-pollinated plant species to the level of the plant-pollinator networks.

Evidence for pollen limitation of a native plant in invaded communities

Animal-pollinated invasive species have frequently been demonstrated to outcompete native species for pollinator attention, which can have detrimental effects on the reproductive success and population dynamics of native species. Many animal-pollinated invasive species exhibit showy flowers and provide substantial rewards, allowing them to act as pollinator 'magnets', which, at a large scale, can attract more pollinators to an area, but, at a smaller scale, may reduce compatible pollen flow to local native species, possibly explaining why most studies detect competition. By performing pollen limitation experiments of populations in both invaded and uninvaded sites, we demonstrate that the invasive plant Lythrum salicaria appears to facilitate, rather than hinder, the reproductive success of native confamilial Decodon verticillatus, even at a small scale, in a wetland habitat in southeastern Ontario. We found no evidence for a magnet species effect on pollinator attraction to invaded sites. Germination experiments confirmed that seeds from invaded sites had similar germination rates to those from uninvaded sites, making it unlikely that a difference in inbreeding was masking competitive effects. We describe several explanations for our findings. Notably, there were no differences in seed set among populations at invaded and uninvaded sites. Our results underscore the inherent complexity of studying the ecological impacts of invasive species on natives.

Whole genome duplication affects evolvability of flowering time in an autotetraploid plant

Whole genome duplications have occurred recurrently throughout the evolutionary history of eukaryotes. The resulting genetic and phenotypic changes can influence physiological and ecological responses to the environment; however, the impact of genome copy number on evolvability has rarely been examined experimentally. Here, we evaluate the effect of genome duplication on the ability to respond to selection for early flowering time in lines drawn from naturally occurring diploid and autotetraploid populations of the plant Chamerion angustifolium (fireweed). We contrast this with the result of four generations of selection on synthesized neoautotetraploids, whose genic variability is similar to diploids but genome copy number is similar to autotetraploids. In addition, we examine correlated responses to selection in all three groups. Diploid and both extant tetraploid and neoautotetraploid lines responded to selection with significant reductions in time to flowering. Evolvability, measured as realized heritability, was significantly lower in extant tetraploids (^b(T) =  0.31) than diploids (^b(T) =  0.40). Neotetraploids exhibited the highest evolutionary response (^b(T)  =  0.55). The rapid shift in flowering time in neotetraploids was associated with an increase in phenotypic variability across generations, but not with change in genome size or phenotypic correlations among traits. Our results suggest that whole genome duplications, without hybridization, may initially alter evolutionary rate, and that the dynamic nature of neoautopolyploids may contribute to the prevalence of polyploidy throughout eukaryotes.

The evolutionary potential of Baker's weediness traits in the common morning glory, Ipomoea purpurea (Convolvulaceae)

PREMISE OF THE STUDY

Many reports have cited Baker's list of weediness traits, or those that exemplify the "ideal" weed, yet few have considered the evolutionary potential of such traits as a group. Thus, it is unknown whether constraints on the evolution of increased weediness, such as a lack of genetic variation or genetic correlations between the traits, are present. Ipomoea purpurea, the common morning glory, is a problematic weed that exhibits many of Baker's ideal weed traits.

METHODS

We used progeny from a half/full-sib breeding design in a series of three greenhouse experiments to assess the presence of genetic variation, narrow sense heritabilities, and genetic correlations in Baker's growth, competition, and fitness "weediness" traits in two populations of I. purpurea.

KEY RESULTS

We uncovered genetic variation underlying reproductive fitness traits and competitive ability in at least one population, but no evidence of genetic variation underlying growth rate in either population. Genetic correlations between many of the weediness characters differed significantly from zero; however, their direction and/or magnitude differed between populations.

CONCLUSIONS

We found that increased weediness in the common morning glory is more likely to occur through selection on reproductive output and competitive ability rather than through selection on growth rate. Assessing Baker's traits in a quantitative genetics framework can provide a solid perspective on their evolutionary potential and a unique framework within which to determine how weeds will respond to different environmental stresses and/or scenarios of global climate change.

Fitness effects of floral plasticity and thermoregulation in a thermally changing environment

To better understand the evolution of phenotypic plasticity and thermoregulation and their potential value for ectotherms in the face of global warming, we conducted field experiments to measure their effects on fitness and their association with reproductive phenology in Plantago lanceolata in a thermally variable environment. We measured the reproductive timing and success of genotypes varying in thermoregulation, as mediated by floral-reflectance plasticity. Results were consistent with the hypothesis that thermoregulation is more adaptive when thermally variable reproductive seasons are shorter and cooler. Strong thermoregulation/plasticity increased reproductive success during the cool portion of the reproductive season but not during the warm portion. Directional selection that favored strongly thermoregulating genotypes early in the season shifted to stabilizing selection that favored genotypes with weaker thermoregulation later in the season. Thermoregulation and reproductive phenology were negatively correlated. Although reproductive onset and duration were similar between genotypes, strong thermoregulators produced more and larger spikes (clutches) early; weak thermoregulators produced more spikes late. Results suggest that with atmospheric warming, the benefit of raising body temperature via thermoregulation when it is cool should decline in extant populations. The negative correlation between thermoregulation and phenology should accelerate the evolutionary shift toward thermoconformity, that is, reduced plasticity.

Phenotypic plasticity and adaptive evolution contribute to advancing flowering phenology in response to climate change

Anthropogenic climate change has already altered the timing of major life-history transitions, such as the initiation of reproduction. Both phenotypic plasticity and adaptive evolution can underlie rapid phenological shifts in response to climate change, but their relative contributions are poorly understood. Here, we combine a continuous 38 year field survey with quantitative genetic field experiments to assess adaptation in the context of climate change. We focused on Boechera stricta (Brassicaeae), a mustard native to the US Rocky Mountains. Flowering phenology advanced significantly from 1973 to 2011, and was strongly associated with warmer temperatures and earlier snowmelt dates. Strong directional selection favoured earlier flowering in contemporary environments (2010-2011). Climate change could drive this directional selection, and promote even earlier flowering as temperatures continue to increase. Our quantitative genetic analyses predict a response to selection of 0.2 to 0.5 days acceleration in flowering per generation, which could account for more than 20 per cent of the phenological change observed in the long-term dataset. However, the strength of directional selection and the predicted evolutionary response are likely much greater now than even 30 years ago because of rapidly changing climatic conditions. We predict that adaptation will likely be necessary for long-term in situ persistence in the context of climate change.

Adaptive value of phenological traits in stressful environments: predictions based on seed production and laboratory natural selection

Phenological traits often show variation within and among natural populations of annual plants. Nevertheless, the adaptive value of post-anthesis traits is seldom tested. In this study, we estimated the adaptive values of pre- and post-anthesis traits in two stressful environments (water stress and interspecific competition), using the selfing annual species Arabidopsis thaliana. By estimating seed production and by performing laboratory natural selection (LNS), we assessed the strength and nature (directional, disruptive and stabilizing) of selection acting on phenological traits in A. thaliana under the two tested stress conditions, each with four intensities. Both the type of stress and its intensity affected the strength and nature of selection, as did genetic constraints among phenological traits. Under water stress, both experimental approaches demonstrated directional selection for a shorter life cycle, although bolting time imposes a genetic constraint on the length of the interval between bolting and anthesis. Under interspecific competition, results from the two experimental approaches showed discrepancies. Estimation of seed production predicted directional selection toward early pre-anthesis traits and long post-anthesis periods. In contrast, the LNS approach suggested neutrality for all phenological traits. This study opens questions on adaptation in complex natural environment where many selective pressures act simultaneously.

Evolutionary constraints on adaptive evolution during range expansion in an invasive plant

Biological invasions may expose populations to strong selection for local adaptation along geographical gradients in climate. However, evolution during contemporary timescales can be constrained by low standing genetic variation and genetic correlations among life-history traits. We examined limits to local adaptation associated with northern migration of the invasive wetland plant purple loosestrife (Lythrum salicaria) using a selection model incorporating a trade-off between flowering time and size at reproduction, and common garden experiments of populations sampled along a latitudinal transect of approximately 1200 km in eastern North America. A strong trade-off between flowering time and size at reproduction caused early-flowering plants to be smaller with reduced seed production in northern populations. Northward spread was associated with a decline in genetic variance within populations and an increase in genetic skew for flowering time and size, with limited genetic variation for small, early-flowering genotypes. These patterns were predicted by our selection model of local adaptation to shorter growing seasons and were not consistent with expectations from non-adaptive processes. Reduced fecundity may limit population growth and rates of spread in northern populations. Identifying genetic constraints on key life-history traits can provide novel insights into invasion dynamics and the causes of range limits in introduced species.

Selection on variance in flowering time within and among individuals

We model the impact of pollinator visitation rate and behavior on the short-term evolution of population flowering phenologies determined by the distributions of flowering times within and among individual plants. Evolution of population flowering phenologies depends on the phenotypic variances and heritabilities of the within-individual mean and variance of flowering time. In the ecological scenarios we investigate selection does not produce a correlation of the mean and variance of individual flowering time. Self-incompatibility causes weak stabilizing selection on flowering time that acts to reduce the within-individual variance in flowering time. Disruptive selection due to pollinator limitation acts mostly to increase the among-individual variance in flowering time. Stabilizing selection due to pollinator attraction, or short reproductive season, acts mostly to decrease the within-individual variance in flowering time. Temporal autocorrelation of environmental stochasticity in pollinator visitation rate strongly selects to increase the within-individual variance in flowering time. These predictions can be tested by measuring the causal factors described above, partitioning the variance in population phenology within and among individuals, and estimating the inheritance of, and selection on, within-individual mean and variance of flowering time.

Displacement of flowering phenologies among plant species by competition for generalist pollinators

We model the evolution of allochronic isolation between sympatric animal-pollinated plant species via displacement of their flowering times. The plant species share generalist pollinators and either produce inviable hybrid seeds or do not hybridize at all. Displacement of flowering times between reproductively isolated species reduces competition for pollinators and the formation of inviable hybrid seeds. Under strong pollen limitation, competition for pollinators causes rapid evolution of allochronic isolation both for hybridizing and nonhybridizing species. Under weak pollen limitation, allochronic isolation evolves rapidly for hybridizing species but more slowly for nonhybridizing species. Positive density-dependent pollinator visitation rate at low flower densities facilitates allochronic isolation under weak pollen limitation. Allochronic isolation among sympatric species sharing generalist pollinators could be common under any intensity of pollen limitation if the flowering season is sufficiently long.

Incipient allochronic speciation due to non-selective assortative mating by flowering time, mutation and genetic drift

We model the evolution of flowering time using a multilocus quantitative genetic model with non-selective assortative mating and mutation to investigate incipient allochronic speciation in a finite population. For quantitative characters with evolutionary parameters satisfying empirical observations and two approximate inequalities that we derived, disjunct clusters in the population flowering phenology originated within a few thousand generations in the absence of disruptive natural or sexual selection. Our simulations and the conditions we derived showed that cluster formation was promoted by limited population size, high mutational variance of flowering time, short individual flowering phenology and a long flowering season. By contrast, cluster formation was hindered by inbreeding depression, stabilizing selection and pollinator limitation. Our results suggest that incipient allochronic speciation in populations of limited size (satisfying two inequalities) could be a common phenomenon.

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.

The strength of phenotypic selection in natural populations

How strong is phenotypic selection on quantitative traits in the wild? We reviewed the literature from 1984 through 1997 for studies that estimated the strength of linear and quadratic selection in terms of standardized selection gradients or differentials on natural variation in quantitative traits for field populations. We tabulated 63 published studies of 62 species that reported over 2,500 estimates of linear or quadratic selection. More than 80% of the estimates were for morphological traits; there is very little data for behavioral or physiological traits. Most published selection studies were unreplicated and had sample sizes below 135 individuals, resulting in low statistical power to detect selection of the magnitude typically reported for natural populations. The absolute values of linear selection gradients |beta| were exponentially distributed with an overall median of 0.16, suggesting that strong directional selection was uncommon. The values of |beta| for selection on morphological and on life-history/phenological traits were significantly different: on average, selection on morphology was stronger than selection on phenology/life history. Similarly, the values of |beta| for selection via aspects of survival, fecundity, and mating success were significantly different: on average, selection on mating success was stronger than on survival. Comparisons of estimated linear selection gradients and differentials suggest that indirect components of phenotypic selection were usually modest relative to direct components. The absolute values of quadratic selection gradients |gamma| were exponentially distributed with an overall median of only 0.10, suggesting that quadratic selection is typically quite weak. The distribution of gamma values was symmetric about 0, providing no evidence that stabilizing selection is stronger or more common than disruptive selection in nature.

Estimating nonlinear selection gradients using quadratic regression coefficients: double or nothing?

The use of regression analysis has been instrumental in allowing evolutionary biologists to estimate the strength and mode of natural selection. Although directional and correlational selection gradients are equal to their corresponding regression coefficients, quadratic regression coefficients must be doubled to estimate stabilizing/disruptive selection gradients. Based on a sample of 33 papers published in Evolution between 2002 and 2007, at least 78% of papers have not doubled quadratic regression coefficients, leading to an appreciable underestimate of the strength of stabilizing and disruptive selection. Proper treatment of quadratic regression coefficients is necessary for estimation of fitness surfaces and contour plots, canonical analysis of the gamma matrix, and modeling the evolution of populations on an adaptive landscape.

Plant reproductive systems and evolution during biological invasion

Recent biological invasions provide opportunities to investigate microevolution during contemporary timescales. The tempo and scope of local adaptation will be determined by the intensity of natural selection and the amounts and kinds of genetic variation within populations. In flowering plants, genetic diversity is strongly affected by interactions between reproductive systems and stochastic forces associated with immigration history and range expansion. Here, we explore the significance of reproductive system diversity for contemporary evolution during plant invasion. We focus in particular on how reproductive modes influence the genetic consequences of long-distance colonization and determine the likelihood of adaptive responses during invasion. In many clonal invaders, strong founder effects and restrictions on sexual reproduction limit opportunities for local adaptation. In contrast, adaptive changes to life-history traits should be a general expectation in both outbreeding and inbreeding species. We provide evidence that evolutionary modifications to reproductive systems promote the colonizing ability of invading populations and that reproductive timing is an important target of selection during range expansion. Knowledge of the likelihood and speed at which local adaptation evolves in invasive plants will be particularly important for management practices when evolutionary changes enhance ecological opportunities and invasive spread.

Re-establishment of clinal variation in flowering time among introduced populations of purple loosestrife (Lythrum salicaria, Lythraceae)

Range expansion during biological invasion requires that invaders adapt to geographical variation in climate, which should yield latitudinal clines in reproductive phenology. We investigated geographic variation in life history among 25 introduced populations of Lythrum salicaria, a widespread European invader of North American wetlands. We detected a strong latitudinal cline in initiation of flowering and size at flowering, which paralleled that reported among native populations. Plants from higher latitudes flowered earlier and at a smaller size than those from lower latitudes, even when raised in a uniform glasshouse. Early flowering was associated with greatly reduced reproductive output, but this was not associated with latitudinal variation in abundance, and probably did not result from a genetic correlation between time to and size at flowering. As introduction to North America c. 200 years ago, L. salicaria has re-established latitudinal clines in life history, probably as an evolutionary response to climatic selection.

Artificial selection shifts flowering phenology and other correlated traits in an autotetraploid herb

There is mounting evidence that plants are responding to anthropogenic climate change with shifts in flowering phenologies. We conducted a three-generation artificial selection experiment on flowering time in Campanulastrum americanum, an autotetraploid herb, to determine the potential for adaptive evolution of this trait as well as possible costs associated with enhanced or delayed flowering. Divergent selection for earlier and later flowering resulted in a 25-day difference in flowering time. Experiment-wide heritability was 0.31 and 0.23 for the initiation of flowering in early and late lines, respectively. Selection for earlier flowering resulted in significant correlated responses in other traits including smaller size, fewer branches, smaller floral displays, longer fruit maturation times, fewer seeds per fruit and slower seed germination. Results suggest that although flowering time shows the potential to adapt to a changing climate, phenological shifts may be associated with reduced plant fitness possibly hindering evolutionary change.

Flowering time and elevated atmospheric CO2

Flowering is a critical milestone in the life cycle of plants, and changes in the timing of flowering may alter processes at the species, community and ecosystem levels. Therefore understanding flowering-time responses to global change drivers, such as elevated atmospheric carbon dioxide concentrations, [CO(2)], is necessary to predict the impacts of global change on natural and agricultural ecosystems. Here we summarize the results of 60 studies reporting flowering-time responses (defined as the time to first visible flower) of both crop and wild species at elevated [CO(2)]. These studies suggest that elevated [CO(2)] will influence flowering time in the future. In addition, interactions between elevated [CO(2)] and other global change factors may further complicate our ability to predict changes in flowering time. One approach to overcoming this problem is to elucidate the primary mechanisms that control flowering-time responses to elevated [CO(2)]. Unfortunately, the mechanisms controlling these responses are not known. However, past work has indicated that carbon metabolism exerts partial control on flowering time, and therefore may be involved in elevated [CO(2)]-induced changes in flowering time. This review also indicates the need for more studies addressing the effects of global change drivers on developmental processes in plants.

Genetic constraints on floral evolution: a review and evaluation of patterns

The characteristics of flowers influence most aspects of angiosperm reproduction, including the agents of pollination and patterns of mating. Thus, a clear view of the forces that mediate floral phenotypic evolution is central to understanding angiosperm diversity. Here, we inform on the capacity for floral phenotype to respond to selection by reviewing published data on heritabilities and genetic correlations for several classes of floral traits (primary sexual, attraction, mating system) in hermaphroditic plants. We find significant heritability for all floral traits but also variation among them, as well as a tendency for heritability to vary with mating system, but not life history. We additionally test predictions stemming from life history theory (eg, negative covariation between male-female traits and flower size-flower number), and ideas concerning the extent and pattern of genetic integration between flowers and leaves, and between the sexes of dioecious and gynodioecious species. We find mixed evidence for life history tradeoffs. We find strong support for floral integration and its relation with floral morphology (actinomorphy vs zygomorphy) and for a decoupling of floral and vegetative traits, but no evidence that modular integration varies with floral morphology. Lastly, we find mixed evidence for a relationship between the level of sexual dimorphism in attraction traits and the between-sex correlation in gender dimorphic plants.

Population structure attributable to reproductive time: isolation by time and adaptation by time

Many populations are composed of a mixture of individuals that reproduce at different times, and these times are often heritable. Under these conditions, gene flow should be limited between early and late reproducers, even within populations having a unimodal temporal distribution of reproductive activity. This temporal restriction on gene flow might be called "isolation by time" (IBT) to acknowledge its analogy with isolation by distance (IBD). IBD and IBT are not exactly equivalent, however, owing to differences between dispersal in space and dispersal in time. We review empirical studies of natural populations that provide evidence for IBT based on heritabilities of reproductive time and on molecular genetic differences associated with reproductive time. When IBT is present, variation in selection through the reproductive season may lead to adaptive temporal variation in phenotypic traits [adaptation by time (ABT)]. We introduce a novel theoretical model that shows how ABT increases as (i) selection on the trait increases; (ii) environmental influences on reproductive time decrease; (iii) the heritability of reproductive time increases; and (iv) the temporal distribution of reproductive activity becomes increasingly uniform. We then review empirical studies of natural populations that provide evidence for ABT by documenting adaptive temporal clines in phenotypic traits. The best evidence for IBT and ABT currently comes from salmonid fishes and flowering plants, but we expect that future work will show these processes are more widespread.

Inbreeding depression in self-incompatible and self-compatible populations of Leavenworthia alabamica

Inbreeding depression is one of the leading factors preventing the evolution of self-fertilization in plants. In populations where self-fertilization evolves, theory suggests that natural selection against partially recessive deleterious alleles will reduce inbreeding depression. The purpose of this study was to evaluate this hypothesis by comparing the magnitude of inbreeding depression in self-incompatible and self-compatible populations of Leavenworthia alabamica. Within-population crosses were conducted to compare the quantity and quality of offspring produced by outcrossing and self-fertilization. These progeny were grown in a common greenhouse and inbreeding depression was measured in germination, survival, biomass, transition rate to flowering, flower number, petal length, pollen grains/anther, pollen viability, and ovule number. In comparison to outcrossing, self-fertilization led to the production of fewer and smaller seeds within self-incompatible populations. Moreover, inbreeding depression was observed in eight of 11 offspring traits within self-incompatible populations of L. alabamica. In contrast, there was significant inbreeding depression only in flower number within self-compatible populations. The results of this study are consistent with the idea that self-fertilization selectively removes partially recessive deleterious alleles causing inbreeding depression in natural plant populations. However, in plant species such as L. alabamica where self-compatibility may evolve in small populations following long-distance dispersal, declines in inbreeding depression may also be facilitated by genetic drift.

Genetic variation in flowering time induces phenological assortative mating: quantitative genetic methods applied to Brassica rapa

It has been argued from first principles that plants mate assortatively by flowering time. However, there have been very few studies of phenological assortative mating, perhaps because current methods to infer paternal phenotype are difficult to apply to natural populations. Two methods are presented to estimate the phenotypic correlation between mates-the quantitative genetic metric for assortative mating-for phenological traits. The first method uses individual flowering schedules to estimate mating probabilities for every potential pairing in a sample. These probabilities are then incorporated into a weighted phenotypic correlation between all potential mates and thus yield a prospective estimate based on mating opportunities. The correlation between mates can also be estimated retrospectively by comparing the regression of offspring phenotype over one parent, which is inflated by assortative mating, to the regression over mid-parent, which is not. In a demonstration experiment with Brassica rapa, the prospective correlation between flowering times (days from germination to anthesis) of pollen recipients and their potential donors was 0.58. The retrospective estimate of this correlation strongly agreed with the prospective estimate. The prospective method is easily employed in field studies that explore the effect of phenological assortative mating on selection response and population differentiation.

Delayed and carryover effects of salinity on flowering in Iris hexagona (Iridaceae)

Saltwater intrusion into wetland ecosystems has destroyed or damaged many native plant populations. Iris hexagona is a salt-sensitive species that exhibits intraspecific variation in salinity tolerance. To investigate the effect of salinity on flowering, we exposed I. hexagona collected from natural populations to salt treatments in a common garden. Experimental salinity additions strongly delayed flowering phenology, but the effect was not apparent until the second year, when less than 4 g/L NaCl delayed flowering up to 3 d. In the field, soil salinity and flowering phenology varied substantially within I. hexagona populations. Iris flowers are receptive to pollinators for 2 d or less, therefore a 3-d delay could affect outcrossing dynamics, and ultimately, the evolutionary ecology of iris populations. Salinity also had a carryover effect; prior salinity exposure delayed flowering in irises that had been replanted in freshwater conditions for 6 mo. This is an important result because it suggests that episodic stress (such as tropical storms) can influence performance well after the stress has disappeared. Our research further underscores the importance of long-term studies because a 1-yr experiment would have failed to reveal the strong effects of salinity that emerged in the second year.