Analysis of trait–performance–fitness relationships reveals pollinator-mediated selection on orchid pollination traits

Premise

The role of pollinators in evolutionary floral divergence has spurred substantial effort into measuring pollinator-mediated phenotypic selection and its variation in space and time. For such estimates, the fitness consequences of pollination processes must be separated from other factors affecting fitness.

Methods

We built a fitness function linking phenotypic traits of food-deceptive orchids to female reproductive success by including pollinator visitation and pollen deposition as intermediate performance components and used the fitness function to estimate the strength of pollinator-mediated selection through female reproductive success. We also quantified male performance as pollinarium removal and assessed similarity in trait effects on male and female performance.

Results

The proportion of plants visited at least once by an effective pollinator was moderate to high, ranging from 53.7% to 85.1%. Tall, many-flowered plants were often more likely to be visited and pollinated. Given effective pollination, pollen deposition onto stigmas tended to be more likely for taller plants. Pollen deposition further depended on traits affecting the physical fit of pollinators to flowers (flower size, spur length), though the exact relationships varied in time and space. Using the fitness function to assess pollinator-mediated selection through female reproductive success acting on multiple traits, we found that selection varied detectably among taxa after accounting for sampling uncertainty. Across taxa, selection on most traits was stronger on average and more variable when pollination was less reliable.

Conclusions

These results support pollination-related trait–performance–fitness relationships and thus pollinator-mediated selection on traits functionally involved in the pollination process.     

Island–mainland difference in <Emphasis Type="Italic">Nicotiana glauca</Emphasis> (Solanaceae) corolla length: a product of pollinator-mediated selection?

A match between floral and pollinator traits, such as that between unique island plants and pollinators, is often thought to be the product of pollinator-mediated selection. I examined whether the floral morphology of an introduced hummingbird-pollinated plant, Nicotiana glauca (tree tobacco, Solanaceae), is under selection by pollinators on the California Channel Islands where it is a recent colonist. I first determined differences in floral morphology and pollinator composition between island and mainland populations of N. glauca. I found that island plants have detectably longer corollas (on average 1 mm) and are visited by hummingbird species with on average 1–2 mm longer bills than common mainland visitors. Corolla length differences were not found to be associated with site abiotic differences. Flower size does not vary consistently with season and corolla width is very consistent across sites. I tested whether island–mainland corolla length differences are the product of pollinator-mediated selection by measuring phenotypic selection and per visit effectiveness. Contrary to expectations, a longer corolla was not consistently associated with higher pollen transfer or seed count on the islands. Per visit effectiveness of longer and shorter-billed hummingbirds did differ; however, effectiveness did not depend on corolla length. Although I failed to detect expected patterns of selection for longer corollas on islands, I cannot rule out weak or past pollinator-mediated selection. It is also possible that despite the apparent match between pollinator and floral traits, island–mainland differences in corolla length are instead due to other environmental effects, selection unrelated to pollinators, or stochastic processes such as 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.     

Floral density and co-occurring congeners alter patterns of selection in annual plant communities*

Although the evolution and diversification of flowers is often attributed to pollinator-mediated selection, interactions between co-occurring plant species can alter patterns of selection mediated by pollinators and other agents. The extent to which both floral density and congeneric species richness affect patterns of net and pollinator-mediated selection on multiple co-occurring species in a community is unknown and is likely to depend on whether co-occurring plants experience competition or facilitation for reproduction. We conducted an observational study of selection on four species of Clarkia (Onagraceae) and tested for pollinator-mediated selection on two Clarkia species in communities differing in congeneric species richness and local floral density. When selection varied with community context, selection was generally stronger in communities with fewer species, where local conspecific floral density was higher, and where local heterospecific floral density was lower. These patterns suggest that intraspecific competition at high densities and interspecific competition at low densities may affect the evolution of floral traits. However, selection on floral traits was not pollinator mediated in Clarkia cylindrica or Clarkia xantiana, despite variation in pollinator visitation and the extent of pollen limitation across communities for C. cylindrica. As such, interactions between co-occurring species may alter patterns of selection mediated by abiotic agents of selection.     

Flower power: its association with bee power and floral functional morphology in papilionate legumes

Background and Aims

A test was made of the hypothesis that papilionate legume flowers filter pollinators according to their ability to exert strength to open flowers to access rewards. In addition, interactions with pollen vectors were expected to explain the structural complexity of the architecture of these flowers since operative flower strength may be determined by a combination of morphological traits which form part of an intrafloral functional module.

Methods

Six papilionate species were studied: Collaea argentina, Desmodium uncinatum, Galactia latisiliqua, Lathyrus odoratus, Spartium junceum and Tipuana tipu. Measurements were made of the strength needed to open keels and the strength that pollinators were capable of exerting. Morphological traits of all petals were also measured to determine which of them could be either mutually correlated or correlated with operative strength and moment of strength and participated in a functional module.

Key Results

It was observed that pollinators were capable in all cases of exerting forces higher and often several times higher than that needed to access floral rewards, and no association could be detected between floral operative strength and strength exerted by the corresponding pollinators. On the other hand, strong and significant correlations were found among morphometric traits and, of these, with operative strength and moment. This was particularly evident among traits of the keel and the wings, presumably involved in the functioning of the floral moveable mechanism.

Conclusions

Though visitors are often many times stronger than the operative strength of the flowers they pollinate, exceptionally weak bees such as Apis mellifera cannot open the strongest flowers. On the other hand, strong correlations among certain petal morphometric traits (particularly between the keel and wings) give support to the idea that an intrafloral module is associated with the functioning of the mechanism of these legume flowers. In addition, the highly significant correlations found across petals support the view of functional phenotypic integration transcending the ontogenetic organization of flower structure.     

The influence of floral symmetry,dependence on pollinators and pollination generalization on flower size variation

Background and Aims

The pollinator-mediated stabilizing selection hypothesis suggests that the specialized pollination system of zygomorphic flowers might cause stabilizing selection, reducing their flower size variation compared with actinomorphic flowers. However, the degree of ecological generalization and of dependence on pollinators varies greatly among species of both flower symmetry types and this may also affect flower size variation.

Methods

Data on 43 species from two contrasting communities (one alpine and one lowland community) were used to test the relationships and interactions between flower size phenotypic variation, floral symmetry, ecological pollination generalization and species'' dependence on pollinators.

Key Results

Contrary to what was expected, higher flower size variation was found in zygomorphic than in actinomorphic species in the lowland community, and no difference in flower size variation was found between symmetry types in the alpine community. The relationship between floral symmetry and flower size variation depended on ecological generalization and species'' dependence on pollinators, although the influence of ecological generalization was only detected in the alpine community. Zygomorphic species that were highly dependent on pollinators and that were ecologically specialized were less variable in flower size than ecologically generalist and selfing zygomorphic species, supporting the pollinator-mediated stabilizing selection hypothesis. However, these relationships were not found in actinomorphic species, probably because they are not dependent on any particular pollinator for efficient pollination and therefore their flower size always shows moderate levels of variation.

Conclusions

The study suggests that the relationship between flower size variation and floral symmetry may be influenced by population-dependent factors, such as ecological generalization and species'' dependence on pollinators.     

A comparative study of floral development inTrillium apetalon andT. kamtschaticum (Liliaceae)

Trillium apetalon Makino is unique amongTrillium in having apetalous flowers. Using scanning electron microscope, the early floral development was observed in comparison with that ofT. kamtschaticum Pallas ex Pursh having petalous flowers. Morphologically petal primordia closely resemble stamen primordia in their more or less narrow and radially symmetric shape and are clearly distinct from sepal primordia with broad bases. Early in floral development sepal primordia are first initiated and subsequently two whorls of three primordia each are formed in rapid sequence, the first three at the corners and the second three at the sides of the triangular floral apex. Based on comparison in position and early developmental processes of their primordia, petals and outer stamens ofTrillium kamtschaticum are equivalent to outer stamens and inner stamens ofT. apetalon. The replacement of petals by outer stamens apparently leads to the loss of petals inTrillium apetalon flowers. Such a replacement can be interpreted in terms of homeosis. The replacement of the petal whorl leads to the serial replacement of the subsequent whorls: outer stamens by inner stamens, and inner stamens by gynoecium inTrillium apetalon. The term ‘serial homeosis’ is introduced for this serial replacement.     

Armament imbalances: match and mismatch in plant-pollinator traits of highly specialized long-spurred orchids

Background

Some species of long-spurred orchids achieve pollination by a close association with long-tongued hawkmoths. Among them, several Habenaria species present specialized mechanisms, where pollination success depends on the attachment of pollinaria onto the heads of hawkmoths with very long proboscises. However, in the Neotropical region such moths are less abundant than their shorter-tongued relatives and are also prone to population fluctuations. Both factors may give rise to differences in pollinator-mediated selection on floral traits through time and space.

Methodology/Principal Findings

We characterized hawkmoth assemblages and estimated phenotypic selection gradients on orchid spur lengths in populations of three South American Habenaria species. We examined the match between hawkmoth proboscis and flower spur lengths to determine whether pollinators may act as selective agents on flower morphology. We found significant directional selection on spur length only in Habenaria gourlieana, where most pollinators had proboscises longer than the mean of orchid spur length.

Conclusions/Significance

Phenotypic selection is dependent on the mutual match between pollinator and flower morphologies. However, our findings indicate that pollinator-mediated selection may vary through time and space according to local variations in pollinator assemblages.     

Association between floral traits and rewards in Erysimum mediohispanicum (Brassicaceae)

Background and Aims

Floral rewards may be associated with certain morphological floral traits and thus act as underlying factors promoting selection on these traits. This study investigates whether some traits that are under pollinator-mediated selection (flower number, stalk height, corolla diameter, corolla tube length and corolla tube width) in the Mediterranean herb E. mediohispanicum (Brassicaceae) are associated with rewards (pollen and nectar).

Methods

During 2005 the phenotypic traits and the visitation rate of the main pollinator functional groups were quantified in 720 plants belonging to eight populations in south-east Spain, and during 2006 the same phenotypic traits and the reward production were quantified in 400 additional plants from the same populations.

Key Results

A significant correlation was found between nectar production rate and corolla tube length, and between pollen production and corolla diameter. Visitation rates of large bees and butterflies were significantly higher in plants exhibiting larger flowers with longer corolla tubes.

Conclusions

The association between reward production and floral traits may be a factor underlying the pattern of visitation rate displayed by some pollinators.Key words: Erysimum, floral traits, nectar, pollen, pollinator visitation rate, reward     

Strong pollinator‐mediated selection for increased flower brightness and contrast in a deceptive orchid

Contrasting flower color patterns that putatively attract or direct pollinators toward a reward are common among angiosperms. In the deceptive orchid Anacamptis morio, the lower petal, which makes up most of the floral display, has a light central patch with dark markings. Within populations, there is pronounced variation in petal brightness, patch size, amount of dark markings, and contrast between patch and petal margin. We tested whether pollinators mediate selection on these color traits and on morphology (plant height, number of flowers, corolla size, spur length), and whether selection is consistent with facilitated or negative frequency‐dependent pollination. Pollinators mediated strong selection for increased petal brightness (Δβ_poll = 0.42) and contrast (Δβ_poll = 0.51). Pollinators also tended to mediate stabilizing selection on brightness (Δγ_poll = –0.27, n.s.) favoring the most common phenotype in the population. Selection for reduced petal brightness among hand‐pollinated plants indicated a fitness cost associated with brightness. The results demonstrate that flower color traits influence pollination success and seed production in A. morio, indicating that they affect attractiveness to pollinators, efficiency of pollen transfer, or both. The documented selection is consistent with facilitated pollination and selection for color convergence toward cooccurring rewarding species.     

Floral and vegetative cues in oil-secreting and non-oil-secreting Lysimachia species

Background and Aims

Unrelated plants pollinated by the same group or guild of animals typically evolve similar floral cues due to pollinator-mediated selection. Related plant species, however, may possess similar cues either as a result of pollinator-mediated selection or as a result of sharing a common ancestor that possessed the same cues or traits. In this study, visual and olfactory floral cues in Lysimachia species exhibiting different pollination strategies were analysed and compared, and the importance of pollinators and phylogeny on the evolution of these floral cues was determined. For comparison, cues of vegetative material were examined where pollinator selection would not be expected.

Methods

Floral and vegetative scents and colours in floral oil- and non-floral oil-secreting Lysimachia species were studied by chemical and spectrophotometric analyses, respectively, compared between oil- and non-oil-secreting species, and analysed by phylogenetically controlled methods.

Key Results

Vegetative and floral scent was species specific, and variability in floral but not vegetative scent was lower in oil compared with non-oil species. Overall, oil species did not differ in their floral or vegetative scent from non-oil species. However, a correlation was found between oil secretion and six floral scent constituents specific to oil species, whereas the presence of four other floral compounds can be explained by phylogeny. Four of the five analysed oil species had bee-green flowers and the pattern of occurrence of this colour correlated with oil secretion. Non-oil species had different floral colours. The colour of leaves was similar among all species studied.

Conclusions

Evidence was found for correlated evolution between secretion of floral oils and floral but not vegetative visual and olfactory cues. The cues correlating with oil secretion were probably selected by Macropis bees, the specialized pollinators of oil-secreting Lysimachia species, and may have evolved in order to attract these bees.     

Floral polymorphism and the fitness implications of attracting pollinating and florivorous insects

Background and Aims

Floral polymorphism is frequently attributed to pollinator-mediated selection. Multiple studies, however, have revealed the importance of non-pollinating visitors in floral evolution. Using the polymorphic annual daisy Ursinia calenduliflora, this study investigated the importance of different insect visitors, and their effects on fitness, in the maintenance of floral polymorphism.

Methods

The spatial structure of a discrete floral polymorphism was characterized based on the presence/absence of anthocyanin floret spots in U. calenduliflora. A 3-year observational study was then conducted in polymorphic populations to investigate differences in visitation rates of dominant visitors to floral morphs. Experiments were performed to explore the floral preference of male and female Megapalpus capensis (the dominant insect visitor) and their effectiveness as pollinators. Next, floral damage by antagonistic florivores and the reproductive success of the two floral morphs were surveyed in multiple populations and years.

Key Results

Floral polymorphism in U. calenduliflora was structured spatially, as were insect visitation patterns. Megapalpus capensis males were the dominant visitors and exhibited strong preference for the spotted morph in natural and experimental observations. While this may indicate potential fitness benefits for the spotted morph, female fitness did not differ between floral morphs. However, as M. capensis males are very efficient at exporting U. calenduliflora pollen, their preference may likely increase the reproductive fitness of the spotted morph through male fitness components. The spotted morph, however, also suffered significantly greater costs due to ovule predation by florivores than the spotless morph.

Conclusions

The results suggest that pollinators and florivores may potentially exert antagonistic selection that could contribute to the maintenance of floral polymorphism across the range of U. calenduliflora. The relative strength of selection imposed by each agent is potentially determined by insect community composition and abundance at each site, highlighting the importance of community context in the evolution of floral phenotypes.     

Pollinator‐mediated selection on floral morphology: evidence for transgressive evolution in a derived hybrid lineage

Hybridization between closely related lineages is a mechanism that might promote substantive changes in phenotypic traits of descendants, resulting in transgressive evolution. Interbreeding between divergent but morphologically similar lineages can produce exceptional phenotypes, but the potential for transgressive variation to facilitate long‐term trait changes in derived hybrid lineages has received little attention. We compare pollinator‐mediated selection on transgressive floral traits in both early‐generation and derived hybrid lineages of the Piriqueta cistoides ssp. caroliniana complex. The bowl‐shaped flowers of morphotypes in this complex have similar gross morphologies and attract a common suite of small insect pollinators. However, they are defined by significant differences in characters that generate pollinator interest and visitation, including floral area and petal separation. In common garden experiments, patterns of pollen deposition in early‐generation recombinant hybrids indicate that Piriqueta's pollinators favour flowers with greater area and reduced petal separation. Changes in floral morphology in derived hybrid lineages are consistent with predictions from selection gradients, but the magnitude of change is limited relative to the range of transgressive variation. These results suggest that hybridization provides variation for evolution of divergent floral traits. However, the potential for extreme transgressive variants to contribute to phenotypic shifts may be limited due to reduced heritability, evolutionary constraints or fitness trade‐offs.     

Effects of floral traits and plant genetic composition on pollinator behavior

Pollinator-mediated selection plays a major role in floral evolution and speciation. Floral traits that influence animal pollinator behavior are the target of pollinator-mediated selection, but can only evolve if floral phenotypes have underlying genetic variation. Thus, understanding the genetic basis of a floral trait is a crucial step in studying pollinator-mediated selection. In this study I tested the effect of quantitative trait loci (QTL) underlying floral traits on pollinator behavior in recombinant inbred lines (RILs) in the common sunflower, Helianthus annuus L. and its crop relative. The indirect effects of QTL on pollinator behavior, mediated by floral phenotypes, were analyzed for six insect visitor types using structural equation modeling (SEM) and path analysis. For three of the six visitor types (large and small bees and non-bee insects) valid models were revealed when all three levels (QTL, floral traits, and pollinator behavior) were incorporated. Nested model without genetics were validated for five of the six visitor types. The results suggest that insect behavior as a reaction to floral phenotypes is affected by the genetic architecture of floral traits. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Heikki Hokkanen     

Pollinators cause stronger selection than herbivores on floral traits in Lobelia cardinalis (Lobeliaceae)

Measures of selection on floral traits in flowering plants are often motivated by the assumption that pollinators cause selection. Flowering plants experience selection from other sources, including herbivores, which may enhance or oppose selection by pollinators. Surprisingly, few studies have examined selection from multiple sources on the same traits. We quantified pollinator-mediated selection on six floral traits of Lobelia cardinalis by comparing selection in naturally and supplementally (hand-) pollinated plants. Directional, quadratic and correlational selection gradients as well as total directional and quadratic selection differentials were examined. We used path analysis to examine how three herbivores--slugs, weevils and caterpillars--affected the relationship between floral traits and fitness. We detected stronger total selection on four traits and correlational selection (γ(ij)) on three trait combinations in the natural pollination treatment, indicating that pollinators caused selection on these traits. Weak but statistically significant selection was caused by weevil larvae on stem diameter and anther-nectary distance, and by slugs on median-flower date. In this study, pollinators imposed stronger selection than herbivores on floral traits in L. cardinalis. In general, the degree of pollen limitation and rate of herbivory are expected to influence the relative strength of selection caused by pollinators or herbivores.     

POLLINATORS DISCRIMINATE AMONG FLORAL HEIGHTS OF A SEXUALLY DECEPTIVE ORCHID: IMPLICATIONS FOR SELECTION

Pollinators have influenced the evolution of many morphological floral traits, although few studies have shown that pollinators have influenced plant height. Chiloglottis trilabra is one of many Australian orchids that deceive and attract male pollinators by mimicking the sex pheromones and morphology of females insects. Orchids in this genus have unusually short flowers whose peduncle elongates dramatically after pollination to approximately twice the original height. In a series of choice experiments in the field, we show that pollinators of C. trilabra strongly discriminate among floral heights, preferring flowers presented at 15 cm-20 cm over flowers presented at lower and higher positions (ranging from 2 cm-100 cm). Our results suggested pollinators have the potential to mediate stabilizing selection for floral height when pollination is limiting. However, the natural height range of the orchid (mean = 10 cm, range 5 cm–15 cm) was lower than the experimentally determined optimum for visitation frequency. This difference may indicate that pollinator-mediated selection does not occur in this species, perhaps because seed set is not sufficiently limited. Alternatively, other life-history factors may counteract pollinator-mediated selection, yielding an evolutionary compromise in height.     

Digest: Floral evolution through the lens of the community context*

How do emergent properties of natural plant communities affect floral evolution? In this issue, Eisen et al. explored this question by studying selection on floral traits in natural communities of Clarkia species. They found that two community properties, namely congeneric species richness and floral density (of conspecifics and heterospecifics), influenced the patterns of selection, although not through the expected means of pollinator-mediated selection. Instead, the authors suggest that additional factors like competition and facilitation between plants are responsible. The results support the hypothesis that, beside pollinators, other factors of the community context can also determine floral evolution.     

Nectar reward and advertisement in hummingbird-pollinated Silene virginica (Caryophyllaceae)

We tested for an association between nectar and various floral traits and investigated their roles as primary and secondary pollinator attractants in hummingbird-pollinated Silene virginica. Our goal was to gain insight into the mechanisms of pollinator-mediated selection that underlies floral trait divergence within the genus. In a field population of S. virginica, we measured five floral and eight vegetative traits and quantified nectar volume, nectar sugar concentration, and total sugar reward (nectar volume × nectar sugar concentration). All three components of nectar reward were positively correlated to flower size, and nectar volume varied significantly among individuals within the population. To ascertain whether the correlation of specific floral traits with nectar reward influences the behavior of the primary pollinator of S. virginica, the ruby-throated hummingbird, Archilochus colubris, we investigated whether A. colubris preferred the expression of floral traits associated with high nectar volume and total sugar reward. We accomplished this by constructing floral arrays consisting of artificial flowers that had equal nectar quantity and total sugar reward but that differed in petal area and corolla tube diameter, which were positively correlated with nectar quantity and total sugar reward in our field study. In observations of visitation frequencies to the various floral-trait combinations, hummingbirds preferentially visited artificial floral phenotypes with larger petal displays, with the greatest preference for floral phenotypes with both larger petals and wider corolla-tube diameters. This association between primary and secondary floral attractants and hummingbird discrimination of floral features supports the concept that the floral traits of S. virginica reflect pollinator-mediated selection by the principal pollinator.     

Using phenotypic manipulations to study multivariate selection of floral trait associations

Background

A basic theme in the study of plant–pollinator interactions is that pollinators select not just for single floral traits, but for associations of traits. Responses of pollinators to sets of traits are inherent in the idea of pollinator syndromes. In its most extreme form, selection on a suite of traits can take the form of correlational selection, in which a response to one trait depends on the value of another, thereby favouring floral integration. Despite the importance of selection for combinations of traits in the evolution of flowers, evidence is relatively sparse and relies mostly on observational approaches.

Scope

Here, methods for measuring selection on multivariate suites of floral traits are presented, and the studies to date are reviewed. It is argued that phenotypic manipulations present a powerful, but rarely used, approach to teasing apart the separate and combined effects of particular traits. The approach is illustrated with data from studies of alpine plants in Colorado and New Zealand, and recommendations are made about several features of the design of such experiments.

Conclusions

Phenotypic manipulations of two or more traits in combination provide a direct way of testing for selection of floral trait associations. Such experiments will be particularly valuable if rooted in hypotheses about differences between types of pollinators and tied to a proposed evolutionary history.Key words: Colour, correlational selection, experiment, floral integration, multivariate selection, phenotypic manipulation, pollination syndrome, pollinator visitation     

Induced mutations affecting pollinator choice in Mimulus lewisii (Phrymaceae)

The flowering plants are one of the most phenotypically varied and wide-ranging groups of organisms on earth, and yet, we have limited understanding of the contribution of animal pollinators to the diversification of floral form. To explore the interaction between variation in floral form and pollinator behavior, we observed the foraging behavior of bumblebees (Bombus impatiens) when presented with both wild-type Mimulus lewisii plants and each of three chemically induced single-locus mutants with altered floral phenotypes, including loss of the three lower petals, loss of nectar guides, and a change in petal color patterning. We found that each of the mutants attracted successful pollinator visits at just 29–80% of the rate relative to wild-type flowers, suggesting that effective recruitment of bumblebee pollinators requires the landing platform provided by the lower petals, and visual cues provided by the nectar guides and petal color pattern. Since single-locus recessive mutations are capable of ablating the lower petals, nectar guides, and color pattern, such changes in floral form provide insight into the driving forces behind plant adaptation.