Pollinator-mediated selection

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Evolution of multiple pollination syndromes. The columns from left to right are examples of bee, hummingbird and moth-pollination syndromes. Each row shows representatives of the same genus. The top row are columbines (Ranunculaceae). The second row are beardtongues (Plantagineaceae). Third row are catchfly (Caryophylaceae). The fourth row are morning glories (Convolvulaceae). The fifth row are larkspurs (Ranunculaceae) and the last row are gilias (Polemoniaceae). Photos by Stickpen, Walter Siegmund, Cstubben, Lionnel, Derek Ramsey, Jebulon, Dinkum, Forest & Kim Starr, S . Zenner, and Jerry Friedman. Pollination syndromes 2.jpg
Evolution of multiple pollination syndromes. The columns from left to right are examples of bee, hummingbird and moth-pollination syndromes. Each row shows representatives of the same genus. The top row are columbines (Ranunculaceae). The second row are beardtongues (Plantagineaceae). Third row are catchfly (Caryophylaceae). The fourth row are morning glories (Convolvulaceae). The fifth row are larkspurs (Ranunculaceae) and the last row are gilias (Polemoniaceae). Photos by Stickpen, Walter Siegmund, Cstubben, Lionnel, Derek Ramsey, Jebulon, Dinkum, Forest & Kim Starr, S . Zenner, and Jerry Friedman.

Pollinator-mediated selection is an evolutionary process occurring in flowering plants, in which the foraging behavior of pollinators differentially selects for certain floral traits. [1] Flowering plant are a diverse group of plants that produce seeds. Their seeds differ from those of gymnosperms in that they are enclosed within a fruit. These plants display a wide range of diversity when it comes to the phenotypic characteristics of their flowers, which attracts a variety of pollinators that participate in biotic interactions with the plant. Since many plants rely on pollen vectors, their interactions with them influence floral traits and also favor efficiency since many vectors are searching for floral rewards like pollen and nectar. Examples of pollinator-mediated selected traits could be those involving the size, shape, color and odor of flowers, corolla tube length and width, size of inflorescence, floral rewards and amount, nectar guides, and phenology. Since these types of traits are likely to be involved in attracting pollinators, they may very well be the result of selection by the pollinators themselves. [2]

Having a floral display that either attracts a variety of pollinators or is efficient in the exchanges that occur during pollination can have advantages for the reproductive success of plants. Thus, pollinator behavior is important to understand in relation to the evolution of flowering plants and in some cases pollinator behavior is thought to lead to specialized pollination syndromes where floral traits have co-evolved with their pollinators in a way that are a direct response to the selection occurring from their pollen vectors. [3] However, many flowering plants don’t display morphology that excludes all pollinators except the one they co-evolved with. The most effective pollinator principle posits that floral traits reflect the adaptation to the pollinator that is efficient at transferring the most pollen. Selection might actually favor some degree of generalization while some flowers can also retain particular traits that allow them to adapt to a certain type of pollinator, but will ultimately be molded by the pollinators that are the most effective and visit the most frequently. [4] This leads to shifts in pollination syndromes and to some genera having a high diversity of pollination syndromes among species, suggesting that pollinators are a primary selective force driving diversity and speciation. [5] [6]

Ophrys apifera is an orchid species that has a highly evolved plant-pollinator relationship. This specific species displays sexual deception and floral mimicry that have resulted from the selection pressures of bees. Bee Orchid.jpg
Ophrys apifera is an orchid species that has a highly evolved plant-pollinator relationship. This specific species displays sexual deception and floral mimicry that have resulted from the selection pressures of bees.

Pollinator-mediated selection requires isolation and therefore cannot function in sympatry. Floral isolation is a consequence of pollinator behavior that reduces inter-lineage pollen transfer, which reduces gene flow and increases the possibility for a transition to different syndromes. [5] Isolation with no gene flow between populations allows for the development of distinct species, thus speciation is a result of reproductive isolation and can be driven by pollinator-mediated selection. [1]

See also

Related Research Articles

Pollinator Animal that moves pollen from the male anther of a flower to the female stigma of a flower

A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.

Coevolution Two or more species influencing each others evolution

In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well as gene-culture coevolution.

Pollination Biological process occurring in plants

Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds, and bats; water; wind; and even plants themselves, when self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species it can produce hybrid offspring in nature and in plant breeding work.

Entomophily Form of pollination by insects

Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, sometimes with conspicuous patterns leading to rewards of pollen and nectar; they may also have an attractive scent which in some cases mimics insect pheromones. Insect pollinators such as bees have adaptations for their role, such as lapping or sucking mouthparts to take in nectar, and in some species also pollen baskets on their hind legs. This required the coevolution of insects and flowering plants in the development of pollination behaviour by the insects and pollination mechanisms by the flowers, benefiting both groups.

Zoophily

Zoophily, or zoogamy, is a form of pollination whereby pollen is transferred by animals, usually by invertebrates but in some cases vertebrates, particularly birds and bats, but also by other animals. Zoophilous species frequently have evolved mechanisms to make themselves more appealing to the particular type of pollinator, e.g. brightly colored or scented flowers, nectar, and appealing shapes and patterns. These plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination.

Nectar guide

Nectar guides are markings or patterns seen in flowers of some angiosperm species, that guide pollinators to their rewards. Rewards commonly take the form of nectar, pollen, or both, but various plants produce oil, resins, scents, or waxes. Such patterns also are known as "pollen guides" and "honey guides", though some authorities argue for the abandonment of such terms in favour of floral guides. Pollinator visitation can select for various floral traits, including nectar guides through a process called pollinator-mediated selection.

Nectar Sugar-rich liquid produced by many flowering plants, that attracts pollinators and insects

Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists, which in turn provide herbivore protection. Common nectar-consuming pollinators include mosquitoes, hoverflies, wasps, bees, butterflies and moths, hummingbirds, honeyeaters and bats. Nectar plays a crucial role in the foraging economics and evolution of nectar-eating species; for example, nectar foraging behavior is largely responsible for the divergent evolution of the African honey bee, A. m. scutellata and the western honey bee.

Flower Reproductive structure in flowering plants

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants. The biological function of a flower is to facilitate reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate outcrossing resulting from cross-pollination or allow selfing when self-pollination occurs.

Ornithophily Pollination by birds

Ornithophily or bird pollination is the pollination of flowering plants by birds. This sometimes coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics, Southern Africa, and on some island chains. The association involves several distinctive plant adaptations forming a "pollination syndrome". The plants typically have colourful, often red, flowers with long tubular structures holding ample nectar and orientations of the stamen and stigma that ensure contact with the pollinator. Birds involved in ornithophily tend to be specialist nectarivores with brushy tongues and long bills, that are either capable of hovering flight or light enough to perch on the flower structures.

Palynivore Group of herbivorous animals

In zoology, a palynivore /pəˈlɪnəvɔːɹ/, meaning "pollen eater" is an herbivorous animal which selectively eats the nutrient-rich pollen produced by angiosperms and gymnosperms. Most true palynivores are insects or mites. The category in its strictest application includes most bees, and a few kinds of wasps, as pollen is often the only solid food consumed by all life stages in these insects. However, the category can be extended to include more diverse species. For example, palynivorous mites and thrips typically feed on the liquid content of the pollen grains without actually consuming the exine, or the solid portion of the grain. Additionally, the list is expanded greatly if one takes into consideration species where either the larval or adult stage feeds on pollen, but not both. There are other wasps which are in this category, as well as many beetles, flies, butterflies, and moths. One such example of a bee species that only consumes pollen in its larval stage is the Apis mellifera carnica. There is a vast array of insects that will feed opportunistically on pollen, as will various birds, orb-weaving spiders and other nectarivores.

Pollination syndrome

Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection. These traits include flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.

Nectar robbing Foraging behavior

Nectar robbing is a foraging behavior utilized by some organisms that feed on floral nectar, carried out by feeding from holes bitten in flowers, rather than by entering through the flowers' natural openings. "Nectar robbers" usually feed in this way, avoiding contact with the floral reproductive structures, and therefore do not facilitate plant reproduction via pollination. Because many species that act as pollinators also act as nectar robbers, nectar robbing is considered to be a form of exploitation of plant-pollinator mutualism. While there is variation in the dependency on nectar for robber species, most species rob facultatively.

Flower constancy

Flower constancy or pollinator constancy is the tendency of individual pollinators to exclusively visit certain flower species or morphs within a species, bypassing other available flower species that could potentially contain more nectar. This type of foraging behavior puts selective pressures on floral traits in a process called pollinator-mediated selection. Flower constancy is different from other types of insect specialization such as innate preferences for certain colors or flower types, or the tendency of pollinators to visit the most rewarding and abundant flowers.

Frequency-dependent foraging by pollinators Animal behavior

Frequency-dependent foraging is defined as the tendency of an individual to selectively forage on a certain species or morph based on its relative frequency within a population. Specifically for pollinators, this refers to the tendency to visit a particular floral morph or plant species based on its frequency within the local plant community, even if nectar rewards are equivalent amongst different morphs. Pollinators that forage in a frequency-dependent manner will exhibit flower constancy for a certain morph, but the preferred floral type will be dependent on its frequency. Additionally, frequency-dependent foraging differs from density-dependent foraging as the latter considers the absolute number of certain morphs per unit area as a factor influencing pollinator choice. Although density of a morph will be related to its frequency, common morphs are still preferred when overall plant densities are high.

Ecological speciation

Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species. Ecological factors can include changes in the environmental conditions in which a species experiences, such as behavioral changes involving predation, predator avoidance, pollinator attraction, and foraging; as well as changes in mate choice due to sexual selection or communication systems. Ecologically-driven reproductive isolation under divergent natural selection leads to the formation of new species. This has been documented in many cases in nature and has been a major focus of research on speciation for the past few decades.

Flowering synchrony is the amount of overlap between flowering periods of plants in their mating season compared to what would be expected to occur randomly under given environmental conditions. A population which is flowering synchronously has more plants flowering at the same time than would be expected to occur randomly. A population which is flowering asynchronously has fewer plants flowering at the same time than would be expected randomly. Flowering synchrony can describe synchrony of flowering periods within a year, across years, and across species in a community. There are fitness benefits and disadvantages to synchronized flowering, and it is a widespread phenomenon across pollination syndromes.

Floral scent

Floral scent, or flower scent, is composed of all the volatile organic compounds (VOCs), or aroma compounds, emitted by floral tissue. Other names for floral scent include, aroma, fragrance, floral odour or perfume. Flower scent of most flowering plant species encompasses a diversity of VOCs, sometimes up to several hundred different compounds. The primary functions of floral scent are to deter herbivores and especially folivorous insects, and to attract pollinators. Floral scent is one of the most important communication channels mediating plant-pollinator interactions, along with visual cues.

Floral biology is an area of ecological research that studies the evolutionary factors that have moulded the structures, behaviour and physiological aspects involved in the flowering of plants. The field is broad and interdisciplinary and involves research requiring expertise from multiple disciplines that can include botany, ethology, biochemistry, entomoloniques. A slightly narrower area of research within floral biology is sometimes termed as pollination biology or anthecology.

Monocotyledon reproduction

The monocots are one of the two major groups of flowering plants, the other being the dicots. In order to reproduce they utilize various strategies such as employing forms of asexual reproduction, restricting which individuals they are sexually compatible with, or influencing how they are pollinated. Nearly all reproductive strategies that evolved in the dicots have independently evolved in monocots as well. Despite these similarities and their close relatedness, monocots and dicots have distinct traits in their reproductive biologies.

Sexual selection is described as natural selection arising through preference by one sex for certain characteristics in individuals of the other sex. Sexual selection is a common concept in animal evolution but, with plants, it is oftentimes overlooked because many plants are hermaphrodites. Flowering plants show many characteristics that are often sexually selected for. For example, flower symmetry, nectar production, floral structure, and inflorescences are just a few of the many secondary sex characteristics acted upon by sexual selection. Sexual dimorphisms and reproductive organs can also be affected by sexual selection in flowering plants.

References

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  2. Mayfield, M (2001). "Exploring the 'Most Effective Pollinator Principle' with Complex Flowers: Bumblebees and Ipomopsis aggregata". Annals of Botany. 88 (4): 591–596. doi: 10.1006/anbo.2001.1500 . ISSN   0305-7364.
  3. Conner, Jeffrey K.; Proctor, Michael; Yeo, Peter; Lack, Andrew (1997). "The Natural History of Pollination". Ecology. 78 (1): 327. doi:10.2307/2266004. ISSN   0012-9658. JSTOR   2266004.
  4. Parachnowitsch, Amy L.; Kessler, André (2010-08-17). "Pollinators exert natural selection on flower size and floral display in Penstemon digitalis". New Phytologist. 188 (2): 393–402. doi: 10.1111/j.1469-8137.2010.03410.x . ISSN   0028-646X. PMID   20723076.
  5. 1 2 Van der Niet, Timotheüs; Peakall, Rod; Johnson, Steven D. (2014). "Pollinator-driven ecological speciation in plants: new evidence and future perspectives". Annals of Botany. 113 (2): 199–212. doi: 10.1093/aob/mct290 . ISSN   1095-8290. PMC   3890394 . PMID   24418954.
  6. Wu, Yun; Zhong, Tao; Zhang, Zhi-Qiang; Li, Qing-Jun (2018-10-01). "Pollinator-mediated selection on floral traits varies in space and between morphs in Primula secundiflora". AoB PLANTS. 10 (5): ply059. doi: 10.1093/aobpla/ply059 . ISSN   2041-2851. PMC   6205359 . PMID   30393517.