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Evolution: a change in the inherited traits of a population from one generation to the next.
Gametes: a reproductive cell that fuses with another gamete during fertilization (e.g., a sperm or egg).
Meiosis: the process by which gametes are produced.
Pollination: the transfer of pollen grains (male gamete) to the structure that contains the ovule (female gamete).
Spermatophytes: those plants that produce seeds.
Co-evolution Helps Plants and Pollinators Work Together
Co-evolution is defined as the mutual evolutionary influence between two species in which each exerts selective pressure on the other. This process has been occurring between plants and pollinators since the Cretaceous period 144 million years ago. Plants have evolved flowers with varying shapes, color, fragrance, location on the plant, and timing to attract pollinators. An example of co-evolution includes the yucca moth (Tegeticula spp., Mesepiola spp., and Parategeticula spp.) and the yucca plant (Yucca spp.). Yucca plants are only suited for the yucca moth to pollinate them due to the shape of the yucca flower. Yucca moths lay their eggs in the yucca flower and the larvae eat the yucca seeds.
A general example of co-evolution has occurred among butterfly species and the plants they pollinate. Butterflies are diurnal with a good sense of vision but a poor sense of smell. Plants that attract butterflies are usually brightly colored with very little fragrance.
A carpenter bee (Xylocopa virginica) visits a blueberry (Vaccinium spp.) flower. Photo by Jerry A. Payne, USDA Agricultural Research Service, www.insectimages.org.
Pollination is an essential part of sexual reproduction in seed-producing plants (spermatophytes), allowing for genetic recombination and the formation of a genetically unique seed. This shuffling of genetic material maximizes the ability of at least some of a plant's offspring to survive in a world of unpredictable environmental changes. Further, the maintenance of genetic variability in a population is necessary for evolution by natural selection to occur, and therefore is the key to the ability of a plant population to adapt to changing environmental pressures.
Some reshuffling of genetic material occurs in meiosis, the process by which gametes are produced. Therefore, even self-pollination, in which pollen grains are transferred from the stamens to the stigma of a single flower (or from one flower to another on the same plant) allows for the maintenance of genetic variation. But genetic variation in the next generation is maximized by cross-pollination, in which pollen from the flower of one plant is transferred to that of another individual plant.
Many plants rely on wind or water for pollination, but must produce large amounts of pollen to ensure the chance interception of pollen by the stigma. For many plants it has proven advantageous to rely on pollination via animals, such as insects, birds, and bats. By relying upon pollination by animals, the plant wastes less pollen compared with pollination by wind or water. On the other hand, the plant may expend additional energy to promote pollination by animals; one example is the production of nectar to reward pollinating animals. In addition, animal pollinators can transfer disease organisms from one plant to another along with pollen.
There are several different types of pollination. The two broad categories of pollination are self-pollination and cross-pollination. Self-pollination, or autogamy, occurs in plants that are self-fertile. It is the transfer of pollen grains from the stamens to the stigma of a single flower or from one flower to another on the same plant. In this type of pollination, a plant has both a stamen and a pistil and pollination results from their direct contact, without the aid of a pollinator. Self-pollination is not very common. Some plants, such as peanuts, peppers, strawberries, and tomatoes have the ability to self-pollinate. Plants found on islands - where pollinators may not be present - have a higher likelihood of being able to self-pollinate than those plants on the mainland. There are several advantages to self-pollination, such as plants not being dependent on the presence of pollinators to reproduce and plants being able to propagate rapidly. The main disadvantage to this form of reproduction is that it can result in decreased genetic diversity, as the parent passes on only its own genes to its offspring. Additionally, many plants that are primarily cross-pollinated have some flowers, or a mechanism within many flowers, that results in self-pollination in case the flower is not cross-pollinated by visiting pollinators.
Cross-pollination, or syngamy, is the movement of pollen from one plant to another. Most plants are designed for cross pollination. Some advantages of cross-pollination include the rapid transfer of beneficial genes to offspring and high genetic diversity throughout the population. However, if pollinators are not present, and the plant does not have the ability to self-pollinate, the plant population may die out due to a lack of offspring to replace adults after they die.
Cross-pollination can occur by animals, wind, or water. Insects, mammals, and birds can all act as animal pollinators. Wind pollination, or anemophily, is the distribution of pollen by wind. Plants that are pollinated with the use of wind usually lack scented flowers, do not produce nectar, produce a large amount of smooth, light pollen, and have feathery stigmas to catch the pollen from the wind. Examples of wind pollinated plants include conifers and ragweeds. Water pollination, or hydrophily, is the distribution of pollen by the flow of water. This type of pollination is uncommon, but has been seen in waterweeds and pondweeds.
A specific type of cross-pollination used by bees (e.g. bumble bees, Bombus spp.) to release pollen is called buzz pollination, or sonication. The bees grab onto a flower and move their flight muscles rapidly. This causes the flower to vibrate and the pollen to become dislodged. Typically, buzz pollinated flowers have tubular anthers with an opening at only one end. The pollen grains are very small and not oily. Examples of buzz pollinated plants include members of the Solanaceae family (e.g., eggplants, potatoes, tomatoes) and some members of the genus Vaccinium (e.g., blueberries, cranberries). Buzz pollination occurs in about 8% of flowering plants worldwide.
References: Buzz Pollination (Stephen Buchmann, USDA Agricultural Research Service), Wind and Water Pollination (USDA Forest Service), Pollination (The University of the Western Cape, The Department of Biodiversity and Conservation Biology), and Biology of Plants: Pollination (Missouri Botanical Gardens, 2006); Self-pollination in island and mainland populations of the introduced hummingbird-pollinated plant, Nicotiana glauca (Solanaceae), Sheile K. Schueller, American Journal of Botany, vol. 91, 2004, pp. 672-681
"The work of pollinators ensures full harvests and seed production from many agricultural crops and provides for healthy plants grown in backyards, community gardens, and other urban areas.
Worldwide, of the estimated 1,330 crop plants grown for food, beverages, fibers, condiments, spices, and medicines, approximately 1,000 (75 percent) are pollinated by animals. It has been calculated that one out of every three mouthfuls of food we eat, and beverages we drink, is delivered to us by pollinators.
More than half the world's diet of fats and oils comes from oilseed crops. Many of these, including cotton, oil palm, canola, and sunflowers, are pollinated by animals.
In the U.S., pollination by insects produces $40 billion worth of products annually.
Pollinators are essential components of the habitats and ecosystems that many wild animals rely on for food and shelter.
Approximately 25 percent of birds include fruit or seeds as a major part of their diet.
Plants provide egg laying and nesting sites for many insects, such as butterflies.
Berries and other fruit form a significant part of the late-summer diet of animals, such as grizzly bears, which fatten themselves in preparation for winter hibernation."
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