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COEVOLUTION

Sometimes two different species develop an intimate association so that, over time, the course of each species' evolution is affected. Coevolution is the interdependent evolution of two or more spe­cies that occurs as a result of their interactions. Flowering plants and their animal pollinators pro­vide an excellent example of Coevolution. Because plants are rooted in the ground, they lack the mo­bility that animals have when mating. Many flow­ering plants rely on animals to help them mate. Bees, beetles, hummingbirds, bats, and other ani­mals transport the male reproductive structures, called pollen, from one plant to another, in effect giving plants mobility. How has this during the millions of years over which these associations developed, flowering plants evolved a number of ways to attract animal pollinators. One of the rewards for the pollinator is food—nectar (a sugary solution) and pollen. Plants often produce food that is precisely correct for one type of pollina­tor. The nectar of flowers that are pollinated by bees, for example, usually contains between 30 per­cent and 35 percent sugar.

bees need in order to make honey. Bees will not visit flowers with lower sugar concentrations their nectar. Pollen also attracts many pollinators. Bees for example use pollen to make bee bread, a nutritious mixture of nectar and pollen that is eaten by their larvae-Plants have also evolved a variety of ways to get the pollinator's attention, most of which in­volve colors and scents. Showy petals visually at­tract the pollinator us a neon sign or golden arches attract a hungry person to a restaurant. Different animal pollinators perceive colors differently. In­sects, for example, see the blue and yellow range of the visible spectrum but don't perceive red as a dis­tinct color. Thus, plants that are pollinated by in­sects often have blue or yellow petals. Insects can also see the ultraviolet range of the electromagnetic spectrum, which is invisible to the human eye; in­sects see ultraviolet as a color culled "bee's purple." Many insect-pollinated (lowers have parts that re­flect ultraviolet (making them appear purple to in­sects) and parts that absorb ultraviolet (making them appear as other colors, such as yellow). This creates patterns on the flower, which direct the in­sect to the center of" the flower where the pollen and nectar are located.

Scents are also an effective way to attract polli­nators. Insects have a well-developed sense of smell, and many insect-pollinated flowers have a strong scent, which may be pleasant but is not al­ways. The carrion plant, for example, is pollinated by carrion flies and smells like rotting flesh. Its pet­als are dappled with a reddish-brown color that looks like dried blood. Flies move from one flower to another, looking for a place to deposit their eggs, and in the process pollen is transferred from one flower to another.

During the time plants were evolving special­ized features to attract pollinators, the animal pollinators coevolved specialized body parts and behav­iors that enabled them to both aid pollination and obtain nectar and pollen as a reward. Many insects have mouthparts that fit into certain flowers much as a lock fits into a key. Thus, even though other insect species may be attracted to the flowers, they cannot obtain the rewards because they lack these specialized mouthparts. The behavior of animals has also coevolved. For example, male wasps, which mature before female wasps, attempt to cop­ulate with certain orchid flowers that resemble fe­male wasps in coloring and shape. As a result, polli­nation is achieved as the males move from flower to flower. When female wasps emerge at a later time, the males finally get to mate for real.