Coevolution & Mimicry

ADAPTATIONS FOR POLLINATION IN FLOWERING PLANTS

Flowering plants depend on external agents to carry pollen from the male parts in the flowers of one plant to the female parts in the flowers of another plant (Fig. 17.20). The flowers of each species are adapted in shape, structure, color, and odor to the particular pollinating agents on which they depend, and they provide an especially clear illustration of the evolution of adaptedness. Evolving together, the plants and their pollinators became more finely tuned to each other's peculiarities - a process often termed coevolution.

There are indeed striking correspondences between the pollinators and the species they pollinate. Bees are attracted innately to bright colors, ultraviolet bull's-eye patterns, and sweet, aromatic, or minty odors; they are active only during the day, and they usually alight on a petal before moving into the part of the flower containing the nectar and pollen. Bee flowers have showy, brightly colored petals that are usually blue or yellow but seldom red (bees can see blue or yellow light well, but they cannot see red at all); indeed, most bee-pollinated flowers have a UV bull's-eye, a sweet, aromatic, or minty fragrance, daytime opening or nectar production, and a special protruding lip or other suitable landing platform. But these observations tell us only that correspondences exist between certain flowers and the preference of bees. They do not indicate how these correspondences may have come about: whether the preferences of the pollinators provided the exclusive selective force on flower morphology and odor, or whether, instead, early flowers provided the selection pressures leading to the innate preferences of modern bees, or whether both factors have been at work. A look at other species of pollinators provides some clues.

Hummingbirds, for example, can see red well but blue only poorly; they have a weak sense of smell; and they ordinarily do not land on flowers, but hover in front of them while sucking the nectar. Flowers pollinated primarily by hummingbirds are usually red or yellow, are nearly odorless, and lack any protruding landing platform. Since flowers of the same genus can have very different morphologies to suit different pollinators (Fig. 17.2 1), it is probably the flowers that have done most of the adapting. However, pollinators have probably been adapting to flowers too, though to a lesser extent; different species of bees, for example, can have very different tongue lengths, suitable for different flower morphologies.

This pattern of coadaptation between pollinators and flowers extends to other nectar-feeding species as well. For example, in contrast to both bees and hummingbirds, moths and bats are generally most active at dusk and during the night, and the flowers they pollinate are mostly white and are open only during the late afternoon and night. These flowers often have a heavy fragrance that helps guide the moths and bats to them.

Moths play a role in a particularly interesting adaptation of plants to their pollinators. The flowers produced by scarlet gilia plants near Flagstaff, Arizona, range from red through pink to white. The dark-red flowers are most effective in attracting hummingbirds, but these pollinators emigrate a month after the season begins; the white flowers are most effective in attracting hawk moths, the pollinators available throughout the blooming season. The plants compensate for this shift in relative pollinator abundance by doubling the production of white flowers late in the season, while at the same time ceasing to produce any red blossoms (Fig. 17.22).

Unlike bees and moths, the short-tongued flies (which feed primarily on carrion, dung, humus, sap, and blood) are attracted by rank rather than sweet odors, and they rely very little on vision in locating food. The flowers of plants that depend on these flies for pollination are usually dull-colored and ill-smelling.

A particularly dramatic example of adaptation for pollination is seen in some species of orchids, whose flowers resemble in shape, odor, and color the females of certain species of wasps, bees, or flies (Fig. 17.23). The male insect is stimulated to attempt to copulate with the flower and becomes covered with pollen in the process. When he later attempts to copulate with another flower, some of the pollen from the first flower is deposited on the second. So complete is the deception that sperm have actually been found inside the orchid flowers after a visit by the male insect.

Flowers pollinated by wind or water rather than animals characteristically lack bright colors, special odors, and nectar. In fact, most of them have no petals, and their sexual parts are freely exposed to the air currents. The pollen grains produced by these flowers are particularly small and light, and it is not unusual for them to be blown hundreds of miles.
We see from this species comparison, then, that the characteristics of flowers are not simply pleasing curiosities of nature that serve no practical function. They are important adaptations that have evolved in response to fundamental selection pressures.

Mimicry

Species not naturally protected by some unpleasant character of their own may closely resemble (mimic) in appearance and behavior some dangerous or unpalatable aposematic species. Such a resemblance can be adaptive: the mimics may suffer little predation because predators cannot distinguish them from their models, which the predators have learned are unpleasant. This phenomenon is called Batesian mimicry (Fig. 17.32).

Convincing evidence for the potential effectiveness of this type of mimicry in protecting the mimic species comes from the elegant experiments of Jane van Z. Brower, then at Oxford. Brower produced an artificial modelmimic system with starlings as the predators and mealworms, which starlings ordinarily eat voraciously, as prey. She painted a tasteless color band on the mealworms, and dipped some in a distasteful solution. She then presented various ratios of noxious (dipped) models and mimics to different groups of birds. After a few unpleasant encounters with the models, the birds learned to recognize and avoid the painted worms, with the result that the mimics among them also escaped predation, particularly when the percentage of mimics presented to the starlings was 60 percent or less.

A few species pursue the opposite approach: aggressive mimics provide lures of apparently palatable prey to attract potential victims (Fig. 17.32C). In addition to Batesian mimicry, which is based on deception-mimicry of a distasteful or dangerous species by individuals of a species that is neither - there is a second kind of mimicry, called Müllerian mimicry, which involves the evolution of a similar appearance by two or more distasteful or dangerous species. In this type of mimicry, individuals of each species act as both model and mimic. The members of each species have some defensive mechanism, but if each species had its own characteristic appearance, the predators would have to learn to avoid each of them separately; the learning process would thus be more demanding, and would involve the death of some individuals of each prey species. Selection favors evolution toward one appearance; the various protected species thus come to constitute a single prey group from the standpoint of the predators, which accordingly learn avoidance more easily.

One striking case of Müllerian mimicry involves the monarch butterfly (Danaus plexippus) and the unrelated viceroy (Limenitis archippus). These two species look very much alike (Fig. 17.33) and are each distasteful to birds, though in different ways: monarchs sequester poisons from the milkweed plant upon which their caterpillars feed, while viceroys synthesize their own bad-tasting chemical. Birds experiencing one member of either species learn to avoid them both. (Until recently, the viceroy was thought to be a Batesian mimic of the monarch because blue jays, the predators used in early tests, are one of the few species of birds that find viceroys only slightly distasteful.)

The selective advantage of Müllerian mimicry may explain the similar markings of many unrelated species of wasps and bees, or of the group of poisonous reptiles known as coral snakes. If avoidance involves any genetic predisposition, then resemblances among the prey animals would facilitate more rapid selection for improved prey-recognition mechanisms in the predators. In fact, there is evidence that some predators have evolved the ability to recognize coral snakes innately; perhaps only Müllerian mimicry can provide a strong enough selection pressure to cause the evolution of such specialized recognition - a recognition that benefits individuals of both predator and prey species.

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