Coevolution and Pollination

Coevolution

Coevolution is the the mutual evolutionary influence between two species (the evolution of two species totally dependent on each other). Each of the species involved exerts selective pressure on the other, so they evolve together. Coevolution is an extreme example of mutualism. Some examples of coevolution include:

Yucca moths and yucca plants: Yucca flowers are a certain shape so only that tiny moth can pollinate them. The moths lay their eggs in the yucca flowers and the larvae (caterpillars) live in the developing ovary and eat yucca seeds.
Acacia ants and acacia trees: Acacias are small, Central American trees in the Leguminosae. They have large, hollow thorns. The acacia ants live in the thorns. On the tips of its leaflets, the plant makes a substance used by the ants as food. The ants defend the tree from herbivores by attacking/stinging any animal that even accidentally brushes up against the plant. The ants also prune off seedlings of any other plants that sprout under “their” tree
Lichens: Lichens are composed of a mixture of fungi and algae. In each “species” of lichen, the alga and fungus are so closely intertwined that whole lichens are classified as species, rather than the component fungus/alga. The type of fungus and alga are species-specific. The alga does photosynthesis and produces sugars for fuel for both. The fungus attaches the whole lichen to its substrate (tree, rock) and holds in water needed by the alga.
Angiosperms, in general: Coevolution is often seen in a number of species of flowering plants that coevolved with specific pollinators (insects, bats, etc). The pollinator gets a reward such as nectar for pollinating the plant. Moth-pollinated plants often have spurs or tubes the exact length of a certain moth’s “tongue.” For example, Charles Darwin predicted the existance of a moth in Madagascar based on the size and shape of a flower he saw there. The moth was actually discovered about 40 years later. The common snapdragons that many people plant in their gardens are designed for a bumblebee of just the right weight to trip the opening mechanism.

Pollination

Wind-pollinated: Some plants (pine, grasses, ragweed) are wind-pollinated. Their reproductive strategy is to produce lots of pollen in hopes that some makes it to the female. These plants expend much energy in making pollen.
Insect-pollinated: Many plants depend on animals to spread their pollen. This is a mutualistic relationship where the plant and the pollinator benefit each other. The plant expends less energy on pollen production and instead produces showy flowers, nectar, and/or odors. Some plants/flowers are more general, while others are more specific.

Evolution of Pollination

The hypothetical evolution of pollination and angiosperms is tied together.

Wind-pollinated early gymnosperms: Early gymnosperms and angiosperms were wind-pollinated. Like modern gymnosperms, the ovule exuded droplets of sap to catch pollen grains.
Switching from wind to insect (beetle) pollinators: Insects (beetles) on the plant found this protein/sugar mix and used it as food. Insects then became dependent on this food source and started carrying pollen from plant to plant. Beetle-pollination must have been more efficient than wind for some species, so there was natural selection for plants that attracted insects.
Nectaries, color, and a folded carpel to enclose seeds: Next to occur would have been the evolution of nectaries, nectar-secreting structures, to lure the pollinators. Development of white or brightly-colored, conspicuous flowers to draw attention to the nectar and/or other food sources would also have occurred. The carpel (female reproductive structure) was originally leaf-shaped. It became folded on itself to enclose and protect the ovule from being eaten by the pollinators (hence Angiosperms). Plants with protected ovules would have been selected over ones with ovules that got eaten.
Further modification of flowers to attract pollinators: Violet with “nectar guides”

Monarch feeding on clustered, small, tube-shaped flowers By the beginning of the Cenozoic Era (65 mya), the first bees, wasps, butterflies, and moths had evolved. The significance in this is that these are insects for which flowers are often the only source of nutrition for the adults.
From this point on, certain plant and insect species have had a profound influence on one another’s evolution. A flower that attracted specific pollinators on a regular basis had an advantage (less wasted pollen) over flowers that attracted “promiscuous” pollinators. It is also an advantage for the pollinator to have its own “private” food source because there is, thus, less competition. The varied shapes, colors, and odors of flowers allowed sensory recognition by pollinators and excluded unwanted, indiscriminate pollinators.

Today, over 65% of Angiosperms are insect-pollinated and 20% of insects, at least at some stage, depend on flowers for their food.

Flower-Pollinator Relationships

For pollination to work, to be effective, a relationship must be established between the pollinator and the blossom to be pollinated, involving:

Pollinator must visit regularly

The pollinator should visit this particular blossom regularly. These visits (whatever the cause) should constitute a regular part of the life activities of the animal. The visitor must perform or at least try to perform certain tasks that are tied in with the structure and function of the blossom. Insects that happen to visit a couple flowers and transfer pollen don’t count as pollinators of that species unless they regularly visit that species of plant for some specific reason.

Plants must provide and advertize a reward

The plant must supply:

Some kind of reward (food?) for the pollinator (nectar, pollen)
Some kind of attractant to advertise the presence of the reward. This could be a direct attractant such as odor, color, shape, or texture, or...

an indirect attractant such as providing prey for predators.
While the exact role as pollinators played by such visitors is unclear, the possibility exists that a more direct insect-blossom relation may develop out of such a behavior.
This may be true of hummingbirds. They eat small insects and spiders and may have originally been attracted to flowers to eat the insects on them.

A means of putting pollen onto the pollinator such that it is effectively transferred to the next flower visited.

The reward offered is not always food. There is a tropical orchid with flowers that look and smell like females of a certain species of wasp. Males of this species emerge a week before the females. A male who smells a flower of this orchid, think it’s a female wasp, gets closer and the flower looks like a female, lands on it and it feels like a female, tries to copulate, gives up in frustration, and goes on to the next thing that smells like a female, and ends up transferring pollen.

Matching Flowers to Pollinators

Adaptations of flowers depend on the type of pollinator on which they depend.

Bees: Bees don’t see red, but do see yellow, blue, and UV. Thus, bee-pollinated flowers are mostly yellow or blue with UV nectar guides (landing patterns) to guide the bee. They usually have a delicate, sweet scent, and a small, narrow floral tube to fit the tongue-length of that species of bee. The flowers are sturdy and irregularly-shaped with a specifically-designed landing platform. For example, snapdragons will only open for a bee of the right weight.
Butterflies: Butterflies are diurnal and have good vision (can see red) but a weak sense of smell. They are perching feeders. Butterfly-pollinated flowers are brightly-colored but odorless. Often, these flowers occur in clusters (Compositae, milkweed) and/or are designed with a “landing platform.” Butterflies walk around on flower clusters probing the blossoms with their tongues. Each flower has a tube of suitable length for the butterfly’s tongue.
Moths: Moths are nocturnal, have a good sense of smell, and are hover-feeders. These flowers are white or pale colors so they are visible at night, and may only be open at night. Typically, these flowers have a strong, sweet scent (again, maybe only at night) and deep tubes to match the length of the appropriate moth’s tongue. The petals are flat or bent back (recurved) so the moth can get in.
Birds: Birds, especially hummingbirds have good eyes which expecially can see red but poor senses of smell. These flowers are brightly-colored, especially red, but have no smell, and have recurved petals so they are out of the way. Hummingbirds are hover-feeders, and these flowers (for example, columbine or fuchsia) are designed to dust the bird’s head (and back) with pollen.
Bats: Bats are nocturnal with a good sense of smell. Those bats which are pollinating species also have good vision and a long, bristly tongue. These flowers are open at night, and are white or light-colored with a musty odor like the bats in order to attract the bats. These flowers must be large and sturdy to withstand insertion of a bat’s head.
Flies and/or ants: Flies are attracted to rotting flesh. These flowers may be nondescript or may be brownish-red with a bad, rotten smell as their main attractant. Some flowers, near the ground, are pollinated by ants or ground beetles.
Non-specific: On the other hand, some plants have not specifically coevolved with a certain pollinator. Canada thistle flowers are visited by a wide variety of bees, beetles, and butterflies.
Similarly, goldenrod flowers attract a variety of beetles, wasps, and other small insects which, in turn, may be food for waiting spiders.

times since 25 Jun 2001.