Pollination involves plants and often insects. (Forget pollination by other animals — such as birds and bats — and wind for now; those situations aren't important in this discussion.) Each species has a vested interest in getting what it needs. The plant, for example, is trying to get its pollen to (or get pollen from) another plant of the same species. To do that, the plant needs to spend effort and resources on attracting pollinators. It has absolutely no interest in helping its pollinator unless, by doing so, it helps itself. For its part, the insect (or pollinator) cares nothing about whether the plant gets pollinated; it's after the reward offered, or promised, by the plant.
Ideally, plants want a pollinator that's very species-specific — that is, attracted only to plants of the same species. After all, what good is a pollinator that dumps your pollen on incompatible plants? With a species-specific pollinator, the plant would be assured that its pollen would go to an individual of the same species.
Consider the orchid species that has nectar 10 inches down in a very long, thin flower. Based on his study of this orchid, Charles Darwin predicted that the pollinator would be a moth with a 10-inch tongue. At the time, no one had ever seen a moth with a tongue that long, and people considered his idea to be pretty ridiculous. But 40 years after Darwin's prediction, exactly such a moth was found — and whaddya know, it turned out to be the pollinator of this particular orchid. In honor of Darwin's prediction, the moth was named Xanthopan morgani subspecies praedicta, just barely beating out Darwin-Hes-the-Man Moth.
The example of the moth with the long tongue shows the benefits of a tight association between the two mutualists. The orchid gets its pollen delivered only to its own species while the moth gets access to resources that the shorter tongued moths can't reach. But there are also potential downsides to being involved in such a tight interaction:
^ If one of the partners is absent, then the other is out of luck. And if one species should go extinct, the other one might not be far behind.
^ It reduces the probability of dispersal to new environments. Imagine an orchid seed carried on the wind to a remote island. The soil's just right, the temperature is perfect, and lots of other plants are doing just fine. But that orchid had better hope that some of those moths get blown over to the island too! Because if none do, then that wandering orchid is doomed.
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