Monday, October 1, 2007


Note: In response to a discussion about invasive species going on over on Jim's blog, here's a little piece I wrote on the concept of coevolution. If there are any real scientists out there in the audience, I'd be thrilled to have your feedback (including, of course, your corrections of any errors I've made). I know that the issues are more complex than I've presented them here.

Have you ever seen the metallic shimmer of a hummingbird dipping its beak into the scarlet flower of a trumpet vine or coral honeysuckle? If you could freeze time for a moment and step up to take a closer look, you would see that the long, slender tongue of the hummingbird is perfectly adapted for extracting nectar from the flower’s deep corolla. Bird and flower seem to be made for each other. In a sense, they are—their perfect fit for each other is the result of a long process known as “coevolution.” Coevolution is said to occur when, in the words of biologist Paul Ehrlich, two or more organisms “influence each other’s evolution.”

What exactly does this mean? How can organisms influence each other’s evolution? To see the entire picture, we have to unfreeze time and watch the hummingbird dart to another red trumpet vine flower. As the hummingbird flits from flower to flower, it collects food for itself in the form of nectar. At the same time, it distributes pollen from the flowers, enabling them to create seeds and reproduce. Both the hummingbird and the flower gain some advantage from the relationship. In other words, their relationship is an example of “mutualism,” in which two organisms gain a mutual benefit from their relationship.

This mutual relationship did not come about by accident. Both hummingbird and flower adapted to each other over the course of evolutionary time. Evolution selected for hummingbirds with long, slender tongues and for flowers that stash their nectar in deep corollas. Hummingbirds with short tongues and trumpet vine flowers with shallow corollas couldn’t compete in a system that favored long tongues and deep corollas.

And there’s more to the story. If you could see with the eyes of a hummingbird, you would find yourself attracted to the color red. It’s no coincidence that in Europe, where there are no native hummingbird species, there are also fewer species of red flowers than there are in the Americas. Red flowers with deep corollas and hummingbirds go together like interlocking pieces in the complex puzzle of coevolution.

An interesting footnote to this story was added recently when a German scientist discovered the fossil remains of an ancient hummingbird near a small town in Germany. This suggests, of course, that hummingbirds did live in Eurasia at one time. For a long time, scientists had recognized that there were a few plants native to Asia and Africa that seemed to adapted to pollination by hummingbirds—plants with red, trumpet-like flowers. How, the scientists wondered, did these flowers evolve in the absence of hummingbirds? Now the missing piece of the coevolutionary puzzle has been found. These flowers probably evolved in a relationship with Eurasian hummingbirds, like the fossilized species found in Germany, and managed to survive even after the hummingbirds themselves had, for whatever reason, become extinct.

Not every example of of a mutual relationship in nature is as stunning as the relationship between hummingbirds and red tubular flowers. My favorite example is hidden underground, among the roots of trees in the forest. The roots of most forest trees are entangled with tiny fungi called mycorrhizal fungi, which help the tree extract nutrients from the soil and protect the tree from diseases. The tree, in turn, provides the fungi with water and nutrients from the atmosphere. This intimate relationship between tree and fungus has evolved over time—like a long courtship that has resulted in a marriage that supports and sustains both partners.

Sometimes a single flower can eloquently tell the story of the intimate and necessary interrelationships in nature. Consider the bloodroot, which still grows in abundance in the hardwood forests of Minnesota. The bloodroot’s intimate relationships with its ecological neighbors—with the roots of trees and with the insects of the forest—suggest a complex evolutionary story in which numerous organisms have adapted to live harmoniously together.

In early April, bloodroot begins to bloom on the wooded bluffs above the Cannon River. The single varicose leaf is still scrolled loosely around the stem as the white petals unfold to reveal a small cluster of stamens, gilded with pollen. It’s the bloodroot’s fleshy rhizome, loaded with stored nutrients, that allows the flower to bloom so early in the season. It’s also this rhizome, with its bright crimson juice, that gives the plant its name: bloodroot, sanguinaria. Native Americans used the red juice from its root for war paint.

In the evening, as the temperature drops, the petals close and appear to glow like a steady white flame at the end of their pale wick, conserving their warmth. Flies, along with some early bees, pollinate the flowers. On the third day, if no pollinators have appeared because of cold weather, the stamens collapse onto the pistil and self-pollinate the flower.

Bloodroot blooms for two days, four at most, before the flowers burn themselves out. The petals fall away, the leaf spreads open, and the ovary begins to swell. In late spring, the mature ovary produces up to seventy seeds, each one with an oil-rich crest called an elaiosome that attracts the attention of ants. The ants carry away the seeds (seed dispersal by ants is called myrmecochory), consume the elaiosome, and leave the seed to germinate. The removal of the elaiosome by the ants facilitates germination; the eliaosome in turn contains a steroid-like chemical that promotes favorable sex ratios in ant populations—it insures that there are enough males and females to go around.

Such intimate relationships abound in nature—between flowers and pollinators, between predators and prey, between plants and seed-dispersing birds and insects, between fungi and angiosperms (that is, flowering plants). These delicate relationships, developed over time through the slow process of evolution, remind us of the fragile interconnections that sustain all life on earth. They remind us that, in the larger scheme of things, we cannot harm the smallest flower without harming ourselves.

1 comment:

Rob Hardy said...

From my sister-in-law, an evolutionary scientist at the U of M:

Hadn't checked your blog in a little while, until this afternoon. I always enjoy it, but these first entries of October especially appeal to me! Just worked my way back to "Interrelationships" - as an evolutionary scientist, I find this very well done. I hadn't heard about the fossil hummingbird find in Germany - very interesting! But I'd be surprised if red, tubular flowers evolve only in
association with hummingbirds. I'm not a pollination biologist (and
sadly, we have none in all of the enormous U. of Minnesota!) - maybe
there is consensus that red,tubular flowers are pollinated only by hummingbirds, but if so, what is pollinating the flowers of that kindin the European flora, or do they now self-pollinate? So this is very intriguing. I have only 1 correction - a spelling error:
elaiosome ... myrmecochory is a very cool phenomenon!

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