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Professor and Students Unlock Secrets of Co-evolution

When visitors travel Nevada's famed "Extraterrestrial Highway," they may see something even stranger than the UFOs they hope to spot - two Willamette students on tall ladders peering down at the flowers that grow on top of Joshua trees. The students are documenting what many consider the Holy Grail of co-evolution - direct evidence of reciprocal natural selection between species.

Undergraduate students Jeff Collins '10 and Tyler Starr '10 are part of a research project spearheaded by biology Professor Chris Smith and profiled in Smithsonian Magazine, The American Naturalist and Evolution, where it was featured as a cover story. Funded with a generous grant from the National Science Foundation, the study is far-reaching both intellectually and geographically, with 43 research sites spread across Arizona, Nevada and California.

Smith and his students are looking at Joshua trees to determine whether reciprocal evolutionary change is shaping the relationship between the tree and its pollinators. The ancient trees are postcard icons of the American Southwest, remarkable not only for their spiny, bizarre appearance (explorer John Fremont called them "repulsive") but also for their highly specialized and unusual pollination pattern.

The trees are pollinated exclusively by two species of yucca moth. The moths, in turn, reproduce by laying their eggs in the creamy white Joshua tree flowers. Upon hatching, the eggs become caterpillars that eat Joshua tree seeds. Thus, the relationship between the Joshua tree and its pollinators is one of complete interdependence; both the moth and the desert plant are entirely reliant upon one another for reproduction.

This system is one of the most remarkable pollination mechanisms in the world - so intriguing that even Darwin weighed in on the questions it raised about natural selection and evolution. It has since become a staple of Biology 101 textbooks.

"We're trying to understand how natural selection shapes this interaction," Smith says. "Once you zoom in and look at the details, it gets very exciting." He and his students have discovered compelling evidence that the two species are evolving together in a reciprocal manner, each influencing the evolution of the other.

Smith and Collins also study the impact of climate change on Joshua trees. "In the course of traveling the Southwest to visit populations of Joshua trees, a pattern struck me almost immediately," Smith says. "Populations in the southern part of the plant's range and those at low elevations are composed mostly of very tall trees. It's hard to miss this when many trees you want to examine require a 40-foot climb up a ladder."

These populations are tall because they are very old, with many believed to be hundreds of years old. There are few seedlings or young trees, meaning no new trees are growing. The populations going extinct are those in the arid south with little rainfall - that is, the populations that are probably experiencing the most severe effects of global warming.

Their loss would be significant, as they are a linchpin of desert ecology, providing a home for birds, and food and moisture for rats, squirrels and jack rabbits. If the smaller mammals don't survive, the ecosystem may also experience a drop in the coyotes, foxes and hawks that depend on them for survival.

Smith works with Collins and Starr through the Science Collaborative Research Program, which prepares undergraduate students for professional careers through collaborative research with faculty. Students communicate their research findings in a formal paper and oral presentation for a campus symposium, as well as an off-campus regional meeting for undergraduate researchers.

"I'm astounded at their abilities," says Smith of his lab assistants. "They are both as talented as some of the best graduate students I've worked with."

"There's so much more I want to ask," says Starr, who, like Collins, plans to further his studies in graduate school. "This definitely brings up a lot of questions even as you answer them. It's a constant search for answers."



08-25-2009