A 30-million-year-old fossil leaf beetle from the Dominican Republic. The palm-feeding genus Delocrania is extinct in the Dominican Republic, but is indistinguishable from modern species of Delocrania (see below) in Central America. Photo by Brian Farrell.

One of every four animals on the planet is a beetle. And a new exhibit at Harvard's Museum of Natural History helps explain why.

"Beetle Boom: Beetle Stories and Beetle Secrets," which runs through April 4 in the Museum's Friend's Gallery, showcases beetles from the museum's insect collection, one of the world's most important with 7.5 million specimens.

The exhibit highlights the diversity of beetle species, with examples from the collections and groundbreaking research on how such diversity came about.

"Beetle Boom" boasts more than 5,000 individual specimens, less than a drop in the worldwide bucket of over 330,000 species. There are scarab beetles, dazzling jewel beetles, patterned harlequin beetles, antlered stag beetles, and even the world's biggest bug: the Goliath beetle.

Assistant Professor of Biology Brian Farrell lectures about palm-feeding leaf beetles at the "Beetle Boom" exhibition at the Museum of Natural History. Photo by Kris Snibbe.

One display may give pause to visiting farmers. It shows live weevils chomping through jars of corn, rice, and oats. "Just ask anyone at Bread and Circus what kind of damage beetles can do to stored grain," said Brian Farrell, J.L. Loeb Associate Professor and associate curator in entomology.

Farrell's research is the focus of "Beetle Boom." He is the first to present significant evidence that the diversity of beetle species is due to a process called co-evolution. His findings suggest that as flowering plants, which include everything from rice to roses, changed and developed new defenses against pests and diseases, beetles changed along with them, coming up with their own adaptations that would allow them to continue feeding on the plants.

The evidence, published in the journal Science, is thought to be the first quantifiable proof that co-evolution affects the diversity of any species. According to evolutionary biologist Douglas Futuyma at the State University of New York, "the magnitude of this work is astonishing."

Some of the thousands of beetles selected for the exhibition. Photo by Frank Siteman.

Farrell's investigation of co-evolution has also answered the question: Why so many species of beetle? Plants eaten by beetles are pressured to develop into a new beetle-resistant species. The new beetle-resistant plant creates a niche for a new kind of beetle, a beetle immune to the new toxins or able to thrive in the plant's habitat. Because nature abhors a vacuum, the niche is eventually filled with a new kind of beetle and the cycle starts again. Simply put, diversity begets diversity.

Using extensive analysis of the DNA, size, and shape of 115 beetle species, Farrell constructed a detailed family tree. By examining this genealogy, he compared beetles that feed on ancient plants called gymnosperms with beetles that feed on angiosperms, or flowering plants, the most diverse group of the plant kingdom. Farrell was able to prove that the beetles that began to feed on the flowering plants in turn became more diverse themselves, in one case a thousand times more so than their gymnosperm-eating relatives.

Though beetles have been his subject, Farrell's focus is not on the beetles themselves. His specialty is the evolution of interactions among organisms. Using similar techniques developed in his study of beetles, Farrell is turning his attention to viruses and vertebrates, seeking parallels to the evolutionary explosion that allowed beetles to take advantage of flowering plants as a food source.

Farrell said nature seems to make it easier for parasites to change so they can use a new host than for them to change so they can use the original host in a different way.

A modern species of Delocrania from Panama. Photo by Brian Farrell.

For example, a beetle that eats flowers of a particular plant seems to evolve more easily to eat the flowers of a different plant than it does to eat the stem or root of the original plant.

"This indirectly favors diversity by permitting parasite coexistence." Farrell said. "Viruses are taking advantage of us. We're the biggest resource on the planet."

British biologist J.B.S. Haldane was once asked by theologians what conclusions he could draw about God from years spent studying life on Earth. Haldane is reported to have said that God must have "an inordinate fondness for beetles."

"The Beetle Boom: Beetle Stories and Beetle Secrets" An exhibit at the Harvard Museum of Natural History explores the world of beetles and why, with 330,000 species, they're the most numerous animal on the planet. Time: Monday to Saturday, 9 a.m. to 5 p.m., Sunday, 1 to 5 p.m. Place: Friends Gallery, Harvard Museum of Natural History, 26 Oxford St. Date: Through April 4