A Taste for Flowers Helped Beetles Conquer the World

July 28, 1998

A Taste for Flowers Helped Beetles Conquer the World

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330,000 Beetles and Counting

By CAROL KAESUK YOON

warming the world in a dazzling array of shapes, colors and sizes from gargantuan Goliath beetles to jewel-like tortoise beetles to dearly familiar ladybugs, beetles, with more species than any other plant or animal group on earth, are the undeniable rulers of the planet.
In fact, so overwhelming is the diversity of these creatures that it is the subject of what has been called evolutionary biology's best known (and perhaps only) one-liner. According to lore, in the middle of this century, the British biologist, J. B. S. Haldane, when asked by a group of theologians what one could glean about the Creator from a study of His creation, is said to have replied, "an inordinate fondness for beetles." Yet despite a longstanding fascination with these armored beasts, biologists have been able to do little more than speculate about how the 330,000 known species of beetles have come to dominate the living world.
Now a study in the latest issue of Science says that the secret to the diversity of beetles lies in what they eat.
Dr. Brian D. Farrell, a curator at the Museum of Comparative Zoology at Harvard University, has shown that groups of beetles that long ago evolved to eat flowering plants spun off thousands of species, many more than the beetles that continued to eat more primitive and less diverse plants. By feasting on flowering plants, which evolved to become the world's most diverse group of plants and includes nearly all the familiar species, among them apple trees, orchids and grasses, the beetles were apparently able themselves to produce many species.
In fact, researchers say the study explains the diversity not only of beetles, but of the many insects that have evolved to chew, suck and otherwise devour flowering plants. Plants and the insects that eat them are two groups whose diversity demands explanation, because together they make up more than half of all known species.
"It's the classic scenario," said Farrell, referring to the well-accepted notion that the evolution of new species adapting to unexplored habitats or new ways of life accounts for much of the diversity of life. "Here was this huge, underexploited resource, the flowering plants. Insects that were able to evolve to make the shift to eat them, enjoyed the fruits, so to speak."
Perhaps most important, the new study provides an answer to one of the most fundamental, and difficult to address, questions in biology: Why are there so many species on earth? The reason appears to be simple: Diversity begets diversity. In fact, it may be no coincidence that the flowering plants are themselves so diverse, because the beetles and other insects attacking them might have provided pressure for the evolution of new, better-defended plant species.
More plants spawn more beetles. More beetles may spawn more plants, as well as more parasites on beetles or more predators, which in turn spawn creatures that eat those predators and on and on.
"It's something we all believe in and we all think is so," said Dr. John N. Thompson, evolutionary biologist at Washington State University and a fellow at the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. "This is one of the best pieces of data we have to show that what we all believe really is so."
Dr. Douglas Futuyma, evolutionary biologist at the State University of New York at Stony Brook, said of the new study: "This is going to make quite an impact. The magnitude of the work is astonishing."
Farrell constructed an evolutionary tree including more than 100 species representing the many kinds of beetles that eat plants. He used DNA sequence data and data on the shapes of beetle species to determine which species are most closely related and which are not.
By examining this family tree, Farrell could look at groups of close relatives and see which had continued eating older, primitive plants known as gymnosperms, which include pine trees and other conifers, cycads and ginkgoes, and which beetles had begun to eat the more recently evolved flowering plants. In every case, groups of beetles that switched to eating flowering plants were more diverse than their gymnosperm-eating counterparts, in one case a thousand times more diverse.
The new study is one of a number in recent years that attest to the power of using family trees of organisms to rigorously test hypotheses. In the past, biologists trying to explain the success of a group of organisms like the beetles would simply focus on one of the group's notable features. For example, a popular theory for why beetles had done so well was that their hard, protective wing coverings -- which allow a beetle to do things like burrow under bark with impunity -- had allowed them to live successfully in many different situations and thus spin off new species. But hard pressed to test such ideas, biologists ended up generating what had been derided by peers as just-so stories.
Even more important than explaining the explosion of beetles, the new study speaks to the more general question of why there are so many species of all types on earth.
Researchers had long theorized that one of the great engines driving the evolution of diversity was a process known as co-evolution, a kind of tit-for-tat process in which species adapt or spin off new species as the species they interact with evolve. A new prey species evolves, so the theory goes, and a new predator evolves to attack it, in turn prompting the evolution of new, better defended prey species, which prompts the evolution of new predators and so on. The process, thought to apply to any number of intimately interacting species, was articulated 34 years ago by Dr. Paul Ehrlich at Stanford University and Dr. Peter Raven at the Missouri Botanical Garden and explains a living world that seems to evolve to ever-greater diversity.
Ehrlich, Bing professor of population studies, said of the new study, "This is a nice confirmation that interactions are important in the diversity of life."
It is particularly nice because, in recent years, the theory of co-evolution had fallen on hard times, in large part because of a study published in 1993 in Science by Dr. Conrad C. Labandeira, paleobiologist at the National Museum of Natural History at the Smithsonian Institution, and Dr. Jack Sepkoski, a paleobiologist at the University of Chicago. The two researchers found that in the fossil record, the numbers of insect families, which are large groups of many insect species, did not increase with the appearance of flowering plants, suggesting that insect diversity was not associated with the diversification of the plants they ate.
But Labandeira, calling the new study "very significant," agreed with Farrell that their respective studies did not conflict. The earlier study, he said, looked at insect families, while the current study looks at the numbers of insect species. Labandeira said that his continuing study of fossil species would probably corroborate the new findings.
Some questions remain about the species-rich beetles. As Dr. May Berenbaum, evolutionary ecologist at the University of Illinois at Urbana-Champaign notes, it remains a mystery why beetles diversified so much more wildly than other insects or even mammals whose mainstay is also the shoots, roots, leaves, flowers or seeds of these plants.
Perhaps the most surprising result was the discovery that while some beetles developed many new species that could eat the evolving new plant species, other beetles were remarkably conservative in their eating habits.
At the base of the beetle family tree, Farrell found a number of ancient lineages of leaf beetles, snout beetles and long-horned beetles that still eat the primitive gymnosperms that they ate 200 million years old when dinosaurs ruled the earth. Farrell said these ancient associations are the oldest plant-insect antagonisms known. In Argentina, where beetles still eat Araucaria, an ancient lineage of gymnosperms, scientists have even turned up fossil Araucaria plants chewed up by ancestors of these beetles.
Researchers described the findings as "astonishing" and "extraordinary." Labandeira said: "It's something that a lot of people probably would not believe. People think that these associations are very volatile."
The reason, said Berenbaum, is that insects are known to be able to evolve extremely quickly. They have been observed to evolve insecticide resistance or to switch to eat plant species newly arrived in their habitat, a stark contrast to the monotonously consistent life style of some ancient beetles.
Farrell simply called these communities of antique insects and plants "a little Triassic Park."
Evidence exists that other ancient lineages of moths and hymenoptera, a group that includes bees and wasps, also eat old lineages of plants, suggesting that many associations between the eaters and the eaten persist intact from the deep past. What makes these associations so stable -- whether it is somehow very difficult for these insects to evolve tastes for new foods, or whether these ancient plants are too good a treat for some insects to give up -- remains a mystery.

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