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How the Butterfly Discovered Daylight

Once upon a time, perhaps some 300 million years ago, a tiny stream-dwelling insect akin to a caddis fly crawled from the water and began to live on mosses and other land plants. Although drab in appearance, the creature had a glorious future: It would become the ancestor of the 160,000 species of moths and butterflies that populate Earth today.

Few insect fossils have been found, so reconstructing the steps in this long evolution has been difficult. A group of biologists has now filled some of the major gaps in the fossil record, with the help of data from the DNA and protein sequences of living insects. On Monday the team, led by Akito Y. Kawahara of the Florida Museum of Natural History in Gainesville, published a paper in Proceedings of the National Academy of Sciences that rewrites key aspects of the narrative of how moths and butterflies evolved.

A central part of this story is the furious evolutionary battle between moths and bats. Bats hunt moths by emitting pulses of sound. But moths gradually adapted, developing the capacity to detect the pulses and respond with evasive flight maneuvers. The bats adapted in turn, and new species arose that used sonar frequencies that the moths could not hear as well. New moths arose in response, equipped with countermeasures: They could broadcast sounds that either jammed the bat’s sonar, probably by throwing off the estimate of the moths’ distance, or advertised that the moth’s tissues were poisonous.

One group of moths escaped this deadly arms race by flying during the day; they eventually gave rise to butterflies. Biologists have long assumed that it was bats that drove butterflies from the night. But Dr. Kawahara’s team has concluded that food, not predation, was the more likely evolutionary prompt.

The team reconstructed the ancient timeline using DNA sequences of contemporary moths and butterflies. They calculated that the ancestral moth emerged some 300 million years ago, at the end of the Carboniferous era, well before the oldest known moth fossil, which is only 200 million years old.

Some 240 million years ago, Dr. Kawahara’s team found, most moths ceased to have chewing jaws and instead developed tubelike mouthparts capable of sucking up sap and water. The team also determined that the earliest butterflies evolved some 98 million years ago — a major surprise, because echolocating bats emerged only much later, some 50 million years ago. Something other than bats must have turned the butterflies into daytime fliers.

The most likely agent of natural selection was the nectar being produced by the many new species of flowering plants, Dr. Kawahara’s team believes. Bees evolved some 125 million years ago, and the plants produced nectar to secure them as pollinators. Because moths had already developed strawlike mouthparts, one group was able to exploit the novel food source, and evolved into butterflies. They switched their flying hours from night to day, Dr. Kawahara said, because nectar is more generally available during the day, when flowers are open.

Living in daylight, the butterflies exchanged the drab, brown livery of many nighttime moths for a rich palette of colors, useful for broadcasting amatory signals to mates and warnings of toxicity to predators.

The new DNA data, sampled from all the major families of butterflies and moths, has helped rewrite another piece of evolutionary history: why hearing developed in moths. Moths evolved ears at least nine different times, in several cases before the evolution of echolocating bats, the DNA data revealed. Perhaps ears helped moths detect the sound of birds’ feet and wings, Dr. Kawahara said. These acoustic faculties thus were already in place to meet the challenge of echolocating bats.

Dr. Kawahara said that the new techniques of genome analysis have enabled him to fulfill a “childhood dream” of reconstructing how these insects came to be. “My personal passion growing up as a child in Japan and the United States was butterflies and moths,” he said.

Maria Heikkila, an evolutionary biologist at the University of Helsinki in Finland, described Dr. Kawahara’s study as “ a step toward a better understanding.” But the dates derived from DNA and fossils are likely to be revised in the future, she said, and a new evolutionary story may emerge.

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