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Uncovering Butterflies' Past
2 October 2009 (All day)
The incredible diversity of butterflies--and the lack of fossils--has long thwarted scientist's attempts to figure out their evolutionary relationships. A new study now reconstructs one butterfly family's history and reveals that it survived one of Earth's great extinctions.
Without a family tree, scientists' estimates for when butterflies first evolved have stretched from 35 million years ago--the age of the few available fossils--to 130 million years ago, about the time the southern super continent Gondwana split up.
So about 10 years ago, evolutionary biologist Niklas Wahlberg of the University of Turku in Finland and colleagues embarked on what Wahlberg calls a "pet project" to create a Nymphalid butterfly family tree. The Nymphalid family, which includes monarchs and many other eye-pleasing butterflies, has about 6000 species.
The researchers started collecting specimens from the family's 540 genera with the help of scientists and amateur butterfly collectors across the globe. When they reached 400 genera, they decided to start analyzing their specimens. They sequenced 10 genetic markers and characterized 235 features of the butterflies' bodies, such as the pattern of veins on their wings, to help build a Nymphalid family tree.
To figure out when groups of genera diverged from others, they needed help from the fossil record. Because there are so few butterfly fossils, Wahlberg and his team looked at the fossil record of flowering plants and made an assumption: If Nymphalids such as monarchs feed on milkweed today, their ancestors probably relied on milkweed's ancestors in the past. By connecting a group's plant of choice with when that plant's ancestor appeared in the fossil record, they could estimate the earliest date those butterflies evolved.
According to this new family tree, Nymphalids first evolved 90 million years go, the scientists report online 30 September in the Proceedings of the Royal Society B. "That basically means they were flying around with the dinosaurs," Wahlberg says. Next, he and his team plotted the number of butterfly lineages that popped up over time and found a stutter in the insects' evolution. About 64 million years ago, the rate at which new Nymphalid species appeared slowed briefly before increasing again. The timing of this evolutionary blip coincides with the so-called Cretaceous-Tertiary extinction event, when some scientists believe a catastrophic asteroid impact killed off the dinosaurs. The darkened skies created by this impact may have also killed flowering plants that Nymphalids depended on, Wahlberg says.
"This is a massive body of work--the diversity of specimens and traits is impressive," says evolutionary ecologist Carol Boggs of Stanford University in Palo Alto, California. Boggs believes that this tree could provide a geographical history of butterflies that would allow scientists to understand how the insects' ancestors adapted as they moved across the world. Comparing butterfly history with plant history could also help researchers study the evolution of plant-insect interactions, says Felix Sperling, an insect systematist at the University of Alberta in Canada. "A tree like this provides us with a road map to study historical relationships," he says.