Nonfiction: Nabokov theory on butterfly evolution is vindicated

Few professional lepidopterists took these ideas seriously during Nabokov’s lifetime. But in the years since his death in 1977,his scientific reputation has grown. And over the past 10 years,a team of scientists has been applying gene-sequencing technology to his hypothesis about how Polyommatus blues evolved. Last week in the Proceedings of the Royal Society of London,they reported that Nabokov was absolutely right.

Nabokov spent much of the 1940s dissecting a confusing group of species called Polyommatus blues. He developed forward-thinking ways to classify the butterflies based on differences in their genitalia. He argued that what were thought to be closely related species were actually only distantly related. He speculated that they originated in Asia,moved over the Bering Strait,and moved south all the way to Chile. While he conceded that the thought of butterflies making a trip from Siberia to Alaska and then all the way down into South America might sound far-fetched,it made more sense to him than an unknown land bridge spanning the Pacific.

Dr Pierce,who became a Harvard biology professor and curator of lepidoptera in 1990,began looking closely at Nabokov’s work while preparing an exhibit to celebrate his 100th birthday in 1999. She was captivated by his idea of butterflies coming from Asia. “It was an amazing,bold hypothesis,” she said. Pierce organised four separate expeditions into the Andes in search of blues. Back at her lab at Harvard,she and her colleagues sequenced the genes of the butterflies and used a computer to calculate relationships between them.

There were several hypotheses for how the butterflies might have evolved. They might have evolved in the Amazon,with the rising Andes fragmenting their populations. If that were true,the species would be closely related to one another. But,she and her colleagues found that the New World species shared a common ancestor that lived about 10 million years ago. Pierce and her colleagues concluded that five waves of butterflies came from Asia to the New World—just as Nabokov had speculated. They also investigated Nabokov’s idea that the butterflies had come over the Bering Strait. The land surrounding the strait was warm 10 million years ago,and has been chilling steadily ever since. They found that the first lineage of Polyommatus blues that made the journey could survive a temperature range that matched the Bering climate of 10 million years ago. The lineages that came later are more cold-hardy,each with a temperature range matching the falling temperatures.CARL ZIMMER