This is a really cool result: a paper published 66 years ago, speculating about the evolutionary history of a group of butterflies, has just been vindicated by a combination of new molecular and ecological work.
But what is even cooler is that the author of the earlier paper was Vladimir Nabokov.
You know Nabokov (1899-1977) as a famous writer, author of, among other books, Lolita, Pnin, Pale Fire, and Speak, Memory. He also taught literature at Cornell from 1948 to 1959; his classroom lectures have been published and they are absolutely superb.
But what you probably didn’t know was that he was also a world-class lepidopterist, specializing in the “alpha taxonomy” (description and publication of new species) of butterflies. From 1945-1948 he was Curator of Lepidoptera at Harvard’s Museum of Comparative Zoology (MCZ), the institution where I got my Ph.D. If you read his novels, you’ll often find a mention of butterflies.
Here’s Nabokov at the MCZ in 1945, from the article on his butterfly work at the New York Public Library Site:
Nabokov’s speciality was the butterfly subfamily Polyommatinae (called “blues” because of their color) in the family Lycaenidae.
Lycaeides melissa, subspecies samuelis. A “blue” from the northeast U.S. The species was first described by Nabokov.
In 1945 Nabokov published a formal description and revision of some lycaenid butterfies in Psyche, the journal of the Cambridge Entomological Club (I’m a member!). Here’s the title of his paper:
The paper is long (62 pages), and largely devoted to describing species based on their genitalia, the character that seems to evolve most rapidly among insects—and many other groups, probably because of sexual selection (see William Eberhard’s excellent Sexual Selection and Animal Genitalia). At the end of the paper he speculates, based on the morphology of the various species, that the “blues” arrived in the New World from Asia over the Bering Strait. He further speculated that there were actually five successive invasions from Asia, each giving rise to a different New World group. And the first invasion produced species that made it to Central and South America, with the North American representatives eventually becoming extinct.
A group of investigators from a bunch of places, led by my friend Naomi Pierce at Harvard, just tested Nabokov’s theory with the modern tools of molecular biology and systematics. Their results, published in a paper in the Proceedings of the Royal Society (free access!) and described by Carl Zimmer in yesterday’s New York Times, show that Nabokov was right on all counts.
Here’s what the authors found:
- Using six genes, they made a molecular phylogeny of 73 species of blues from all five “sections” described by Nabokov. This analysis showed that, indeed, the five separate invasions posited by Nabokov each produced a monophyletic group (i.e., a group descended from a single ancestral species). The butterflies also descended, as Nabokov posited, from Asian ancestors, and certainly came to the New World by migration over the Bering Strait.
- The oldest group was, as Nabokov suggested, the butterflies in the neotropics: Central and South America. The other groups were younger, and in the precise age sequence that Nabokov posited had arrived from Asia (and then radiated in North America).
- The branches of the phylogeny were dated using a “molecular clock.” They were found to have begun about 11 million years ago, and continued until about 1 million years ago. During the earliest part of that period the Bering Strait was not continuous between Asia and North America, and so some of the ancestral blues must have crossed an expanse of ocean.
- The authors then determined the temperature-tolerance ranges of each group based on the climate where its members are now living. From that, they reconstructed the temperature tolerance of each group’s ancestors. (You can do this for any measurable trait, morphological or physiological, using a method called “ancestral character state reconstruction”.) And they found that the temperatures tolerated by each successive invader declined over time. The earliest invader tolerated higher temperatures than the next, and so on for all five invasions.
- The spiffy result: the temperature tolerances posited for each ancestor matched very nicely the temperatures thought to have prevailed in “Beringia” (the region on either side of the Bering Strait) at the time of the invasions. Here’s part of the authors’ Figure 1, showing the temperature tolerances of ancestors during each of the five invasions (vertical bars), and the temperatures posited to have existed in Beringia, all over the 11 million years from the first to the last invasion:
Note the good match between the actual temperatures in Beringia and the temperatures tolerated by the ancestral invader of each of the five groups. Clearly, the butterflies now in the neotropics tolerated high but not low temperatures, and so were likely driven to the south as the climate became colder. This also implies that each group has pretty much retained the ancestral temperature tolerance of its ancestor, though the ancestral five groups did diverge from one another in tolerance.
What a lovely piece of work, and how nice that the modern work, using all the highfalutin tools of molecular biology, systematics, ancestral state reconstruction, and so on, managed to confirm the speculations of one itinerant zoologist/author equipped only with a microscope and a bunch of butterfly genitals!
R. Vila, C. D. Bell, R. Macniven, B. Goldman-Huertas, R. H. Ree, C. R. Marshall, Z. Bálint, K. Johnson, D. Benyamini, and N. E. Pierce. 2011. Phylogeny and palaeoecology of Polyommatus blue butterflies show Beringia was a climate-regulated gateway to the New World. Proc. R. Soc. B published online before print January 26, 2011, doi:10.1098/rspb.2010.2213