Is evolution like a soccer game, with long periods of stability interrupted by brief flashes of exciting activity? Or is it like a treadmill, perennially churning? In the early 1970’s, evolutionary biologist Leigh Van Valen sat down to answer that question, manually graphing “survivorship curves” for all organism groups which were sufficiently well represented in the fossil record at the time. His conclusion – so controversial at the time he started his own journal to publish it – was dubbed the Red Queen Hypothesis, one of the most famous literary metaphors in evolutionary science.
In Lewis Carroll’s Through the Looking Glass, Alice experiences the strange physics of the Red Queen’s country, where no progress is made no matter how fast you run. “Now here, you see, it takes all the running you can do, to keep in the same place,” the Red Queen tells Alice. Van Valen, who passed away October 16, used that episode and quote in his legendary 1973 paper to encapsulate his findings on the mercurial, unceasing nature of evolution.
Looking at the fossil record for several different biological genera (groups of similar species), the University of Chicago biologist saw the same pattern again and again: a steady decline in survival over time as species from each genera fizzle out and go extinct. To explain this uniform decline, Van Valen proposed the law of constant extinction, the tenet that species have an equal probability of going extinct at any time, never becoming more vulnerable or more resistant to being snuffed out.
But how could this be, if evolution is the story of species becoming gradually more adapted to their environment and, presumably, less likely to go extinct? Van Valen’s Red Queen Hypothesis filled that gap, proposing that no species could obtain perfect “fitness” because that goal was a moving target. While a species adapts to best fit its environment, the environment isn’t standing still. Gradual climate change, competition between species for the same food source, simultaneous evolution by the predators and prey of a species – all these elements and more are constantly shifting, making evolution a constant process rather than an occasional flurry.
“The Red Queen hypothesis was that the biological environment was constantly in flux in a way that was detrimental to the organisms living in it,” said David Jablonski, professor of geophysical sciences at the University of Chicago and a close colleague of Van Valen. “It’s like the Red Queen’s race in that everyone is madly scrambling, getting better all the time, but no one is gaining ground. A species can’t get far enough ahead of the pack such that it would be extinction-proof.”
Vivid examples of this constant turnover are found in nature through the “arms race” of hosts and parasites or predator and prey. Biologist Sean B. Carroll wrote about one such competition last week in the New York Times, discussing the ongoing tug of war between king cobras and mongooses evolving stronger venom and better venom resistance. Studies of the Red Queen relationship between snails and parasitic worms offered an elegant argument for why organisms reproduce sexually rather than asexually; the genetic reshuffling produced through sexual reproduction helps snails avoid parasitic infection.
Yet when Van Valen first proposed his theory and hypothesis, the evolutionary biology establishment resisted. After several rejections from the high-profile journals of the day, Van Valen went ahead and started his own, Evolutionary Theory, which premiered with “A New Evolutionary Law” as the lead article. After its publication, the theory continued to create controversy in the field, with some critics arguing that mass extinction events (such as the Cretaceous-Tertiary extinction event thought to end the time of the dinosaurs) were the primary drivers of evolution, invalidating Van Valen’s idea of perpetual motion. But Jablonski points out that Van Valen addressed that very complaint in his original paper, portraying such events as important, but rare, exceptions to the rule.
“That didn’t mean there were no external factors like rocks falling out of the sky or ice ages or rises or falls in sea level. He made it very clear that there were these physical challenges as well,” Jablonski said. “The interesting and sophisticated thing about this theory is that he saw both things taking place: this constant biological struggle and the intermittent perturbations that reset the system.”
In the nearly 40 years since it was published, the Red Queen Hypothesis has gained acceptance in the scientific community and been adapted to for uses beyond Van Valen’s original intent. The idea of relentless competition between organisms driving perpetual evolution has informed the study of host-parasite relationships and even offered clues to why organisms sexually reproduce rather than merely clone themselves, Jablonski said. A paper published in Nature earlier this year by scientists from England and New Mexico not only confirmed aspects of Van Valen’s theory, it found that it also contributed to a regular rate of speciation, the opposite pole to extinction.
Restless throughout his career, Van Valen subsequently moved on to a diverse array of other scientific questions, including the connection between brain size and intelligence and whether HeLa cells should be considered a separate species. Throughout his time at the University of Chicago, Jablonski said, he remained an open-minded scientist and teacher who prioritized creative and rigorous thinking, an attitude characterized by the conclusion to his 1973 paper.
“This is a novel way of looking at the world, one with which I am not yet comfortable,” he wrote. “But I have not yet found evidence against it, and it does make visible new paths and may even approach reality.”