Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live by Marlene Zuk Page B

experiment seems destined to be shrouded in mystery, but regardless, once Bumpus saw that about half the birds recovered after they were warmed, while the others died, he knew he had a gold mine on his hands. Natural selection requires some individuals to survive and reproduce while others fail, and here was an opportunity to see whether the sparrows that gasped to life in his laboratory had any characteristics that the others did not. So Bumpus methodically measured the size of bills, wings, legs, and other body parts in the survivors and those not so fortunate, and then compared the two groups. They differed substantially; birds that were either unusually large or unusually small fared worse than those clustered around the average size of the group, and Bumpus postulated that stabilizing selection, a kind of natural selection that winnows out the extremes and favors those in the middle, had been at work.
    His results have been scrutinized many times since he published them, and scientists still argue about the best way to analyze his measurements, but the fact remains that Bumpus’s sparrows are among the earliest examples of evolution occurring before our eyes, at least within the local population of house sparrows. Bumpus had not only confirmed that evolution happens in the wild; he had shown that it could, literally, occur overnight.
    How many darwins does it take to screw in a lightbulb?
    After Bumpus, several other scientists tried their hand at calculating rates of evolution, and at coming up with a unit of measurement that could be applied to it. It’s fine to say that the sparrows’ size changed after a storm, or that the silent crickets came to predominate after just five years, but how do you compare, say, a 10 percent increase in leg length after thirty generations with a 40 percent decrease in swimming speed after a hundred generations? Which is faster?
    The scientist who first tried to formalize measuring the rate of evolution was the great British geneticist J. B. S. Haldane. Born into an aristocratic family, Haldane became an enthusiastic scientist at an early age, performing physiology experiments on himself (he once drank hydrochloric acid to see how it would affect his muscles), as well as a public intellectual, writing a dystopian science fiction novel published in 1924, and an avid Marxist. Haldane’s paper outlining the technique for assessing how fast evolution occurs is regrettably much less colorful than his fiction, but it does suggest using the percentage of change in a characteristic—for example, leg length—and combining that measure with the characteristic’s standard deviation, a statistical measurement of how much a trait varies within a population. 3 He suggested a unit of measurement called, appropriately enough, a “darwin,” which means that if we had lightbulbs being installed at two different times, we could, at least in theory, ask the question in the title of this section.
    Haldane’s method, however, is best applied to changes that take a long time, like the interval between the appearance of different kinds of teeth in fossil horses from different time periods (the example he used in his 1949 paper). Long after Haldane’s death from cancer in 1964 (while ill he wrote a poem called “Cancer’s a Funny Thing”), paleontologist Phil Gingerich proposed a different unit, which he dubbed the haldane. 4 Both measures are currently used by scientists, although neither has made it into the popular vocabulary.
    If that is how evolution is measured, what do we mean when we say that evolution is fast? These days, evolution is considered rapid if a population shows a genetic change over tens of generations or fewer, or sometimes as much as a hundred generations. Rapid evolution is sometimes called “contemporary evolution,” to emphasize that it happens within a modern time span, or “evolution in ecological timescales,” to emphasize that evolution can be important to