Biston betularia, industrial melanism
In Defense of Darwin and a Former Icon of EvolutionMICHAEL MAJERUS PROFILE: (Majerus at wikipedia) - Death of Mike Majerus
Fiona Proffitt - Science, Vol 304, Issue 5679, 1894-1895 , 25 June 2004
After a severe drubbing, the famous example of the peppered moth is getting refurbishedCAMBRIDGE, U.K.--Michael Majerus's fascination for "bugs," as he calls all insects, was ignited at the tender age of 4. His mother can recall the exact moment: It was late summer, and he caught his first butterfly--a red admiral resting on a white chrysanthemum--with his bare hands. From that point on, he was hooked. At night when other children were in bed, the young Majerus roamed the English countryside tending his moth traps. Now 50 and in his 25th year of teaching evolutionary genetics at the University of Cambridge, Majerus still runs his moth traps most nights.
Majerus's research has focused on sexual selection, sex-ratio manipulation, and the evolution of melanism (the darkening of body color) in various moths, butterflies, and ladybirds. But over the past few years, half of his working life has been occupied by one controversial species, the peppered moth, Biston betularia, and an infamous study that's been attacked by both evolutionary biologists and anti-evolutionists.
Through his research, Majerus found himself embroiled in the scientific debate over the evolutionary forces behind melanism in the peppered moth. Experiments by British lepidopterist Bernard Kettlewell in the 1950s claimed to show that bird predation, coupled with pollution, was responsible for a color shift in the moth population. But problems with Kettlewell's methodology led some scientists to doubt his conclusions. Majerus was not the first to point out the flaws, but by doing so, he inadvertently set off a wave of anti-evolutionist attacks. While acknowledging that Kettlewell made mistakes, Majerus believes Kettlewell was right in his conclusions and has taken it upon himself to prove it.
As Majerus shows off some of the roughly 100,000 peppered moth pupae he'll rear for his latest experiment, it's clear that he's prepared to go to great lengths to make his case. Once the moths begin to emerge in May, Majerus begins a daily grind. He releases them at dusk and gets up at dawn to observe their fate: counting how many are plucked from their resting places by birds, and how many survive to see another night. He will continue this routine into August, as he has done for the past three summers. All he needs, he reckons, is another 2 years' worth of data--a total of some 4000 moth observations--to settle the controversy over whether bird predation is the major selective force in favoring one color form of the peppered moth over another.
Small and unobtrusive, the peppered moth doesn't look like the star in an evolutionary drama. But the rise and fall of the almost-black melanic form (carbonaria) in tandem with changing pollution levels has become the most famous example of evolution in action. Through his pioneering experiments, Kettlewell claimed to have demonstrated that melanic peppered moths were more common in industrialized areas because they escaped the attention of predatory birds when resting against soot-blackened, lichen-free bark. Because more of the darker ones survived to produce the next generation, he argued, entire populations grew darker.
But doubts emerged over Kettlewell's methodology in recent decades as researchers failed to replicate some of his results. His predation experiments were chiefly criticized for their artificiality: He placed the moths on exposed parts of trees in broad daylight, when they don't normally fly, rather than allowing them to settle naturally; he released them in large numbers, thereby inflating moth densities and possibly creating a magnet for predatory birds; and he used a mixture of lab-reared and wild-caught moths without checking to see whether they behaved the same way. Majerus summarized these criticisms in a book on the evolution of melanism in 1998 and stated that the simplified textbook story of the peppered moth was inaccurate, while asserting that Kettlewell's conclusions were qualitatively sound. Majerus had no idea at the time what a furor his book would cause.
Jerry Coyne--a highly respected evolutionary geneticist at the University of Chicago--concluded in his review of Majerus's book in the 5 November 1998 issue of Nature that "for the time being, we must discard Biston as a well-understood example of natural selection in action, although it is clearly a case of evolution." Coyne's words carried weight, and the anti-evolutionists were quick to twist them into an argument against natural selection itself. "Coyne might have just thought he was stirring things up. ... As it happened, he was the lightning conductor," observes Mark Ridley, an evolutionary biologist at the University of Oxford. Coyne thought the flaws in Kettlewell's experiments were sufficient to cast doubt on the idea that bird predation was the agent of natural selection in this case, although he says his "biological intuition is that predation is probably a major cause."
Majerus dismisses most critics of the bird predation hypothesis as being people who write about the peppered moth without having a "feel for the organism." Many biologists start with theories and then test them in the field. Majerus takes the opposite and less fashionable approach: "There's another way of doing science, which is first find your organism and look at it incredibly closely, for a long time, in great detail and see what questions it asks you," he says.
Colleagues describe Majerus as a brilliant natural historian and a great communicator. He's made numerous appearances on radio, on television, and in the popular press, and he's on the circuit as an after-dinner speaker. "He doesn't have to try very hard to get a lot of people interested in what he's saying," says former graduate student Matt Tinsley. "Mike likes having a small crowd of students around him and telling stories."
It's a talent Majerus hopes to put to good use in defending the reputation of Kettlewell and the peppered moth in a road show, which he aims to take around Britain--and possibly the United States--later this year. He is motivated by growing concern over attacks on Kettlewell's character, most notably writer Judith Hooper's scathing account of the men behind the peppered moth story in her 2002 book Of Moths and Men: The Untold Story of Science and the Peppered Moth, which helped fuel an anti-evolutionist campaign to remove Biston from school textbooks. "A lot of [the campaign] is pointed at the peppered moth as being the example that Darwinism is debunked," says Majerus, who wants to make a public stand against teaching creationism and "intelligent design" in biology classes. "To have people believe the biology of the planet is controlled by a Creator, I think that's dangerous."
After decades of moth-watching, Majerus is convinced that Kettlewell was right and that bird predation is the primary agent of natural selection on the peppered moth. "But that can never be enough," he says, "because I'm also a scientist. ... We're miles beyond reasonable doubt, but it's not scientific proof."
Majerus's experiment is designed to avoid the mistakes Kettlewell made when comparing the proportion of typical and melanic peppered moths that escape the attention of predatory birds. He's releasing a small number of moths, at night, and letting them choose their own hiding places within specially designed mesh sleeves, which he removes at dawn. Like Kettlewell, he's using a mixture of lab-reared and wild-caught moths, but his design allows him to test for potential differences between the two. Majerus is determined to get "a definite answer" on the bird predation issue.
Although Majerus expects to confirm Kettlewell's conclusions, he claims not to care which way the results go: Any findings, he thinks, would make a splash by settling the controversy. But peppered moth expert and evolutionary geneticist Bruce Grant of the College of William and Mary in Williamsburg, Virginia, doubts that Majerus will silence the critics. "To do the job the right way is going to be too labor-intensive and it's just not worth it. ... Right now, I think there are other things that need doing more."
Time is running out for studying the melanic peppered moth, which, with declining pollution levels, is expected to make up only 1% of the British peppered moth population by 2019. And for Majerus, there are other fish to fry: "I don't want to get stuck with peppered moths for the rest of my career," he says, but he doesn't see his bug obsession waning anytime soon.