II.
CITYSCAPES
We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages.
CHARLES DARWIN, On the Origin of Species (1859)
I don’t think so.
HOMEY D. CLOWN, In Living Color (1990)
7
THESE ARE THE FACTS
Albert Brydges Farn was born in 1841. His entry in The Aurelian Legacy, a sort of Who’s Who for British collectors of Lepidoptera (butterflies and moths), describes him as “an all-round naturalist,” “a man of vigor, courage, and rather boisterous good humor.” It also calls him “a sportsman,” which in those days did not imply he would be seen jogging along country lanes or playing rugby with the village lads, but rather referred to his reputation for potting at bats with a .22 rifle and having bagged a legendary thirty snipe with thirty consecutive shots on Lord Walsingham’s Estate. Farn, clearly, liked to kill stuff.
Most of what he killed, though, were butterflies and moths, which he pinned, mounted, labeled, identified, and organized with great precision. When, in 1921, he died, he left what many considered the finest private lepidopterological collection in Britain at the time. Sadly, the collection was auctioned off piecemeal, and bits of it ended up in different places. Some, says Adam Hart of the University of Gloucestershire, are now “in the bowels of the Natural History Museum” in London. Though he does not know for sure, Hart likes to think that among them are some specimens of the Annulet moth (Charissa obscurata), which Farn collected near Lewes in the 1870s.
It’s a rather drab species, the Annulet, paling in comparison to some of Farn’s prize possessions, like the spectacular Purple Emperor (Apatura iris) that he caught in South Wales. Or the rows and rows of pinned European Map butterfly (Araschnia levana), a pretty black-orange-white species from continental Europe, illegally introduced into the Forest of Dean in 1912 only to be single-handedly exterminated by Farn, who disapproved of all exotic species, pretty or not. Despite its humdrum appearance, though, the Annulet is Farn’s claim to fame. A fame, however, that came 130 years late.
In 2009, Hart, a professor of science communication, was visiting the Gloucester City Museum and Art Gallery in preparation for a class. “I was going through some backroom stuff to look for teaching specimens,” he says. There, he came across a print-out of a letter dated November 18, 1878. The letter had been written by Farn, and the print-out was there because the museum owns a copy of an annotated book that had once belonged to Farn, and the librarian had taken an interest in him. The reason that this particular letter survives and had even been transcribed and put online is, however, not because of the author, but because of the addressee: Charles Darwin.
By 1878, the aging Darwin was one of the most famous scientists in England. A new generation had grown up since On the Origin of Species had been published, and his name as Mr. Evolution was firmly established. Colleagues from all over the world corresponded with him and Darwin kept a meticulous administration of the letters he received and sent—not for social but for scientific reasons. What his correspondents conveyed to him was crucial for his work. As the archivists of the Darwin Correspondence Project at Cambridge University (where much of Darwin’s library is kept) evocatively explain, “[h]e went back over some letters again and again as he worked on different subjects, scrawling on them in different colored pencil, and cut them up so that he could file the pieces with relevant notes or stick them into his experiment book. Letters were dissected like specimens, every useful bit of information sucked out of them and then reincarnated in his publications.”
As far as we know, Albert Farn wrote to Darwin only once. The letter survived in Darwin’s library, the Darwin Correspondence Project duly transcribed it, placed the text online and it was a print-out of this text that Hart found lying around. It is only a brief note, and it seems that Darwin never did anything with it or replied to it.
Farn writes:
My dear Sir,
The belief that I am about to relate something which may be of interest to you, must be my excuse for troubling you with a letter.
Perhaps among the whole of the British Lepidoptera, no species varies more, according to the locality in which it is found, than does [the Annulet moth]. They are almost black on the New Forest peat; gray on limestone; almost white on the chalk near Lewes; and brown on clay, and on the red soil of Herefordshire.
Do these variations point to the “survival of the fittest”? I think so.
It was, therefore with some surprise that I took specimens as dark as any of those in the New Forest on a chalk slope; and I have pondered for a solution. Can this be it?
It is a curious fact, in connexion with these dark specimens, that for the last quarter of a century the chalk slope, on which they occur, has been swept by volumes of black smoke from some lime-kilns situated at the bottom: the herbage, although growing luxuriantly, is blackened by it.
I am told, too, that the very light specimens are now much less common at Lewes than formerly, and that, for some few years, lime-kilns have been in use there.
These are the facts I desire to bring to your notice.
I am, Dear Sir, Yours very faithfully,
A. B. Farn
“I must admit it was a bit of a eureka moment,” says Hart. “This letter had been lying around for so long but no one had realized its significance!” That significance, as Hart pointed out to the evolutionary biology community in a 2010 article in Current Biology, was of course that Farn’s observation may have been the first recorded case of ongoing natural selection. What Farn was suggesting was that the light-colored Annulets, originally well camouflaged on the pale limestone, had now become sitting ducks against the soot-blackened background, and were being picked off by birds and other predators. Meanwhile, a genetic mutant with dark wings had appeared and had been “naturally selected” because it did not stand out as much as its pale ancestors. If Farn was correct, it would be the very first observation of evolution in action. As Farn rightly anticipated, Darwin should have been thrilled. So why did he ignore Farn’s letter?
Of course, it is possible that Darwin just couldn’t be bothered on that November 18, 1878. Maybe he was tending to his orchids, or playing with his grandchildren, or laid up with one of his fits of general malaise. But of course we prefer to see a deeper significance in his lack of response. If it means anything, my guess would be that Darwin underestimated the power of his own discovery, natural selection, and that he found it hard to imagine that its work could be observed on the timescale of years or decades. After all, in Chapter IV of On the Origin of Species, he wrote, “We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages.”
In the preceding pages of his great book, Darwin had laid out the foundations of his theory in four easy, steadfast steps. One—there is variation: in many (sometimes near-imperceptible) ways, each individual is different from the next one. Two—this variation is heritable: offspring resemble their parents. Three—there is surplus: most offspring do not survive. Four—there is selection: survival is not random but favors those who are best suited to the world they live in. To Darwin’s mind—and to everyone since who has fully grasped the enormity of this insight—natural selection is a law of nature. As Darwin wrote, “natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good.”
And yet, despite that “daily and hourly,” Darwin did not actually believe that natural selection could be observed in real time. Maybe this was because he lacked the mathematical prowess to calculate exactly how long it would take for natural selection to do its thing. It lasted until the 1920s for mathematical biologists like J.B.S. Haldane and Ronald Fisher to do this. With Darwin’s theory cast in algebraic formulas, it became possible to see whether his pessimism was well founded or not.
As it turned out, it was n
ot. Darwin’s mistake probably was that he imagined natural selection to be a linear process. He may have thought to himself, let’s imagine a population of 100,000 pale-winged moths. Then, a black-winged mutant appears that enjoys a teeny-weeny advantage. Say, an imperceptible 1 percent—meaning that for every 100 black-winged moths born, surviving and reproducing, there would be 99 pale-winged ones. It’s that small a difference. So how long would it take for all those 100,000 white-wings with one black-winged mutant thrown in, to evolve into a fully black-winged moth population and all pale-winged gone? Forever, right? Wrong—it takes only about a few hundred generations.
That is because natural selection is not a linear process. In the beginning, when the black-wings are still rare, they increase only very slowly, one moth at a time. But when the frequency of black-wings has gone up to a few percent, the process speeds up, because all those thousands of black-winged moths enjoy the same advantage, and pour their joint offspring into the total gene pool, which becomes duskier by the day.
You can see this for yourself by doing an online simulation. Radford University, for example, has a website where you can enter the population size, the advantage of a mutant (the so-called selection coefficient), and the start-off frequency of the mutant, and you can just watch a virtual population evolve in a nice, S-shaped curve. Play around with the settings and you’ll see that it doesn’t make much difference whether your moth population is 10,000 or 100,000 or even 1 million winged souls: in all cases they evolve into a black-winged moth in less than a thousand generations, while enjoying only a 1 percent advantage. Make that selection coefficient 5 percent and it all happens in just 200 generations. For some moth species, 200 generations is less than a century. So, at least in theory, even very weak natural selection can have dramatic effects before the hand of time has begun to mark any longish lapses.
It seems Darwin never really entertained the notion that such evolutionary agility was a possibility. Although … In the first four editions of On the Origin of Species, he still writes emphatically, “I do believe that natural selection will always act very slowly.” But in the fifth edition, published ten years after the first, he changed “always” for “generally,” so he may have begun to doubt that natural selection is so tardy a process. Be that as it may, Darwin missed a trick by not picking up on Farn’s tip, and it was left to the next generation to reveal the breakneck evolution of “industrial melanism.” Not in the Annulet moth, but in Biston betularia. The “peppered moth,” as it is called, is literally a textbook example of urban evolution, and you’ve probably heard about it in school. But there have been so many recent twists and turns to the story that I hope you’ll forgive me for re-telling it.
8
URBAN MYTHS
We may think of rapid urban growth as a thing of today, but between 1770 and 1850, the city of Manchester grew as explosively as any twenty-first-century megalopolis: from 24,000 to 350,000 inhabitants. The city’s coal-powered textile industry sucked in workers from the surrounding countryside and spewed pollution into it. Immense quantities of soot, sulfur, and nitrous gases billowed from its smoke stacks, which darkened the sky, blocked out the sun, and on windless days caused a haze so thick that people could barely make out their neighbors across the street. A constant mist of soot particles settled on everything: the houses, the pavements, even the trees in the rural environs of the city.
Imagine an autumn day in 1819. In a forest just outside Manchester, something like this must have happened. A caterpillar of the peppered moth (Biston betularia) is inching down the stem of a soot-blackened birch tree, heading for the ground to begin its pupation. As proper inchworms do, it clamps the bark with its true legs (at the front), then pulls its squishy false legs (at the rear end of its long stick-like body) up to its regular legs, and folds its body into the shape of an omega. Then, it releases its true legs, holding on to the bark with only its false legs, and stretches forward, brings up the rear, does the omega, stretches, brings up, releases, grabs, pulls up … tirelessly until it reaches the foot of the tree.
Although the caterpillar is, by definition, immature, its testes are already formed inside and busy creating sperm cells for when, after pupation, the animal will metamorphose into a sexually active adult peppered moth, its wings characterized by a sprinkling of black marks on a white background. Or at least, that is how its parents, and all the other peppered moths in Britain until that day, looked. But as our caterpillar takes a final bound onto the grass beneath its tree, something odd happens in one testicular cell. Something that is to change the course of peppered moth evolution. While the cellular machinery is separating the chromosomes and packaging them into what is going to be a sperm cell, a bit of DNA releases itself from one of the chromosomes. It is a so-called transposon, a “jumping gene,” able to cut itself out of a chromosome and reinsert itself somewhere else. And that is precisely what this transposon does. Unbeknown to the caterpillar, who is busy pushing its head between the grass roots, so-called transposase enzymes cut the 22,000-letter-long runt of DNA loose from its original position and insert it smack in the middle of cortex, a gene controlling wing pigmentation in moths.
While the caterpillar burrows into the soil, turns itself into a pupa, hibernates and finally hatches into a moth, the mutated sperm cell sits quietly, biding its time. It obediently joins thousands of other, non-mutant, sperm cells in one of the moth’s sperm packages, is ejaculated into a female peppered moth upon one of the male moth’s successful copulations, and, by sheer luck, manages to fertilize one of her eggs. The fertilized egg develops into a young caterpillar, built from cells that now all carry a copy of the mutated cortex gene. For that whole summer, the mutant caterpillar, together with all its siblings, munches away at birch foliage until it is its time again to burrow into the ground and pupate.
But as the chrysalis lies there quietly beneath the grass roots, its dormant appearance belies a revolution going on inside. In the animal’s developing wings, still encased inside the auburn shell of the pupa, the transposon wedged into the cortex gene proves to be a spanner in the works that normally produce that delicate white-and-black speckled wing pattern. Instead, when the moth emerges, scrambles up, and clings to a branch of birch, its unfolding and hardening wings turn out to be a pure anthracite black. Not unlike the tinge of the soot clinging to that very same branch.
Our black-winged Biston betularia survives and reproduces. It spawns a small but slowly growing band of black descendants. Some of these are noticed by early nineteenth-century Mancunian entomologists, but the first catch that makes it into the scientific literature is one that is netted and pinned in 1848 by Manchester moth collector R.S. Edleston. From then on, things escalate. The black moths proliferate. In the 1860s, in some parts of Manchester, the black mutants are becoming commoner than the pale ones. From their Manchester stronghold, the black gene also filters into other parts of England. In the 1870s, black moths are seen in Staffordshire, some 40 miles south of Manchester, and in Yorkshire to the northeast. By the late nineteenth century, the original gene for pale wings had almost gone from many of the British populations of Biston betularia, some parts of the rural south excepted. The continent and North America succumbed not much later.
Moth boffins are baffled, and several debates are raging in the British entomological journals, with speculation ranging from changes in humidity and food to “the powerful impression of surrounding objects on the female” during procreation (these are the days before genes and how they work are discovered). But it is the eminent Victorian lepidopterologist J.W. Tutt who, in his 1896 book British Moths, eloquently promulgated the concept of what we now know as industrial melanism:
Let us see whether we can understand how this has been brought about! […] In our woods in the south the trunks are pale and the moth has a fair chance of escape, but put the Peppered Moth with its white ground color on a black tree trunk, and what would happen? It would, as you say, be very conspicuous, and would fall prey
to the first bird that spied it out. But some of these Peppered Moths have more black about them than others, and you can easily understand that the blacker they are the nearer they will be to the color of the tree trunk, and the greater will become the difficulty of detecting them. So it really is; the paler ones the birds eat, the darker ones escape.
“So it really is.” Most evolutionary biologists today would agree with Tutt’s explicit (and Albert Farn’s implicit) explanation for industrial melanism. Acid rain killed the lichens, soot turned the naked branches black, and the mottled coloration of the peppered moth, the Annulet, and many other insect species was rendered ineffective as camouflage. New or existing mutants with darker bodies, which previously would never gain a foothold, now proved better at not being seen against the darkening background. And natural selection did the rest.
But the general acceptance of this process has had a history as peppered as Biston betularia’s wings. After all, more was required than just the regarded opinion of an esteemed lepidopterist. For Tutt’s just-so story to be adopted as the first rubber-stamped case of real-time evolution, proof and verification were needed.
First to take a stab at the peppered moth was the mathematical biologist J.B.S. Haldane. In 1924, he used the time (50 years) it had taken for the dark moths to take over Manchester to calculate the selection coefficient, that is, the relative disadvantage of the pale moths compared with the dark ones. He came up with a value of about 50 percent, meaning that for every two pale moths surviving bird attacks and breeding, there were three dark ones. At the time, many of Haldane’s colleagues balked at the suggestion that selection could ever be that strong. Moreover, the camouflage–bird connection was tenuous: birds in the wild had never been seen eating peppered moths at all. It took another thirty years to move the debate on.
Darwin Comes to Town Page 7