The Best American Science and Nature Writing 2010 Page 16
Darwin, who assumed that evolution plodded along at a glacially slow rate, observable only in the fossil record, would be equally delighted by another discovery. In those same Galápagos finches, modern Darwins can watch evolution occur in real time. In 1973 Peter and Rosemary Grant, now of Princeton University, began annual observations of the finch populations on the tiny Galápagos island of Daphne Major. They soon discovered that the finches in fact evolved from one year to the next, as conditions on the island swung from wet to dry and back again. For instance, Daphne Major initially had only two regularly breeding ground finches, one of which was the medium ground finch (G fortis), which fed on small seeds. When severe drought struck the island in 1977 and small seeds became scarce, the medium finches were forced to switch to eating bigger, harder seeds. Those with larger beaks fared better and survived to pass on the trait to their offspring.
Another shift took place after a competitor arrived in 1982: the large ground finch (G magnirostris), which also eats large, tough seeds. For many years the two species coexisted, and in 2002 both became unusually abundant. But then drought struck, and by 2005, only thirteen large and eighty-three medium ground finches remained alive. Remarkably, instead of adjusting to the drought by eating bigger seeds, as they had twenty-eight years before, the surviving medium finches experienced a marked reduction in the size of their beaks, as in competition with their larger cousins they struggled to carve out a niche by surviving on very small seeds. A finch with a smaller beak is not a new species of finch, but Peter Grant reckons it might take only a few such episodes before a new species is established that would not choose to reproduce with its parent species.
The variation seen among the Galápagos finches is a classic example of "adaptive radiation," each species evolving from a common ancestor to exploit a special kind of food. Another famous radiation took place on a different set of islands—islands of water rather than land. The lakes and rivers of Africa's Great Rift Valley contain some 2,000 species of cichlid fish that have evolved from a few ancestors, some in an instant of geologic time. For example, Lake Victoria, the largest of those lakes, was completely dry just 15,000 years ago. Its 500 diverse species of cichlids have all evolved since then from a handful of species of uncertain origin. Like the finches, cichlid fish species have adapted to diets in different habitats, such as rocky or sandy patches of lake beds. Some species eat algae and have densely packed teeth suited to scraping and pulling plant matter, while others feed on snails and have thick, powerful jaws capable of crushing open their shells. And what gene is responsible for thickening those jaws? The gene for the protein BMP4—the same gene that makes the Galápagos ground finch's beak deep and wide. What better evidence for Darwin's belief in the commonality of all species than to find the same gene doing the same job in birds and fish, continents apart?
In The Origin of Species, Darwin tactfully left unspoken how his theory would extend that commonality to include humankind. A decade later he confronted the matter head-on in The Descent of Man. He would be delighted to know that a certain gene, called FOXP2, is critical for the normal development of both speech in people and song in birds. In 2001 Simon Fisher and his colleagues at the University of Oxford discovered that a mutation in this gene causes language defects in people. He later demonstrated that in mice, the gene is necessary for learning sequences of rapid movement; without it, the brain does not form the connections that would normally record the learning. In human beings, presumably, FOXP2 is crucial to learning the sophisticated flickers of lips and tongue with which we express our thoughts.
Constance Scharff of the Free University of Berlin then discovered that this very same gene is more active in a part of the brain of a young zebra finch just when the bird learns to sing. With fiendish ingenuity, her group infected finches' brains with a special virus carrying a mirror-image copy of part of the FOXP2 gene, which stifled the gene's natural expression. The result was that birds not only sang more variably than usual but also inaccurately imitated the songs of adults—in much the same way as children with mutant FOXP2 genes produce variable and inaccurately copied speech.
Today's Darwins see in detail how pressures such as competition and a changing environment can forge new species. But Darwin also proposed another evolutionary driver: sexual selection. In Lake Victoria, cichlid fish have vision adapted to the light in their surrounding environment—at greater depths, where available light is shifted toward the red end of the spectrum, their visual receptors are biased toward red light, while closer to the surface they see better in blue. Ole Seehausen of the University of Bern and the Swiss Federal Institute for Aquatic Science and Technol ogy has found that male cichlids have evolved conspicuous colors to catch the female eye: typically red nearer the lake bottom and blue at shallower depths. The blue and red populations appear to be genetically diverging—suggesting they represent two separate species in the making.
If natural selection is survival of the fittest (a phrase coined by the philosopher Herbert Spencer, not by Darwin), then sexual selection is reproduction of the sexiest. It has the delightful effect of generating weapons, ornaments, songs, and colors, especially on male animals. Darwin believed that some such ornaments, such as stags' antlers, helped males fight each other for females; others, such as peacocks' tails, helped males "charm" (his word) females into mating. It was, in truth, an idea born of desperation, because useless beauty worried him as an apparent exception to the ruthlessly practical workings of natural selection. He wrote to the American botanist Asa Gray in April 1860 that "the sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!"
His notion of sexual selection was politely ignored by most Victorian opinion, which was mildly scandalized by the thought of females actively choosing a mate rather than submitting coyly to the advances of males. Even biologists dropped the idea for roughly a century, because they became obsessed with arguing that traits evolve to suit the species rather than to suit the individual. But we now know Darwin was right all along. In all sorts of species, from fish and birds to insects and frogs, females approach the males with the most elaborate displays and invite them to mate.
Darwin did not speculate much on why a female would choose an ornamented male. It is a question that still excites biologists, because they have two equally good answers to it. One is simply fashion: when females are choosing gorgeous males, other females must follow suit or risk having sons that do not attract females. The other is more subtle. The tail of a peacock is an exhausting and dangerous thing for the bird to grow. It can be done well only by the healthiest males: parasites, starvation, and careless preening will result in duller plumage. So bright plumage constitutes what evolutionary biologists call an "honest indicator of fitness." Substandard peacocks cannot fake it. And peahens, by instinctively picking the best males, thereby unknowingly pass on the best genes to their offspring.
In one of his flights of fancy, Darwin argued that sexual selection might account for human racial differences: "We have seen that each race has its own style of beauty ... The selection of the more attractive women by the more powerful men of each tribe, who would rear on an average a greater number of children, [would] after the lapse of many generations modify to a certain extent the character of the tribe." The jury is still out on that particular idea, but there are hints that Darwin might be at least partly right.
Take blue eyes. Darwin, like many Europeans, had blue eyes. In early 2008, Hans Eiberg and his colleagues at the University of Copenhagen announced that they had found the genetic mutation common to all people having pure blue eyes. The mutation is a single letter change, from A to G, on the long arm of chromosome 15, which dampens the expression of a gene called OCA2, involved in the manufacture of the pigment that darkens the eyes. By comparing the DNA of Danes with that of people from Turkey and Jordan, Eiberg calculated that this mutation happened only about 6,000-10,000 years ago, well after the invention of agriculture, in a particular individual somewhere a
round the Black Sea. So Darwin may have gotten his blue eyes because of a single misspelled letter in the DNA in the baby of a Neolithic farmer.
Why did this genetic change spread so successfully? There is no evidence that blue eyes help people survive. Perhaps the trait was associated with paler skin, which admits more of the sunlight needed for the synthesis of vitamin D. That would be especially important as people in less sunny northern climates became more de pendent on grain as a food source, which is deficient in vitamin D. On the other hand, blue-eyed people may have had more descendants chiefly because they happened to be more attractive to the opposite sex in that geographic region. Either way, the explanation leads straight back to Darwin's two theories—natural and sexual selection.
Intriguingly, the spelling change that causes blue eyes is not in the pigment gene itself but in a nearby snippet of DNA scripture that controls the gene's expression. This lends support to an idea that is rushing through genetics and evolutionary biology: evolution works not just by changing genes but by modifying the way those genes are switched on and off. According to Sean Carroll of the University of Wisconsin at Madison, "The primary fuel for the evolution of anatomy turns out not to be gene changes, but changes in the regulation of genes that control development."
The notion of genetic switches explains the humiliating surprise that human beings appear to have very few, if any, special human genes. Over the past decade, as scientists compared the human genome with that of other creatures, it has emerged that we inherit not just the same number of genes as a mouse—fewer than 21,000—but in most cases the very same genes. Just as you don't need different words to write different books, so you don't need new genes to make new species: you just change the order and pattern of their use.
Perhaps more scientists should have realized this sooner than they did. After all, bodies are not assembled like machines in factories; they grow and develop, so evolution was always going to be about changing the process of growth rather than specifying the end product of that growth. In other words, a giraffe doesn't have special genes for a long neck. Its neck-growing genes are the same as a mouse's; they may just be switched on for a longer time, so the giraffe ends up with a longer neck.
Just as Darwin drew lessons from both fossil armadillos and living rheas and finches, his scientific descendants combine insights from genes with insights from fossils to understand the history of life. In 2004 Neil Shubin of the University of Chicago and his colleagues found a 375-million-year-old fossil high in the Canadian Arctic—a creature that fit neatly in the gap between fish and land-living animals. They named it Tiktaalik, which means "large freshwater fish" in the local Inuktitut language. Although it was plainly a fish with scales and fins, Tiktaalik had a flat, amphibian-style head with a distinct neck and bones inside its fins corresponding to the upper and lower arm bones and even the wrists of land animals: a missing link if ever there was one. It may even have been able to live in the shallows or crawl in the mud when escaping predators.
Equally intriguing, however, is what Tiktaalik has taught Shubin and his colleagues in the laboratory. The fossil's genes are lost in the mists of time. But, inspired by the discovery, the researchers studied a living proxy—a primitive bony fish called a paddlefish—and found that the pattern of gene expression that builds the bones in its fins is much the same as the one that assembles the limbs in the embryo of a bird, a mammal, or any other land-living animal. The difference is only that it is switched on for a shorter time in fish. The discovery overturned a long-held notion that the acquisition of limbs required a radical evolutionary event.
"It turns out that the genetic machinery needed to make limbs was already present in fins," says Shubin. "It did not involve the origin of new genes and developmental processes. It involved the redeployment of old genetic recipes in new ways."
Though modern genetics vindicates Darwin in all sorts of ways, it also turns the spotlight on his biggest mistake. Darwin's own ideas on the mechanism of inheritance were a mess—and wrong. He thought that an organism blended together a mixture of its parents' traits, and later in his life he began to believe it also passed on traits acquired during its lifetime. He never understood, as the humble Moravian monk Gregor Mendel did, that an organism isn't a blend of its two parents at all, but the composite result of lots and lots of individual traits passed down by its father and mother from their own parents and their grandparents before them.
Mendel's paper describing the particulate nature of inheritance was published in an obscure Moravian journal in 1866, just seven years after The Origin of Species. He sent it hopefully to some leading scientists of the day, but it was largely ignored. The monk's fate was to die years before the significance of his discovery was appreciated. But his legacy, like Darwin's, has never been more alive.
TIM FLANNERY The Superior Civilization
FROM The New York Review of Books
ANTS ARE SO MUCH A PART of our everyday lives that unless we discover them in our sugar bowl we rarely give them a second thought. Yet those minuscule bodies voyaging across the kitchen counter merit a closer look, for as the entomologists Bert Holldobler and Edward O. Wilson tell us in their latest book, they are part of a superorganism. Superorganisms such as some ant, bee, and termite colonies represent a level of organization intermediate between single organisms and the ecosystem: you can think of them as comprised of individuals whose coordination and integration have reached such a sophisticated level that they function with some of the seamlessness of a human body. The superorganism whose "hand" reaches into your sugar bowl is probably around the size of a large octopus or a garden shrub, and it will have positioned itself so that its vital parts are hidden and sheltered from climatic extremes while it still has easy access to food and water.
The term "superorganism" was first coined in 1928 by the great American ant expert William Morton Wheeler. Over the ensuing eighty years, as debates around sociobiology and genetics have altered our perspectives, the concept has fallen into and out of favor, and Holldobler and Wilson's book is a self-professed and convincing appeal for its revival. Five years in the making, The Superorganism draws on centuries of entomological research, charting much of what we know of the evolution, ecology, and social organization of the ants.
For all its inherent interest to an intelligent lay reader, it's a technical work filled with complex genetics, chemistry, and entomological jargon such as, for example, "gamergate," "eclosed," and "anal trophallaxis." Occasional lapses add to the lay reader's difficulties. The etymology of "gamergate" ("married worker"), for example, which is so useful in understanding the term, is given only many pages after it's first introduced. I fear that The Superorganism may reach a smaller audience than it deserves, which is a great pity, for this is a profoundly important book with immediate relevance for anyone interested in the trends now shaping our own societies.
Ants first evolved around 100 million years ago, and they have since diversified enormously. With 14,000 described species, and perhaps as many still awaiting discovery, they have colonized every habitable continent and almost every conceivable ecological niche. They vary enormously in size and shape. The smallest are the leptanilline ants, which are so rarely encountered that few entomologists have ever seen one outside of a museum. They are possibly the most primitive ants in existence, and despite being less than a millimeter in length they are formidable hunters. Packs of these Lilliputian creatures swarm through the gaps between soil particles in search of venomous centipedes much larger than themselves, which form their only prey. The largest ant in existence, in contrast, is the bullet ant, Dinoponera quadriceps (of which Holldobler and Wilson give abundant details, yet frustratingly neglect to inform us precisely how large these formidable-sounding creatures are). Inhabitants of the Neotropics—South and Central America—bullet ants belong to a great group known as the ponerines.
In explaining what a superorganism is, Holldobler and Wilson draw up a useful set of "functional parallels" b
etween an organism (such as ourselves) and the superorganism that is an ant colony. The individual ants, they say, function like cells in our body, an observation that's given more piquancy when we realize that, like many of our cells, individual ants are extremely short-lived. Depending upon the species, between 1 and 10 percent of the entire worker population of a colony dies each day, and in some species nearly half of the ants that forage outside the nest die daily. The specialized ant castes—such as workers, soldiers, and queens—correspond, they say, to our organs; and the queen ant, which in some instances never moves, but which can lay twenty eggs every minute for all of her decade-long life, is the equivalent of our gonads.
Pursuing the same reasoning, Holldobler and Wilson argue that the nests of some ants correspond to the skin and skeleton of other creatures. Some ant nests are so enormous that they are akin to the skeletons of whales. Those of one species of leafcutter ant from South America, for example, can contain nearly two thousand individual chambers, some with a capacity of fifty liters, and they can involve the excavation of forty tons of earth and extend over hundreds of square feet. Coordination within such giant colonies, which can house 8 million individual ants, occurs through ant communication systems that are extraordinarily sophisticated and are the equivalent of the human nervous system. Not all ant species have reached this level of organization. Indeed, one of the most successful groups of ants, the ponerines, rarely qualifies for superorganism status.
Parallels between the ants and ourselves are striking for the light they shed on the nature of everyday human experiences. Some ants get forced into low-status jobs and are prevented from becoming upwardly mobile by other members of the colony. Garbage dump workers, for example, are confined to their humble and dangerous task of removing rubbish from the nest by other ants who respond aggressively to the odors that linger on the garbage workers' bodies.