Charles Darwin's wife, Emma, was terrified that they would be separated for eternity, because she would go to heaven and he would not. Emma confessed her fears in a letter that Charles kept and treasured, with his reply to her scribbled in the margin: "When I am dead, know that many times, I have kissed and cryed over this."

Close as they were, the two could hardly bear to talk about Darwin's view of life. And today, those of us who live in the United States, by many measures the world's leading scientific nation, find ourselves in a house divided. Half of us accept Darwin's theory, half of us reject it, and many people are convinced that Darwin burns in hell. I find that old debate particularly strange, because I've spent some of the best years of my life as a science writer peering over the shoulders of biologists who actually watch Darwin's process in action. What they can see casts the whole debate in a new light--or it should.

Darwin himself never tried to watch evolution happen. "It may metaphorically be said," he wrote in the Origin of Species,

that natural selection is daily and hourly scrutinising, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers.... We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages.

Darwin was a modest man who thought of himself as a plodder (one of his favorite mottoes was, "It's dogged as does it"). He thought evolution plodded too. If so, it would be more boring to watch evolution than to watch drying paint. As a result, for several generations after Darwin's death, almost nobody tried. For most of the twentieth century the only well-known example of evolution in action was the case of peppered moths in industrial England. The moth had its picture in all the textbooks, as a kind of special case.

Then, in 1973, a married pair of evolutionary biologists, Peter and Rosemary Grant, now at Princeton University, began a study of Darwin's process in Darwin's islands, the Galapagos, watching Darwin's finches. At first, they assumed that they would have to infer the history of evolution in the islands from the distribution of the various finch species, varieties, and populations across the archipelago. That is pretty much what Darwin had done, in broad strokes, after the Beagle's five-week survey of the islands in 1835. But the Grants soon discovered that at their main study site, a tiny desert island called Daphne Major, near the center of the archipelago, the finches were evolving rapidly. Conditions on the island swung wildly back and forth from wet years to dry years, and finches on Daphne adapted to each swing, from generation to generation. With the help of a series of graduate students, the Grants began to spend a good part of every year on Daphne, watching evolution in action as it shaped and reshaped the finches' beaks.

At the same time, a few biologists began making similar discoveries elsewhere in the world. One of them was John A. Endler, an evolutionary biologist at the University of California, Santa Barbara, who studied Trinidadian guppies. In 1986 Endler published a little book called Natural Selection in the Wild, in which he collected and reviewed all of the studies of evolution in action that had been published to that date. Dozens of new field projects were in progress. Biologists finally began to realize that Darwin had been too modest. Evolution by natural selection can happen rapidly enough to watch.

Now the field is exploding. More than 250 people around the world are observing and documenting evolution, not only in finches and guppies, but also in aphids, flies, grayling, monkeyflowers, salmon, and sticklebacks. Some workers are even documenting pairs of species--symbiotic insects and plants--that have recently found each other, and observing the pairs as they drift off into their own world together like lovers in a novel by D.H. Lawrence.

The Grants' own study gets more sophisticated every year. A few years ago, a group of molecular biologists working with the Grants nailed down a gene that plays a key role in shaping the beaks of the finches. The gene codes for a signaling molecule called bone morphogenic protein 4 (BMP4). Finches with bigger beaks tend to have more BMP4, and finches with smaller beaks have less. In the laboratory, the biologists demonstrated that they could sculpt the beaks themselves by adding or subtracting BMP4. The same gene that shapes the beak of the finch in the egg also shapes the human face in the womb.

Some of the most dramatic stories of evolution in action result from the pressures that human beings are imposing on the planet. As Stephen Palumbi, an evolutionary biologist at Stanford University, points out, we are changing the course of evolution for virtually every living species everywhere, with consequences that are sometimes the opposite of what we might have predicted, or desired.

Take trophy hunting. Wild populations of bighorn mountain sheep are carefully managed in North America for hunters who want a chance to shoot a ram with a trophy set of horns. Hunting permits can cost well into the six figures. On Ram Mountain, in Alberta, Canada, hunters have shot the biggest of the bighorn rams for more than thirty years. And the result? Evolution has made the hunters' quarry scarce. The runts have had a better chance than the giants of passing on their genes. So on Ram Mountain the rams have gotten smaller, and their horns are proportionately smaller yet.

Or take fishing, which is economically much more consequential. The populations of Atlantic cod that swam for centuries off the coasts of Labrador and Newfoundland began a terrible crash in the late 1980s. In the years leading up to the crash, the cod had been evolving much like the sheep on Ram Mountain. Fish that matured relatively fast and reproduced relatively young had the better chance of passing on their genes; so did the fish that stayed small. So even before the population crashed, the average cod had been shrinking.

We often seem to lose out wherever we fight hardest to control nature. Antibiotics drive the evolution of drug-resistant bacteria at a frightening pace. Sulfonamides were introduced in the 1930s, and resistance to them was first observed a decade later. Penicillin was deployed in 1943, and the first penicillin resistance was observed in 1946. In the same way, pesticides and herbicides create resistant bugs and weeds.

Palumbi estimates that the annual bill for such unintended human-induced evolution runs to more than $100 billion in the U.S. alone. Worldwide, the pressure of global warming, fragmented habitats, heightened levels of carbon dioxide, acid rain, and the other myriad perturbations people impose on the chemistry and climate of the planet--all change the terms of the struggle for existence in the air, in the water, and on land. Biologists have begun to worry about those perturbations, but global change may be racing ahead of them.

To me, the most interesting news in the global evolution watch concerns what Darwin called "that mystery of mysteries, the origin of species."

The process whereby a population acquires small, inherited changes through natural selection is known as microevolution. Finches get bigger, fish gets smaller, but a finch is still a finch and a fish is still a fish. For people who reject Darwin's theory, that's the end of the stow: no matter how many small, inherited changes accumulate, they believe, natural selection can never make a new kind of living thing. The kinds, the species, are eternal.

Darwin argued otherwise. He thought that many small changes could cause two lines of life to diverge. Whenever animals and plants find their way to a new home, for instance, they suffer, like emigres in new countries. Some individuals fail, others adapt and prosper. As the more successful individuals reproduce, Darwin maintained, the new population begins to differ from the ancestral one. If the two populations diverge widely enough, they become separate species. Change on that scale is known as macroevolution.

In Origin, Darwin estimated that a new species might take between ten thousand and fourteen thousand generations to arise. Until recently, most biologists assumed it would take at least that many, or maybe even millions of generations, before microevolutionary changes led to the origin of new species. So they assumed they could watch evolution by natural selection, but not the divergence of one species into separate, reproductively isolated species. Now that view is changing too.