A curious aspect of the theory of evolution is that everybody thinks he understands it.

—Jacques Monod

If anything is true about nature, it is that plants and animals seem intricately and almost perfectly designed for living their lives. Squids and flatfish change color and pattern to blend in with their surroundings, becoming invisible to predator and prey. Bats have radar to home in on insects at night. Hummingbirds, which can hover in place and change position in an instant, are far more agile than any human helicopter, and have long tongues to sip nectar lying deep within flowers. And the flowers they visit also appear designed—to use hummingbirds as sex aids. For, while the hummingbird is busy sipping nectar, the flower attaches pollen to its bill, enabling it to fertilize the next flower that the bird visits. Nature resembles a well-oiled machine, with every species an intricate cog or gear.

What does all this seem to imply? A master mechanic, of course. This conclusion was most famously expressed by the eighteenth-century English philosopher William Paley. If we came across a watch lying on the ground, he said, we would certainly recognize it as the work of a watchmaker. Likewise, the existence of well-adapted organisms and their intricate features surely implied a conscious, celestial designer—God. Let's look at Paley's argument, one of the most famous in the history of philosophy:

When we come to inspect the watch, we perceive... that its several parts are framed and put together for a purpose, e.g. that they are so formed and adjusted as to produce motion, and that motion so regulated as to point out the hour of the day; that, if the different parts had been differently shaped from what they are, if a different size from what they are, or placed after any other manner, or in any other order than that in which they are placed, either no motion at all would have been carried on in the machine, or none which would have answered the use that is now served by it____Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater and more, and that in a degree which exceeds all computation.

The argument Paley put forward so eloquently was both common-sensical and ancient. When he and his fellow "natural theologians" described plants and animals, they believed that they were cataloging the grandeur and ingenuity of God manifested in his well-designed creatures.

Darwin himself raised the question of design—before disposing of it— in 1859.

How have all those exquisite adaptations of one part of the organization to another part, and to the conditions of life, and of one distinct organic being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and missletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives though the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.

Darwin had his own answer to the conundrum of design. A keen naturalist, who originally studied to be a minister at Cambridge University (where, ironically, he occupied Paley's former rooms), Darwin well knew the seductive power of arguments like Paley's. The more one learns about plants and animals, the more one marvels at how well their designs fit their ways of life. What could be more natural than inferring that this fit reflects conscious design? Yet Darwin looked beyond the obvious, suggesting—and supporting with copious evidence—two ideas that forever dispelled the idea of deliberate design. Those ideas were evolution and natural selection. He was not the first to think of evolution—several before him, including his own grandfather Erasmus Darwin, floated the idea that life had evolved. But Darwin was the first to use data from nature to convince people that evolution was true, and his idea of natural selection was truly novel. It testifies to his genius that the concept of natural theology, accepted by most educated Westerners before 1859, was vanquished within only a few years by a single 500-page book. On the Origin of Species turned the mysteries of life's diversity from mythology into genuine science.

So what is "Darwinism"?1 This simple and profoundly beautiful theory, the theory of evolution by natural selection, has been so often misunderstood, and even on occasion maliciously misstated, that it is worth pausing for a moment to set out its essential points and claims. We'll be coming back to these repeatedly as we consider the evidence for each.

In essence, the modern theory of evolution is easy to grasp. It can be summarized in a single (albeit slightly long) sentence: Life on Earth evolved gradually beginning with one primitive species—perhaps a self-replicating molecule—that lived more than 3.5 billion years ago; it then branched out over time, throwing off many new and diverse species; and the mechanism for most (but not all) of evolutionary change is natural selection.

When you break that statement down, you find that it really consists of six components: evolution, gradualism, speciation, common ancestry, natural selection, and nonselective mechanisms of evolutionary change. Let's examine what each of these parts means.

The first is the idea of evolution itself. This simply means that a species undergoes genetic change over time. That is, over many generations a species can evolve into something quite different, and those differences are based on changes in the DNA, which originate as mutations. The species of animals and plants living today weren't around in the past, but are descended from those that lived earlier. Humans, for example, evolved from a creature that was ape-like, but not identical to modern apes.

Although all species evolve, they don't do so at the same rate. Some, like horseshoe crabs and gingko trees, have barely changed over millions of years. The theory of evolution does not predict that species will constantly be evolving, or how fast they'll change when they do. That depends on the evolutionary pressures they experience. Groups like whales and humans have evolved rapidly, while others, like the coelacanth "living fossil," look almost identical to ancestors that lived hundreds of millions of years ago.

The second part of evolutionary theory is the idea of gradualism. It takes many generations to produce a substantial evolutionary change, such as the evolution of birds from reptiles. The evolution of new features, like the teeth and jaws that distinguish mammals from reptiles, does not occur in just one or a few generations, but usually over hundreds or thousands—even millions—of generations. True, some change can occur very quickly. Populations of microbes have very short generations, some as brief as twenty minutes. This means that these species can undergo a lot of evolution in a short time, accounting for the depressingly rapid rise of drug resistance in disease-causing bacteria and viruses. And there are many examples of evolution known to occur within a human lifetime. But when we're talking about really big change, we're usually referring to change that requires many thousands of years. Gradualism does not mean, however, that each species evolves at an even pace. Just as different species vary in how fast they evolve, so a single species evolves faster or slower as evolutionary pressures wax and wane. When natural selection is strong, as when an animal or plant colonizes a new environment, evolutionary change can be fast. Once a species becomes well adapted to a stable habitat, evolution often slows down.

The next two tenets are flip sides of the same coin. It is a remarkable fact that while there are many living species, all of us—you, me, the elephant, and the potted cactus—share some fundamental traits. Among these are the biochemical pathways that we use to produce energy, our standard four-letter DNA code, and how that code is read and translated into proteins. This tells us that every species goes back to a single common ancestor, an ancestor who had those common traits and passed them on to its descendants. But if evolution meant only gradual genetic change within a species, we'd have only one species today—a single highly evolved descendant of the first species. Yet we have many: well over ten million species inhabit our planet today, and we know of a further quarter million as fossils. Life is diverse. How does this diversity arise from one ancestral form? This requires the third idea of evolution: that of splitting, or, more accurately, speciation.

Look at figure 1, which shows a sample evolutionary tree that illustrates the relationships between birds and reptiles. We've all seen these, but let's examine one a bit more closely to understand what it really

figure l. An example showing common ancestors in reptiles. X and Y are species that were the common ancestors between later-evolved forms.

means. What exactly happened when node X, say, split into the lineage that leads to modern reptiles like lizards and snakes on the one hand and to modern birds and their dinosaurian relatives on the other? Node X represents a single ancestral species, an ancient reptile, that split into two descendant species. One of the descendants went on its own merry path, eventually splitting many times and giving rise to all dinosaurs and modern birds. The other descendant did the same, but produced most modern reptiles. The common ancestor X is often called the "missing link" between the descendant groups. It is the genealogical connection between birds and modern reptiles—the intersection you'd finally reach if you traced their lineages all the way back. There's a more recent "missing link" here, too: node Y, the species that was the common ancestor of bipedal meat-eating dinosaurs like Tyrannosaurus rex (all now extinct) and modern birds. But although common ancestors are no longer with us, and their fossils nearly impossible to document (after all, they represent but a single species out of thousands in the fossil record), we can sometimes discover fossils closely related to them, species having features that show common ancestry. In the next chapter, for example, we'll learn about the "feathered dinosaurs" that support the existence of node Y.

What happened when ancestor X split into two separate species? Nothing much, really. As we'll see later, speciation simply means the evolution of different groups that can't interbreed—that is, groups that can't exchange genes. What we would have seen had we been around when this common ancestor began to split is simply two populations of a single reptilian species, probably living in different places, beginning to evolve slight differences from one another. Over a long time, these differences gradually grew larger. Eventually the two populations would have evolved sufficient genetic difference that members of the different populations could not interbreed. (There are many ways this can happen: members of different animal species may no longer find each other attractive as mates or, if they do mate with each other, the offspring could be sterile. Different plant species can use different pollinators or flower at different times, preventing cross-fertilization.)

Millions of years later, and after more splitting events, one of the descendant dinosaur species, node Y, itself split into two more species, one eventually producing all the bipedal, carnivorous dinosaurs and the other producing all living birds. This critical moment in evolutionary history—the birth of the ancestor of all birds—wouldn't have looked so dramatic at the time. We wouldn't have seen the sudden appearance of flying creatures from reptiles, but merely two slightly different populations of the same dinosaur, probably no more different than members of diverse human populations are today. All the important changes occurred thousands of generations after the split, when selection acted on one lineage to promote flight and on the other to promote the traits of bipedal dinosaurs. It is only in retrospect that we can identify species Y as the common ancestor of T. rex and birds. These evolutionary events were slow, and seem momentous only when we arrange in sequence all the descendants of these diverging evolutionary streams.

But species don't have to split. Whether they do depends, as we'll see, on whether circumstances allow populations to evolve enough differences that they are no longer able to interbreed. The vast majority of species—more than 99 percent of them—go extinct without leaving any descendants. Others, like gingko trees, live millions of years without producing many new species. Speciation doesn't happen very often. But each time one species splits into two, it doubles the number of opportunities for future speciation, so the number of species can rise exponentially. Although speciation is slow, it happens sufficiently often, over such long periods of history, that it can easily explain the stunning diversity of living plants and animals on Earth.

Speciation was so important to Darwin that he made it the title of his most famous book. And that book did give some evidence for the splitting. The only diagram in the whole of The Origin is a hypothetical evolutionary tree resembling figure 1. But it turns out that Darwin didn't really explain how new species arose, for, lacking any knowledge of genetics, he never really understood that explaining species means explaining barriers to gene exchange. Real understanding of how speciation occurs began only in the 1930s. I'll have more to say about this process, which is my own area of research, in chapter 7.

It stands to reason that if the history of life forms a tree, with all species originating from a single trunk, then one can find a common origin for every pair of twigs (existing species) by tracing each twig back through its branches until they intersect at the branch they have in common. This node, as we've seen, is their common ancestor. And if life began with one species and split into millions of descendant species through a branching process, it follows that every pair of species shares a common ancestor sometime in the past. Closely related species, like closely related people, had a common ancestor that lived fairly recently, while the common ancestor of more distantly related species, like that of distant human relatives, lived farther back in the past. Thus, the idea of common ancestry—the fourth tenet of Darwinism—is the flip side of speciation. It simply means that we can always look back in time, using either DNA sequences or fossils, and find descendant lineages fusing at their ancestors.

Let's examine one evolutionary tree, that of vertebrates (figure 2). On this tree I've put some of the features that biologists use to deduce evolutionary relationships. For a start, fish, amphibians, mammals, and reptiles all have a backbone—they are "vertebrates"—so they must have descended from a common ancestor that also had vertebrae. But within vertebrates, reptiles and mammals are united (and distinguished from fish and amphibians) by having an "amniotic egg"—the embryo is surrounded by a fluid-filled membrane called the amnion. So reptiles and mammals must have had a more recent common ancestor that itself possessed such an egg. But this group also contains two subgroups, one with species that all have hair, are warm-blooded, and produce milk (that is, mammals), and another with species that are cold-blooded, scaly, and produce watertight eggs (that is, reptiles). Like all species, these form a nested hierarchy: a hierarchy in which big groups of species whose members share a few traits are subdivided into smaller groups of species sharing more traits, and so on down to species, like black bears and grizzly bears, that share nearly all their traits.

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