Why Evolution Is True (3 page)

Although Darwin was the first to compile evidence for the theory, since his time scientific research has uncovered a stream of new examples showing evolution in action. We are observing species splitting into two, and finding more and more fossils capturing change in the past—dinosaurs that have sprouted feathers, fish that have grown limbs, reptiles turning into mammals. In this book I weave together the many threads of modern work in genetics, paleontology, geology, molecular biology, anatomy, and development that demonstrate the “indelible stamp” of the processes first proposed by Darwin. We will examine what evolution is, what it is not, and how one tests the validity of a theory that inflames so many.

We will see that while recognizing the full import of evolution certainly requires a profound shift in thinking, it does not inevitably lead to the dire consequences that creationists always paint when trying to dissuade people from Darwinism. Accepting evolution needn’t turn you into a despairing nihilist or rob your life of purpose and meaning. It won’t make you immoral, or give you the sentiments of a Stalin or Hitler. Nor need it promote atheism, for enlightened religion has always found a way to accommodate the advances of science. In fact, understanding evolution should surely deepen and enrich our appreciation of the living world and our place in it. The truth—that we, like lions, redwoods, and frogs, all resulted from the slow replacement of one gene by another, each step conferring a tiny reproductive advantage—is surely more satisfying than the myth that we were suddenly called into being from nothing. As so often happens, Darwin put it best:

Chapter 1

What Is Evolution?

—Jacques Monod

 

 

 

I
f 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:

The argument Paley put forward so eloquently was both commonsensical 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.

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 five-hundred-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 apelike, 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.

FIGURE 1
. An example of common ancestry in reptiles. X and Y are species that were the common ancestors between later-evolved forms.

 

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 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 ances-tral
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 each other. 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.)