Why Evolution Is True (5 page)

Organisms aren’t just at the mercy of the luck of the mutational draw, but are also constrained by their development and evolutionary history. Mutations are changes in traits that already exist; they almost never create brand-new features. This means that evolution must build a new species starting with the design of its ancestors. Evolution is like an architect who cannot design a building from scratch, but must build every new structure by adapting a preexisting building, keeping the structure habitable all the while. This leads to some compromises. We men, for example, would be better off if our testes formed directly outside the body, where the cooler temperature is better for sperm.
⁴
The testes, however, begin development in the abdomen. When the fetus is six or seven months old, they migrate down into the scrotum through two channels called the inguinal canals, removing them from the damaging heat of the rest of the body. Those canals leave weak spots in the body wall that make men prone to inguinal hernias. These hernias are bad: they can obstruct the intestine, and sometimes caused death in the years before surgery. No intelligent designer would have given us this tortuous testicular journey. We’re stuck with it because we inherited our developmental program for making testes from fishlike ancestors, whose gonads developed, and remained, completely within the abdomen. We begin development with fishlike internal testes, and our testicular descent evolved later, as a clumsy add-on.

So natural selection does not yield perfection—only improvements over what came before. It produces
the fitter,
not the
fittest.
And although selection gives the appearance of design, that design may often be imperfect. Ironically, it is in those imperfections, as we’ll see in chapter 3, that we find important evidence for evolution.

This brings us to the last of evolutionary theory’s six points:
processes other than natural selection can cause evolutionary change.
The most important is simple random changes in the proportion of genes caused by the fact that different families have different numbers of offspring. This leads to evolutionary change that, being random, has nothing to do with adaptation. The influence of this process on important evolutionary change, though, is probably minor, because it does not have the molding power of natural selection. Natural selection remains the only process that can produce adaptation. Nevertheless, we’ll see in chapter 5 that genetic drift may play some evolutionary role in small populations and probably accounts for some nonadaptive features of DNA.

These, then, are the six parts of evolutionary theory.
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Some parts are intimately connected. If speciation is true, for instance, then common ancestry must also be true. But some parts are independent of others. Evolution might occur, for example, but it need not occur gradually. Some “mutationists” in the early twentieth century thought that a species could instantly produce a radically different species via a single monster mutation. The renowned zoologist Richard Goldschmidt, for example, once argued that the first creature recognizable as a bird might have hatched from an egg laid by an unambiguous reptile. Such claims can be tested. Mutationism predicts that new groups should arise instantly from old ones, without transitions in the fossil record. But the fossils tell us that this is not the way evolution works. Nevertheless, such tests show that different parts of Darwinism can be tested independently.

Alternatively, evolution might be true, but natural selection might not be its cause. Many biologists, for instance, once thought that evolution occurred by a mystical and teleological force: organisms were said to have an “inner drive” that made species change in certain prescribed directions. This kind of drive was said to have propelled the evolution of the huge canine teeth of saber-toothed tigers, making the teeth get larger and larger, regardless of their usefulness, until the animal could not close its mouth and the species starved itself to extinction. We now know that there’s no evidence for teleological forces—saber-toothed tigers did not in fact starve to death, but lived happily with oversized canines for millions of years before they went extinct for other reasons. Yet the fact that evolution might have different causes was one reason why biologists accepted evolution many decades before accepting natural selection.

So much for the claims of evolutionary theory. But here’s an important and commonly heard refrain: Evolution is only a theory, isn’t it? Addressing an evangelical group in Texas in 1980, presidential candidate Ronald Reagan characterized evolution this way: “Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science and is not yet believed in the scientific community to be as infallible as it once was believed.”

The key word in this quote is “only.”
Only
a theory. The implication is that there is something
not quite
right
about a theory—that it is a mere speculation, and very likely wrong. Indeed, the everyday connotation of “theory” is “guess,” as in, “My theory is that Fred is crazy about Sue.” But in science the word “theory” means something completely different, conveying far more assurance and rigor than the notion of a simple guess.

According to the
Oxford English Dictionary,
a
scientific
theory is “a statement of what are held to be the general laws, principles, or causes of something known or observed.” Thus we can speak of the “theory of gravity” as the proposition that all objects with mass attract one another according to a strict relationship involving the distance between them. Or we talk of the “theory of relativity,” which makes specific claims about the speed of light and the curvature of space-time.

There are two points I want to emphasize here. First, in science, a theory is much more than just a speculation about how things are: it is a well-thought-out group of propositions meant to explain facts about the real world. “Atomic theory” isn’t just the statement that “atoms exist”; it’s a statement about how atoms interact with one another, form compounds, and behave chemically. Similarly, the theory of evolution is more than just the statement that “evolution happened”: it is an extensively documented set of principles—I’ve described six major ones—that explain
how and why
evolution happens.

This brings us to the second point. For a theory to be considered scientific, it must be
testable
and
make verifiable predictions.
That is, we must be able to make observations about the real world that either support it or disprove it. Atomic theory was initially speculative, but gained more and more credibility as data from chemistry piled up supporting the existence of atoms. Although we couldn’t actually see atoms until scanning-probe microscopy was invented in 1981 (and under the microscope they do look like the little balls we envision), scientists were already convinced long before that atoms were real. Similarly, a good theory makes predictions about what we should find if we look more closely at nature. And if those predictions are met, it gives us more confidence that the theory is true. Einstein’s general theory of relativity, proposed in 1916, predicted that light would be bent as it passed by a large celestial body. (To be technical, the gravity of such a body distorts space-time, which distorts the path of nearby photons.) Sure enough, Arthur Eddington verified this prediction in 1919 by showing, during a solar eclipse, that light coming from distant stars was bent as it went by the sun, shifting the stars’ apparent positions. It was only when this prediction was verified that Einstein’s theory began to be widely accepted.

Because a theory is accepted as “true” only when its assertions and predictions are tested over and over again, and confirmed repeatedly, there is no one moment when a scientific theory suddenly becomes a scientific fact. A theory becomes a fact (or a “truth”) when so much evidence has accumulated in its favor—and there is no decisive evidence against it—that virtually all reasonable people will accept it. This does not mean that a “true” theory will never be falsified. All scientific truth is provisional, subject to modification in light of new evidence. There is no alarm bell that goes off to tell scientists that they’ve finally hit on the ultimate, unchangeable truths about nature. As we’ll see, it is possible that despite thousands of observations that support Darwinism, new data might show it to be wrong. I think this is unlikely, but scientists, unlike zealots, can’t afford to become arrogant about what they accept as true.

In the process of becoming truths, or facts, scientific theories are usually tested against
alternative
theories. After all, there are usually several explanations for a given phenomenon. Scientists try to make key observations, or conduct decisive experiments, that will test one rival explanation against another. For many years, the position of the earth’s landmasses was thought to have been the same throughout the history of life. But in 1912, the German geophysicist Alfred Wegener came up with the rival theory of “continental drift,” proposing that continents had moved about. Initially, his theory was inspired by the observation that the shapes of continents like South America and Africa could be fitted together like pieces of a jigsaw puzzle. Continental drift then became more certain as fossils accumulated and paleontologists found that the distribution of ancient species suggested that the continents were once joined. Later, “plate tectonics” was suggested as a mechanism for continental movement, just as natural selection was suggested as the mechanism for evolution: the plates of the earth’s crust and mantle floated about on more liquid material in the earth’s interior. And although plate tectonics was also greeted with skepticism by geologists, it was subject to rigorous testing on many fronts, yielding convincing evidence that it is true. Now, thanks to global positioning satellite technology, we can even see the continents moving apart, at a speed of two to four inches per year, about the same rate that your fingernails grow. (This, by the way, combined with the unassailable evidence that the continents were once connected, is evidence against the claim of “young-earth” creationists that the earth is only six to ten thousand years old. If that were the case, we’d be able to stand on the west coast of Spain and see the skyline of New York City, for Europe and America would have moved less than a mile apart!)

When Darwin wrote
The Origin,
most Western scientists, and nearly everyone else, were creationists. While they might not have accepted every detail of the story laid out in Genesis, most thought that life had been created pretty much in its present form, designed by an omnipotent creator, and had not changed since. In
The Origin,
Darwin provided an alternative hypothesis for the development, diversification, and design of life. Much of that book presents evidence that not only supports evolution but at the same time refutes creationism. In Darwin’s day, the evidence for his theories was compelling but not completely decisive. We can say, then, that evolution was a theory (albeit a strongly supported one) when first proposed by Darwin, and since 1859 has graduated to “facthood” as more and more supporting evidence has piled up. Evolution is still called a “theory,” just like the theory of gravity, but it’s a theory that is also a fact.

So how do we test evolutionary theory against the still popular alternative view that life was created and remained unchanged thereafter? There are actually two kinds of evidence. The first comes from using the six tenets of Darwinism to make
testable predictions.
By predictions, I don’t mean that Darwinism can predict how things will evolve in the future. Rather, it predicts what we should find in living or ancient species when we study them. Here are some evolutionary predictions:

 

 

In addition to these predictions, Darwinism can also be supported by what I call
retrodictions:
facts and data that aren’t necessarily predicted by the theory of evolution but
make sense only in light of the theory of evolution.
Retrodictions are a valid way to do science: some of the evidence supporting plate tectonics, for example, came only after scientists learned to read ancient changes in the direction of the earth’s magnetic field from patterns of rocks on the seafloor. Some of the retrodictions that support evolution (as opposed to special creation) include patterns of species distribution on the earth’s surface, peculiarities of how organisms develop from embryos, and the existence of vestigial features that are of no apparent use. These are the subjects of chapters 3 and 4.