Evolution, in biology, is the idea that all organisms are descended from a single common ancestor that arose in the deep geological past. Estimates of the number of species currently on earth vary between 10,000,000 and 100,000,000, and many more have succumbed to extinction over the 3.5-billion-year history of life on earth. According to the idea of evolution, all those species, past and present, are related by the process of ancestry and descent known as "evolution."

The terms theory of evolution and the closely similar evolutionary theory refer to two separable aspects of evolution: (1) the very idea that all organisms are descended from a single common ancestor (our general definition of evolution, above); and (2) ideas about how the evolutionary process operates. This article deals with both aspects.

The word theory in common language is often associated with guesswork or vague ideas; for example, each of us may have our own "theory" of why the Yankees lost the 2001 World Series to the Arizona Diamondbacks. In science, however, the word has a more formal meaning: a scientific theory is a body of one or more hypotheses that have been rigorously analyzed and tested, and rather than being "falsified" (that is, demonstrated to have been incorrect), the idea has so far appeared to have stood all tests—and therefore is the strongest explanation so far available for a particular set of natural phenomena. Thus plate tectonics is a theory, as are quantum mechanics, special relativity, and a host of related, well-confirmed ideas in science.

In biology, evolution in the first sense—that all organisms are descended from a single common ancestor—has been tested over and over again, and has been repeatedly confirmed. Biologists agree that life has evolved—the very few exceptions being creationists, who refuse to accept the idea of evolution, based on their religious views. Creationists are especially common in the United States, Canada, and Australia; creationism is usually, though not exclusively, associated with some aspects of conservative Protestant Christianity. "Scientific creationists" attempt to disprove evolution with supposedly scientific evidence and argumentation—though their arguments have long since been disproven by scientists. One of the standard creationist claims is that the very idea of evolution is not scientific, as no one was there in the remote past (many creationists, however, dispute the 4.65 billion year age for the origin of the earth and follow an interpretation of Genesis that sees the earth as only some 10,000 years old). If no one was there to observe the evolution of life, and if evolution cannot be tested in the laboratory (but it can!—see below), then, creationists claim, the idea is not a truly scientific one.

To be scientific, an idea must be testable, which means that if an idea ("hypothesis") is true, we must be able to make predictions from it regarding what we would expect to observe. Failure to make these predicted observations would "falsify" the idea—the cardinal procedure of hypothesis testing in all of science. So we ask: What observations would we expect to make in the natural world if the hypothesis that evolution has occurred—that all organisms are descended from a single common ancestor—is correct?

There are two such predictions. The first one, pointed out by Charles Darwin in his epochal book On the Origin of Species, published in England in 1859, is that if all organisms are descended from a single common ancestor, we should expect to find a pattern of progressive similarity linking up all of life. In other words, closer relatives should resemble each other more (that is, share more features in common) than either one does with more remote relatives. Like the ever-widening circles in a pool after a rock is thrown in, we should find features that link up small groups

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