The first edition of Charles Darwin’s The Origin of Species published in 1859 did not contain the word evolution (though evolved was the last word). By the sixth edition the word evolution was used 15 times, yet it was not defined. Nor did he specify what he called “the great principle of evolution”. Alas, it is much the same today. One might say (in the manner of Mark Twain on the weather), “Everyone talks about evolution but no one defines it.”
Intelligent design proponents have noted several uses of the word evolution:
- Change over time — small-scale change in a population of organisms over time, often called “microevolution”.
- Universal common descent — the view that all organisms are related and are descended from a single common ancestor.
- Natural selection and random mutation as the main cause or mechanism of change during the history of life — the idea that an unguided process of natural selection acting upon random mutations is sufficient to produce the new forms of life that appear during that history as well as the appearance of design that living forms manifest.
Only definition #3 fits that of a scientific theory. Definition #2 has to do with the scope of a theory of evolution. Definition #1 is a very general and trivial statement, which is often used to make critics of evolution seem to oppose a truism.
Critics of evolution most commonly criticize the adequacy of definition #3. Definition #2 is actually the most deficient: it confuses a scientific theory with its scope.
Compare Isaac Newton’s law of universal gravitation. He did not claim that gravitation was by definition universal. Instead, he claimed that the scope of the law that he developed was universal. However, since the emergence of relativity and quantum mechanics, Newton’s law of gravitation is known to be limited. It is not universal.
Newton’s laws of motion are an example of what Werner Heisenberg calls a “closed theory.” While it was superseded, it is still valid within certain limits. An “open theory” is what he calls a theory whose limits are unknown.
In The Origin of Species Darwin proposed universal common descent by a general, basically philosophical, argument:
Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. We see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree. Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.
Natural selection was the only mechanism he proposed to explain universal common descent, as if it were sufficient. Darwin was indirect in asserting the connection between natural selection and universal common descent, all the better to avoid arguing directly for its sufficiency. But evolution as universal common descent is not established by any mechanisms. They are independent assertions and must be established independently.
Science progresses from open to closed theories in Heisenberg’s sense. Every mechanism has its limits, which eventually will be discovered, though the limits of each mechanism are rarely admitted by the scientific community. Meanwhile the search for more mechanisms goes on, and the search for alternative theories is derided.