The theory of evolution is not as easily tested as most, simply because of the time-scales involved in watching it happen. Since by definition any interference in the process would no longer be natural selection, little can be done to hurry it along. That said, if you accept the existence of natural selection, you already accept evolution, so I assume the question you are really asking is “how can we do an experiment to prove it has been going on for billions of years, rather than for 6000 years, as creationists claim?”
Since we do not at this time have the technology to look backward in time, we cannot gather evidence of long term evolution by direct observation, but science gives us a loophole. By definition, a scientific theory makes specific predictions of what will happen in a given set of circumstances. Like a crime scene detective, if we can find evidence of those predictions having come true at some time in the past, we can treat that as evidence that the theory is right. It isn’t as reliable as setting up a controlled experiment and watching it happen, but if you consistently find the evidence that your theory predicted, over many thousands of cases, you can accept this as proof that the theory is correct. In the case of evolution, there is a mountain of evidence that matches the predictions made by the theory.
First, as you pointed out, speciation has been observed by scientists numerous times, in birds, insects, lizards, and bacteria, so we know what it looks like when it happens, and can predict what it would have looked like in the past.
Next, the theory of evolution predicts that since all life are cousins, the more closely related cousins should be more alike and have more similar DNA and biochemistry, while the most distant ones should be more different, and have less similar DNA and biochemistry. This means that even starting from a single cell of life, given billions of years of slow speciation, we should have an extremely complex biosphere, with incredibly diverse life forms that appear to form a relatively smooth continuum of shapes and biochemistries. In fact, we do find this.
Third, the theory predicts that there will be intermediate forms of creatures that connect ancestral species to later species. If this were true, you would expect to find intermediate species represented in the fossil record. In fact, hundreds of years of study of the fossil record has produced thousands of examples of intermediate species.
Fourth, evolution predicts that the fossil record, with all its intermediate species, will be sorted by age in the rocks they are found in. In fact, we also find this to be true.
Fifth, evolution predicts that since there is no specific goal or timetable to the process, we should see different rates of change in different organisms in different environments. A well adapted species in a stable environment could remain unchanged for millions or even billions of years, while one in a volatile environment could accumulate significant changes in a comparatively short time. In fact, there are hundreds of examples of this. Fossils show that creatures like crocodiles and sharks have remained nearly unchanged for millions of years, while animals in small, difficult island environments have been observed by scientists to gain noticeable changes during a single human lifetime.
Sixth, and most important to your question, while evolution itself contains no direct prediction of the time evolutionary changes will take, geology has dozens of methods for determining the age of rocks, based on things like sedimentary deposition rates, chemical makeup, tectonic activity, and several different kinds of radioactive decay dating, all of which agree with one another, and all of which date the fossil record to having covered billions of years.
Is any of this absolute proof? No, but the pile of evidence is huge and keeps getting bigger. Eventually, we have to accept that if it looks like a duck, walks like a duck, and quacks like a duck, thaere’s a pretty good chance it is a duck!
Posted: January 22nd 2010
See all questions answered by Blaise