Is the theory of evolution testable? This is a necessary requirement for any respectable scientific theory, and it seems that it would be very difficult to figure out a way to test evolution. After all, we can't go back and watch the development of our modern species. That kind of history doesn't repeat itself. So direct observation of past evolution isn't possible.
We can observe modern evolutionary processes in action and we have, many times. However, this doesn't answer the questions about testing the evolutionary past.
In fact, we can test the theory of evolution-and the test comes from one of the most progressive and up-to-the-minute facets of modern science: biotechnology.
In the second half of the 20th century, we have made tremendous advances in our understanding of the molecular basis of life. Our knowledge of DNA a substance whose significance the people of Darwin's day didn't even imagine has provided some profound insight into the real nature of life.
On a more practical level, this continually increasing knowledge of DNA and the product of its information protein has given us a priceless tool to test Darwin's theory in a way completely independent of any available to him or to others of his day by directly comparing the genetic information in different species.
To put a modern spin on Darwin's ideas, if we are correct about evolution, then once two species split from their common ancestor they should gradually accumulate more and more genetic differences, most of them silent or neutral in nature. So the degree of genetic difference between two species should be a rough measure of the length of time since their evolutionary lines separated.
So here's a test of a major prediction made by evolutionary theory. Say we consider three species call them A, B and C. By more traditional taxonomic means, we determine that A and B are more closely related to each other (shared a more recent common ancestor) than either of them is related to C. Now we compare their DNAs (or the proteins produced by the recipes coded into the DNA). If we are correct about evolution, then we should find that the DNAs of A and B are more similar to each other than either is similar to C's DNA.
One of the greatest triumphs of evolutionary theory is that it has passed this test over and over again. In fact, though this has been done many, many times, the theory has never failed the test.
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Updated 25 September 2004