PZ Myers did an excellent job yesterday of dismantling the latest from intelligent-design advocate Jonathan Wells. Wells wrote a piece on WorldNetDaily called “Why Darwinism is Doomed.” It is based some new research identifying an important gene involved in the human brain–which I blogged about last month. Wells claims that this discovery is a serious threat to evolution. Why? Because scientists have only just found the gene, because many genes are involved in building the human brain, and because finding a gene that’s different in humans than in other animals doesn’t actually reveal what caused that difference. Myers shows why all three arguments are wrong.
If you rely on Wells, you will come away with an impression of how scientists look for new genes that is precisely the opposite of reality. The authors of the new paper did not simply scan through the human genome in the hopes of finding something interesting. The genome is too big and mysterious for that approach. It’s like searching the Pacific for an island. If you just wander the ocean randomly, you may die before landfall.
What made this particular search especially challenging was that the gene in question does not make a protein. Protein-encoding DNA makes up only two percent of the genome, and non-coding DNA is a murky realm of virus-like stretches of DNA, broken genes, and some genes that actually serve an important function. It is very difficult to look at a single genome and pick out functionally important segments of DNA from the “junk.”
So how did the scientists find the new gene? They began with the fact that functionally important stretches of DNA will show signatures of evolution. A functionally unimportant stretch of DNA will acquire random mutations over millions of years. But if a segment of DNA has an important function, most mutations will be harmful. So the scientists compared non-coding DNA from chimpanzees, mice, and rat, for segments of DNA showed significant signs of negative selection. They identified 35,000 segments, all of which have resisted acquiring mutations for hundreds of millions of years. Then they looked at the human versions of each one. They found evidence that 49 of those regions had experience strong natural selection only in the human lineage. They ranked the DNA regions by the strength of natural selection and then investigated the ones at the top of the list. Lo and behold, they discovered that these DNA regions encoded genes that produce RNA, which turns out to do interesting things in the brain.
These scientists did not find a new gene and then try to make up a story to fit the theory of evolution. Evolution guided them to the genes. And they are hardly alone. Last year I blogged twice about scientists who find new genes and other important elements of DNA by looking for signatures of evolution. (Here and here.) Far from showing that “Darwinism is doomed,” the new research shows just how vital evolution is to understanding how the human genome works.
Originally published September 28, 2006. Copyright 2006 Carl Zimmer.