New Analyses Bolster Central Tenets of Evolution Theory
Monday, September 26, 2005
When scientists announced last month they had determined the exact order of all 3 billion bits of genetic code that go into making a chimpanzee, it was no surprise that the sequence was more than 96 percent identical to the human genome. Charles Darwin had deduced more than a century ago that chimps were among humans' closest cousins.
But decoding chimpanzees' DNA allowed scientists to do more than just refine their estimates of how similar humans and chimps are. It let them put the very theory of evolution to some tough new tests.
If Darwin was right, for example, then scientists should be able to perform a neat trick. Using a mathematical formula that emerges from evolutionary theory, they should be able to predict the number of harmful mutations in chimpanzee DNA by knowing the number of mutations in a different species' DNA and the two animals' population sizes.
"That's a very specific prediction," said Eric Lander, a geneticist at the Broad Institute of MIT and Harvard in Cambridge, Mass., and a leader in the chimp project.
Sure enough, when Lander and his colleagues tallied the harmful mutations in the chimp genome, the number fit perfectly into the range that evolutionary theory had predicted.
Their analysis was just the latest of many in such disparate fields as genetics, biochemistry, geology and paleontology that in recent years have added new credence to the central tenet of evolutionary theory: That a smidgeon of cells 3.5 billion years ago could -- through mechanisms no more extraordinary than random mutation and natural selection -- give rise to the astonishing tapestry of biological diversity that today thrives on Earth.
Evolution's repeated power to predict the unexpected goes a long way toward explaining why so many scientists and others are practically apoplectic over the recent decision by a Pennsylvania school board to treat evolution as an unproven hypothesis, on par with "alternative" explanations such as Intelligent Design (ID), the proposition that life as we know it could not have arisen without the helping hand of some mysterious intelligent force.
Today, in a courtroom in Harrisburg, Pa., a federal judge will begin to hear a case that asks whether ID or other alternative explanations deserve to be taught in a biology class. But the plaintiffs, who are parents opposed to teaching ID as science, will do more than merely argue that those alternatives are weaker than the theory of evolution.
They will make the case -- plain to most scientists but poorly understood by many others -- that these alternatives are not scientific theories at all.
"What makes evolution a scientific explanation is that it makes testable predictions," Lander said. "You only believe theories when they make non-obvious predictions that are confirmed by scientific evidence."
Lander's experiment tested a quirky prediction of evolutionary theory: that a harmful mutation is unlikely to persist if it is serious enough to reduce an individual's odds of leaving descendants by an amount that is greater than the number one divided by the population of that species.
The rule proved true not only for mice and chimps, Lander said. A new and still unpublished analysis of the canine genome has found that dogs, whose numbers have historically been greater than those of apes but smaller than for mice, have an intermediate number of harmful mutations -- again, just as evolution predicts.