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Darwin proposed that natural selection could gradually transform a species. Scientists have observed thousands of cases of natural selection in action. They've documented that beaks of finches on the Galápagos Islands have gotten thicker when droughts forced the birds to crack tough seeds to survive. They've observed bacteria develop resistance to drugs that were believed to be invincible. Now biologists are applying DNA-sequencing technology to natural selection, which lets them identify the individual genetic changes that boost reproductive success.
As populations adapt to their surroundings, they can gradually evolve into new species. "We now have, I think, a good understanding of how new species arise that is, how biological diversity is created," says Jerry Coyne, an evolutionary biologist at the University of Chicago and the author of the new book Why Evolution Is True. "Darwin made little inroad into the problem, despite the title of his magnum opus."
Biologists have also found plenty of evidence to support Darwin's other major claim: that different species share a common ancestry. Over the past 15 years, for example, paleontologists have found several fossils of whales with legs, linking modern whales to their terrestrial ancestors. Besides studying fossils, biologists can discover the genealogy of species by looking at their DNA. The fossil record points to hippos and other hoofed mammals as being the closest living relatives of whales. So does their DNA. Our own DNA contains clues to the bonds we share with the rest of life it turns out, for instance, that we are closer kin to mushrooms than to sunflowers.
It's been 1.5 billion years or more since our ancestors split off from our fungal cousins. How did the genome of our ancestor change so that it could produce two-legged primates? One part of the answer is that mutations over time altered genes that encode proteins, and some of those changes have been favored by natural selection. But that does not mean that our genome the sum total of our human DNA is a finely tuned collection of protein-coding genes. In fact, a lot of mutations that all humans carry neither helped nor harmed our ancestors. They spread just by chance. And a lot of our genome is not made up of protein-coding genes. In fact, 98.8% of it is not. Some of that 98.8% consists of "pseudogenes" genes that once encoded proteins but no longer can because of a crippling mutation. They are the molecular equivalent of a vestigial tail, allowing us to see evolution's track.
Biologists are a long way from understanding the entire genome, but as they get to know its parts better, they're getting a more precise comprehension of one of the most important features of evolution: how complex organs evolve. The notion that something as intricate as an eye could have evolved, Darwin wrote, "seems, I freely confess, absurd in the highest degree." But he argued that new complex organs could evolve through a series of intermediate forms.