DNA weaves together the tapestry of life on Earth. It mutates, which can lead to variation among organisms. Watch how natural selection relies on DNA to mold organisms to suit their environments. Play the Evolution Lab: http://www.pbs.org/nova/labs/lab/evolution/ .
If Darwin came back today, what would he be most excited by in modern science? I think it would be the whole world of genetics. That’s the answer to all the big questions he had – how does variation emerge? And how could that be transmitted? Let’s return to our brown bears stranded in the Arctic to consider the impact of genetics on our understanding of evolution. Each bear is made up of cells. And if we take a brown bear cell, and tunnel into its nucleus, we find DNA – the molecule with the genetic instructions for building, in this case, a brown bear, written in a four-letter code. Now, the thing about DNA—it’s not perfect. When it’s copied, mistakes get made. Mutations, in other words, that sometimes affect an organism’s traits and that sometimes can be passed from parent to offspring. So the variation at the heart of evolution—it’s genetic variation. Slight differences in DNA that—for example—could make some bears a bit lighter in color, a bit more insulated against the cold, and a bit more capable of digesting fattier foods like seals. Evolution is essentially any change in the genetic composition of a population. Mutations are random, so they’re not always helpful. But the bears with mutations that gave them some advantage for Arctic living survived and reproduced more often than bears without them. They passed the genes responsible for those adaptations onto their cubs. Over generations, more bears inherited and elaborated on these and other changes in the DNA. The eventual result – a polar bear. And when we tunnel into its cells, we find polar bear DNA. Think of DNA as the raw material that—across billions of years—evolution has molded and built into countless forms of life. We are all related, tied to one another by the most spectacular of double helical threads.