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Unintelligent DesignHow birds got to fly.

By Arthur AllenPosted Wednesday, March 7, 2007, at 5:25 PM ET

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Daniel Engber explained how to identify the calls of evolutionarily less-fortunate bird species. Millions of years of evolution have not taught birds what to do when trapped in a wood stove. Jacob Weisberg argued that gains in evolutionary thought are leaving increasingly less room for God, though the Catholic Church doesn't necessarily oppose evolution. David Greenberg explained why we worry about the intelligent design/evolution debate: The Scopes trial never settled it.

It's genome size, not body size, that matters. Most salamanders are smaller than birds, but their genomes are often 90 times larger. If you've ever seen a salamander move (they don't get around much), you can see why they don't need small cells. Having larger cells also means that salamanders have fewer neurons per square centimeter of brainpan. Thus "bird-brained" is unfair to birds (though "amphibian-brained" doesn't have the same ring, I grant you).

There are other advantages to smaller genomes and cells. Smaller red blood cells give an organism a higher surface-to-volume ratio for the exchange of gases like oxygen and carbon dioxide. Birds need a very high metabolism to fly. So, it's not surprising, for example, that the ostrich has a larger genome than the finch.

So, why should anyone besides evolutionary biologists care about all of this? Because it reveals the combination of random and selective changes that make up the history of life. There's nothing like one of these studies to show that if God is in charge of this show, he's very vague in his stage directions.

Also, helping us parse evolution is one way in which molecular biology is continually reshaping our ideas about life's building blocks. When scientists first learned about the smallness of animal genomes several decades back, they were puzzled that small, insignificant creatures like salamanders had much more DNA per cell than people. This phenomenon was described as the "C-value paradox," with the C-value signifying the size of the genome. At the time, scientists figured that DNA and genes were synonymous—in other words, that all of our DNA had the purpose of encoding proteins. Hence the paradox: Humans were obviously more complex than onions, yet the onion has five times more DNA in each cell than a human being does.

Gradually, scientists realized that most DNA did not code proteins (the current estimate, in human DNA, is about 1.5 percent). People, apes, birds, and corn all have similar numbers of genes. But they have varying amounts of what many scientists, having no better explanation for the stuff, used to call "junk DNA." Why some of God's creatures have more DNA than others is pretty much anyone's guess, but a great deal of research is now focused on learning what the various types of "junk DNA" do or do not do. The junkyard includes things like jumping genes, which can replicate on their own and float around the chromosome, and pseudogenes, relics of old genes that went through so many mutations, they lost their coding mojo. "In many ways, these various elements can be conceived of as living in the ecosystem of the genome," says T. Ryan Gregory, a professor of integrative biology at the University of Guelph in Ontario. Like an ecosystem, the genome is a jungle of random and selective evolution. Sometimes bits of dead genes come back to life, in a sense, by getting themselves inserted into a spot on a chromosome in a way that disrupts the function of newer genes or affects their expression in the cell.