Lehigh biochemistry professor Michael Behe and his cronies in the intelligent design community have attempted to poke holes in evolutionary theory using an idea dubbed "irreducible complexity"—the notion that complex systems with interdependent parts could not have evolved through Darwinian trial and error and must be the work of a creator, since the absence of any single part makes the whole system void. However, a paper published in the April 7th issue of Science provides the first experimental proof that "irreducible complexity" is a misnomer, and that even the most complex systems come into being through Darwinian natural selection.
"We weren't motivated by irreducible complexity," said Joe Thornton, an evolutionary biologist at the University of Oregon and a co-author of the paper. "How complexity evolved is a longstanding issue in evolutionary biology per se, and it's once we saw our results that we realized the implications for the social debate."
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Thornton's team has been studying one example of a complex system in which each part defines the function of the other: the partnerships between hormones and the proteins on cell walls, or receptors, that bind them. The researchers looked specifically at the hormone aldosterone, which controls behavior and kidney function, and its receptor.
"[This pairing] is a great model for the problem of the evolution of complexity," said Thornton. "How do these multi-part systems—where the function of one part depends on the other part—evolve?"
Thornton and his co-investigators used computational methods to deduce the gene structure of a long-gone ancestor of aldosterone's receptor. They then synthesized the receptor in the lab. After recovering the ancient receptor—which they estimate to be a 450-million-year-old receptor that would have been present in the ancestor of all jawed vertebrates—Thornton's team tested modern day hormones that would activate it. Although aldosterone did not evolve until many millions of years after the extinction of the ancient hormone receptor, Thornton found that it and the ancient receptor were compatible.
This cross-generational partnership is made possible, Thornton explained, by the similarity in form between aldosterone and the ancient hormone that once partnered with the receptor.
"The story is basically that a new hormone evolved later and exploited a receptor that had a different function previously to take part in a new partnership," said Thornton.
The principal at work in the evolution of complex systems is molecular exploitation: when an individual component casts around for other materials that might work together with it, even though those elements might have evolved as parts of other systems.
"Evolution assembles these complex systems by exploiting parts that are already present for other purposes, drawing them into new complexes and giving them new functions through very subtle changes in their sequences and in their structures," Thornton said.
While the mutually dependent parts do not evolve to be perfectly complementary to one another, after molecular exploitation, they cleave together and create an illusion of irreducible complexity.
"Such studies solidly refute all parts of the intelligent design argument," wrote Christoph Adami, of the Keck Graduate Institute of Applied Life Sciences, in an introduction to the Science paper. "Those 'alternate' ideas, unlike the hypotheses investigated in these papers, remain thoroughly untested. Consequently, whatever debate remains must be characterized as purely political."