Advocates of Intelligent Design invoke a concept called “irreducible complexity”, in which the function of a complex system (e.g., a molecular system in a living organism) depends on all of its components working together, implying that building the system as a whole must occur to achieve the function of the system, and implying that gradual stepwise Darwinian evolution could not have built the system. Research reported in the journal, Science, provides contrary evidence.
Science 7 April 2006: Vol. 312. no. 5770, pp. 97 – 101
Evolution of Hormone-Receptor Complexity by Molecular Exploitation
Jamie T. Bridgham, Sean M. Carroll, Joseph W. Thornton*
According to Darwinian theory, complexity evolves by a stepwise process of elaboration and optimization under natural selection. Biological systems composed of tightly integrated parts seem to challenge this view, because it is not obvious how any element’s function can be selected for unless the partners with which it interacts are already present. Here we demonstrate how an integrated molecular system—the specific functional interaction between the steroid hormone aldosterone and its partner the mineralocorticoid receptor—evolved by a stepwise Darwinian process.
Using ancestral gene resurrection, we show that, long before the hormone evolved, the receptor’s affinity for aldosterone was present as a structural by-product of its partnership with chemically similar, more ancient ligands. Introducing two amino acid changes into the ancestral sequence recapitulates the evolution of present-day receptor specificity.
Our results indicate that tight interactions can evolve by molecular exploitation—recruitment of an older molecule, previously constrained for a different role, into a new functional complex.
Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, OR 97403, USA.
* To whom correspondence should be addressed. E-mail: [email protected]