Ian Musgrave has a nice description of how a complex system evolved. The same kind of system has been used by Michael Behe as an example of an irreducible complex system. Obviously it has now been reduced.
The Panda’s Thumb is the online virtual pub of the online virtual University of Ediacara.
Here’s the reference:
Science 7 April 2006:
Vol. 312. no. 5770, p. 15
Highly integrated biological systems can provide a challenge in terms of understanding how such systems arose during the course of evolution. One such system involves the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR), which arose from a gene duplication deep in the vertebrate lineage. MR binds to and is activated by the hormone aldosterone, which only appeared much later, in the lineage leading to tetrapods, raising the question of how each component “anticipated” the presence of the other. Bridgham et al. (p. 97) analyze the evolution of this system by reconstructing the ancestral hormone receptor (AncCR) that gave rise to MR and GR and show that it already had substantial affinity for aldosterone, as a consequence of its affinity for other more ancient hormones, possibly including 11-deoxycorticosterone. Furthermore, two single-nucleotide mutations in the AncCR yield a GR-like receptor with present-day low affinity for aldosterone.
Science magazine is published by the American Association for the Advancement of Science (AAAS).
Carl Zimmer has an excellent commentary on this research and the reaction of Intelligent Design advocates to it.