Evolution of IC: Evolution of Hormone-Receptor Complexity
By Ian Musgrave
Posted April 14, 2006
Update: Ive added some substantial comments 95979 and 96005 that might be lost in the comment roll, but add some important perspective to Behe's arguments.
Michael Behe is known as the author of the concept of Irreducible
Complexity (IC, but see [note 1]). However, he has given several
different, not entirely consistent, definitions of IC. Everyone is
familiar with the "multiple parts" definition, fewer will be familiar
with the "neutral mutational steps" definition (1) and fewer still with
the idea that amino acids interactions themselves are IC (2, see my critique of this). Indeed, Behes recent paper with David Snoke (3) relied on a combination of the last two definitions (see our critique), and Behe also used the latter definition in the Dover trial (3).
A paper just out in the journal Science has effectively refuted the claims of the Behe and Snoke paper (4)
Behe has claimed that he will accept that a system is not IC if
someone can give a mutation-by-mutation account, with selective
coefficients for each mutation, for that system. Now in general, this
is just not practical, because it is very difficult to reconstruct
these systems even for bacterial antibiotic resistance, where we can
keep a close eye on the mutations, and can readily measure the effect
of the mutations. In fact, for resistance to cefotaxamine, 5 mutations
are required, and there are 18 high probability pathways to this
resistance (5), so we have little idea of the historical path the
system took. Reconstructing the actual mutation-by-mutation path of any
given feature, especially in organisms that are not as easy to grow or
test as bacteria (and given that many intermediate organisms may be
extinct), is a daunting task. By requesting a ludicrous amount of
detail, Behe has tried to insulate himself from the evidence for
In a magnificent paper, Bridgham, Carroll and Thornton (BCT, 4) have
tackled the daunting task of reconstructing evolutionary pathways head
on, reconstructing ancient proteins in the process. The article looks
at one of Behes key examples, protein-binding sites for small
molecules. The ability of proteins to bind small molecules is critical
for organisms to function, because enzymes bind small molecules as part
of metabolism, receptors proteins bind small hormone molecules to
initiate cell signaling and many small molecules bind to a variety of
proteins to modify their functions. For example, in the Behe and Snoke
paper (3), they examine the ability of the oxygen carrying protein
haemoglobin to bind the organic phosphate molecule
2,3-diphosphoglycerate (DPG, which modifies the oxygen binding capacity
The BCT paper looks at the binding site of the receptors for some
steroid hormone receptors, those for mineralocorticoids (MR) and those
for glucacorticoids (GR). Receptors are proteins whose amino acid
chains fold in such a way as to produce a pocket where particular small
molecules (hereafter called ligands) bind. These pockets can be
incredibly selective, and the term "lock and key" is often used. The
small molecule fits into the protein receptor like a key into a lock
(this is an oversimplification, as the molecules are flexible, and are
"floppy" keys and locks, and the electrostatic charge and lipid
solubility of the molecule comes into play, but it helps visualization).
Lock and key binding illustrated with the molecule adrenaline (right)
Now, modern tetrapods (amphibians, reptiles, birds and mammals) have
separate receptors for the steroid hormones cortisol (GR, which
modulates metabolism, inflammation and immunity) and aldosterone (MR,
modulates salt balance amongst other things). Hagfish and Lampreys
("primitive" jawless fish with cartilaginous skeletons) have only one
receptor, which is activated by both aldosterone and cortisol. Sharks
and such have two receptors, both of which are activated by both
aldosterone and cortisol. Finally, bony fish and tetrapods have two
receptors, one which is activated by aldosterone, and one which is
activated by cortisol. What were the molecular steps which brought this
The aldosterone receptor (as helix and strings) showing aldosterone
(the white collection of balls) binding in the pocket. On the right is
a close-up, with the amino acids that are mutated to make the GR
receptor shown as balls as well.
BCT approach this in a very elegant way. Using phylogenetic analysis
of the existing sequences of GR and MR receptors they were able to
reconstruct the sequence of the receptor ancestral to the GR and MR.
They expressed this reconstructed gene in cells, and tested the
sensitivity to aldosterone and cortisol. The ancestral receptor
responded to both aldosterone and cortisol. By a combination of
phylogenetic analysis and mutagenesis they isolated two mutations that
converted the ancestral receptor into a GR (serine to proline at
position 106 in the chain, and lysine to glutamine at position 111). By
studying their properties, and comparing them to MR and GR receptors
from hagfish, sharks, bony fish and tetrapods, they determined that the
seriene to proline mutation came first, followed by the lysine to
Click Image to enlarge
Fig. 4. Evolution of specific aldosterone-MR signaling
by molecular exploitation. (A) Synthesis pathway for corticosteroid
hormones. Ligands for the ancestral CR and extant MRs are underlined;
cortisol, the ligand for the tetrapod GR, is overlined. The terminal
addition of aldosterone is in green. Asterisks, steps catalyzed by the
cytochrome P-450 11ß-hydroxylase enzyme; only the tetrapod enzyme can
catalyze the step marked with a green asterisk. (B) MRs aldosterone
sensitivity preceded the emergence of the hormone. The vertebrate
ancestor did not synthesize aldosterone (dotted circle), but it did
produce other corticosteroids (filled circle); it had a single receptor
with affinity for both classes of ligand. A gene duplication (blue)
produced separate GR and MR. Two changes in GRs sequence (red)
abolished aldosterone activation but maintained cortisol sensitivity
[see ©]. In tetrapods, synthesis of aldosterone emerged due to
modification of cytochrome P-450 11ß-hydroxylase. mya, million years
ago. © Mechanistic basis for loss of aldosterone sensitivity in the
GRs. Phylogenetically diagnostic amino acid changes that occurred
during GR evolution were introduced into AncCR-LBD by mutagenesis.
Dose-response is shown for aldosterone (green), DOC (blue), and
cortisol (red). The double mutant (bottom right) has a GR-like
phenotype. Arrows shows evolutionary paths via a nonfunctional (red) or
functional (green) intermediate. From Bridgham JT, Carroll SM, Thornton
JW. Evolution of hormone-receptor complexity by molecular exploitation.
Science. 2006 Apr 7;312(5770):97-101. under "Fair Use" and non-profit
Deriving the ancestral sequence, determining the mutations, their
temporal sequence and their relative selectability is all in all a
virtuoso performance. And it directly addresses the issue Behe raised
with the evolution of the DPG binding site (3, see our critique).
Behe and the Discovery Insitute have reacted quickly and negatively
to this paper. But in doing so they display a curious amnesia. Behe
I certainly would not classify their system as IC. The IC systems I
discussed in Darwins Black Box contain multiple, active protein
factors. Their "system", on the other hand, consists of just a single
protein and its ligand."
Yet this "system" is precisely the thing that Behe uses in his
exemplar for the Behe and Snoke paper, the binding of DPG to
haemoglobin. And Behe has said in testimony to the Dover trial (3) that
the Behe and Snoke paper on evolution of binding sites is about
irreducible complexity. So if the evolution of the DPG binding site
(where you only need two mutations to make a functioning DPG binding site)
is an example of IC, then the evolution of the aldosterone binding site
is also (note 2). As the BCT paper specifically cites the Behe and
Snoke paper, you would expect they would look at the ideas contained in
the paper, not "Darwins Black Box". Behe has had a long history of
citing examples of molecular IC. He has even called disulfide bond
"irreducibly complex" (2). So his disavowal of an example that directly
addresses the Behe and Snoke paper (3) is particularly disingenuous.
Stephen Meyer also argues on this line.
Contrary to what the authors assume receptor-hormone pairs do not
constitute irreducibly complex systems. The receptor-hormone pair is
only a small component of a signal transduction circuit that regulates
other complex physiological processes. For such pairs to have any
selective or functional advantage many other protein components have to
be present, including the other components of a signal transduction
circuit and the physiological processes that such circuits regulate.
Yet, and I emphasise this again, Behe himself has argued in sworn testimony to the Dover trial that they do constitute IC systems (3). Don't the DI fellows follow each others work?
Once again we see that the Intelligent design promoters are willing
to move the goal posts to avoid refutation of their ideas. Molecular IC
is Behes own invention, and he cant ignore his idea when it is
Notes and references.
[Note 1] Actually, evolutionary biologist H.J. Muller came up with
irreducible complexity in 1918, as a prediction of Darwinian evolution.
See H. J. Muller, "Reversibility in Evolution Considered from the
Standpoint of Genetics," Biological Reviews 14 (1939): 261-80. In 1986
another evolutionary biologist, Cairns-Smith, also described IC systems
produced by evolutionary processes.
[Note 2] Technically, the GR receptor is an example of
subfunctionalisation, where a generalist receptor becomes more
selective. This is quite important in evolution. Behes exemplar,
binding DPG in haemoglobin, is a result of a reduction of specificty,
the ATP binding site now binds DPG. When Meyer complains that the BCT
paper does not generate a new protein fold family, well, neither does
Behes DPG example, which Behe claims demonstrates IC. (ironically,
there is also a recent paper showing in more detail how protein fold
familes evolve. Zeldovich KB, Berezovsky IN, Shakhnovich EI. Physical
origins of protein superfamilies. J Mol Biol. 2006 Apr
(1) "An irreducibly complex evolutionary pathway is one that
contains one or more unselected steps (that is, one or more
necessary-but-unselected mutations). The degree of irreducible
complexity is the number of unselected steps in the pathway." http://www.arn.org/docs/behe/mb_indefenseofblood...
"Thus in a real sense the disulfide bond is irreducibly complex,
although not nearly to the same degree of complexity as systems made of
"The problem of irreducibility in protein features is a general one.
Whenever a protein interacts with another molecule, as all proteins do,
it does so through a binding site, whose shape and chemical properties
closely match the other molecule."
(3) Behe MJ & Snoke DW (2004) Simulating evolution by gene
duplication of protein features that require multiple amino acid
residues. Protein Science.
There are none that use that phrase, but as I indicated in my direct testimony, that I
regard my paper with Professor David Snoke as to be arguing for the
irreducible complexity of things such as complex protein binding sites. Emphasis added IFM
"So the point is that those little colored squares [amino acids] are
enormously complex in themselves, and further the ability to get them
to bind specifically to their correct partners also requires much more
additional information. It is not a single step phenomenon. You have to
have the surfaces of two proteins to match."
(4) Bridgham JT, Carroll SM, Thornton JW. Evolution of
hormone-receptor complexity by molecular exploitation. Science. 2006
Apr 7;312(5770):97-101. [PubMed ] [Abstract] [Full Text (subscribers only)]
See also commentary Adami C. Evolution. Reducible complexity. Science. 2006 Apr 7;312(5770):61-3. [summary] [Full Text (subscribers)]
(5) Weinreich DM, Delaney NF, Depristo MA, Hartl DL. Darwinian
evolution can follow only very few mutational paths to fitter proteins.
Science. 2006 Apr 7;312(5770):111-4. [PubMed] [Abstract] [Full text (subscribers only)]
[I corrected a few typos -- Nick]
Originally posted at The Panda's Thumb