In an earlier response to someone elses question you said:

"I accept that such coevolution is the most likely (and indeed only
supported) idea. And since it's been observed in some systems and is
plausible in others, it's reasonable to assume that is what is
happening. If I go to a factory and see a BWM assembled by robots, I
don't have to see every factory in the world manufacturing every product
to accept that it is possible to build both complex and simple things
in this way."

Just wondering what the systems it has been observed in were? It would be useful when dealing with questions to be able to point to a specific example (debating with creationist friends is tiring)!

In the context there I was talking about two systems within an organism co-eveolving though of course the term generall referes to two different species evolving in tandem (like a host species becoming resistant to an infectious bacteria and that become more infections in response, or an orchid and bee specialising to use each other).

There are enough obvious examples (though the difficult questioner was talking specifically about biochemical pathways, but I'm a palaeontologist, so I'll stick to hard anatomy). One that's easy enough to see in living species and fossils is that things like buffalo and elk with big horns or antlers have bigger neural spines (the bits of vertebrae that stick up) in the neck. These support bigger muscles to help hold up the heavy head with all that extra bone on it. Obviously it would be a disadvantage ofr a neck to grow huge muscles to support nothing, and huge horns with no extra support wouldn't get you very far. So these things must have developed together, with increasingly big horns being developed in tandem with increasingly big neural spines.

I think the previous questioner was trying to get a more complex and integrated systems and while that's not really in my line, things like the immune system and blood clotting factors have been tracked in enormous detail and really are very complex. They are well worth reading up on.

As a final note, I'd suggest simply not arguing with creationists. It is, in my experience, both frustrating and futile. But good luck!

Agreed and just to reiterate my point from the previous discussions. One should not take the 2 "finished articles" as the starting point and say "isn't it amazingly unlikely they both evolved by chance at the same time". It is far more helpful to think of an initial mutation that provided a selective advantage and as a result a whole host of other related "supportive" mutations are also selected for to maximise that initial advantage. That is by far the most likely scenario as to how complex biochemical pathways evolved.

Right and trying to backtrack that from the final derived outcome (my computer analogy - see below)  is a worthwhile exercise, but extremelty hard and there will be gaps that we cannot fill.


With respect to protein structure and function evolution, there are specific examples of co-evolution of G protein-coupled receptors (GPCRs) and their corresponding endogenous peptide ligands. A wonderful example are the neurohypophysial hormones vasopressin and oxytocin, and their receptors. Vasopressin controls the absorption of water in our kidneys by binding to the vasopressin V2 receptor, and has numerous other actions including effects on behaviour by activating brain vasopressin V1a and V1b receptors. Vasopressin is related to oxytocin that has a number of roles in reproduction and behaviour. The two hormones differ by only 2 out of 9 amino acids, and the one oxytocin receptor is structurally closely-related to the three vasopressin receptors. Vasopressin will also activate the oxytocin receptor whereas oxytocin only usually binds the vasopressin receptors with low affinity.

The vasopressin and oxytocin genes have evolved by gene duplication and subsequent mutations hundreds of millions of years ago from a single ancestral gene that encodes lysine-conopressin in molluscs - vasopressin shares 5 out of 9 amino acids with this peptide, while oxytocin and lys-conopressin have 6 out of 9 amino acids in common. Lys-conopressin has vasopressin-like and oxytocin-like (e.g., reproductive) effects. Not only that, the two lys-conopressin receptors have high homology (around 40%) to the mammalian vasopressin and oxytocin receptors but do not bind vasopressin and oxytocin with high affinity. A large number of receptors and their cognate vasopressin/oxytocin-like hormones have been identified in species such as mammals (e.g., pig vasopressin differs by 1 amino acid from our vasopressin), marsupials (the eastern gray kangaroo has a ‘super’ vasopressin-like molecule called phenypressin which differs by 1 amino acid from our vasopressin, as well as the pig vasopressin and a molecule related to oxytocin), non-mammalian tetrapods and invertebrates. What you are struck by is the homology and differences in gene and protein structure during phylogeny.

The 7-transmembrane structure found in all GPCRs (around 800 in the human genome that are the target of 30-60% of current pharmaceutical agents) is a feature of eukaryotic organisms but can also be identified in prokaryotic genomes (e.g., the light-sensitive bacteriorhodopsins). During invertebrate evolution the GPCR families expanded massively by gene duplication and independent evolution of the gene copies. Other evolutionary mechanisms are also evident, e.g., there has been a large-scale degeneration in olfactory GPCR genes in animals such as whales and dolphins, and population genetics suggest that there is evolutionary selection on certain human GPCRs - see http://www.ncbi.nlm.nih.gov/pubmed/20708652

Last edited by Steve Lolait (9th Jun 2011 06:03:31)