You would be amazed by how conserved adaptation is. For example, in the evolution of a receptor,
Nature publishes paper on the edge of evolution, Part 2 | Uncommon Descent - The Weblog of Michael BeheUsing clever synthetic and analytical techniques, Bridgham et al (2009) show that the more recent hormone receptor protein that they synthesized, a GR-like protein, cant easily revert to the ancestral structure and activity of an MR-like protein because its structure has been adjusted by selection to its present evolutionary task, and multiple amino acid changes would be needed to switch it back. That is a very general, extremely important point that deserves much more emphasis. In all cases not just this one natural selection is expected to hone a protein to suit its current activity, not to suit some future, alternate function. And that is a very strong reason why we should not expect a protein performing one function in a cell to easily be able to evolve another, different function by Darwinian means. In fact, the great work of Bridgham et al (2009) shows that it may not be do-able for Darwinian processes even to produce a protein performing a function very similar to that of a homologous protein.
Before reading their paper even I would have happily conceded for the sake of argument that random mutation plus selection could convert an MR-like protein to a GR-like protein and back again, as many times as necessary. Now, thanks to the work of Bridgham et al (2009), even such apparently minor switches in structure and function are shown to be quite problematic. It seems Darwinian processes cant manage to do even as much as I had thought.
Comparatively, people may have previously conceded that the intelligent mechanism for adaptation (rivaling Darwin's random variation) would be acting through enzyme moderation but not anti-biotic resistance. From research, it can be shown that even the latter is under the influence of that intelligent mechanism. If there was anything you would have granted random mutation it would have been anti-biotic resistance through the high mutation rate of microbes, but it fails. Though adaptation has risen as the fountain of intellectual fulfillment in materialism, it may turn out to be one of the most compelling cases for design. This one might just be the one to flip the earth on its axis- stay tuned.