Dear XJ,
Your example is a nice illustration of a trait that cannot be explained by evolution through random mutation and selection, and rightfully you wonder:
Is it theoretically plausible to posit such a phenomenon (evolutionary "momentum"), given the known mechanisms of mutation? What about any speculative mechanisms?
Is there any evidence of such phenomena?
PB: You've had several responses from neodarwians who try to keep up the appearance of randomness of mutations, and they have to since otherwise the theory falls. Hoever, by now they should know better, since it has been demonstrated that a class of mutations operates in the genome in a non-random fashion. For examples see my threads: "molecular genetic evidence against random mutation", "Dr Page's best example of common descent readily -and better- explained by the GUToB", and "More non-random evolution".
Any questions? Don't hesitate to ask,
Best wishes,
Peter