Never having taken a biology course much less one on evolutionary theory I can't say I know anything much about Dollo's proposition other than that he posed it over 100 years ago and that Richard Dawkins supposedly characterized it more as a statement about probabilities.
Evolution is one of those fuzzy areas of science where we have collected many facts. I remember hearing one of my history professors pre-empting near-inevitable religious arguments from his freshman students against evolution by saying simply, "We know that evolution occurs: just look at dogs. We controlled their evolution." The same could be said of cattle, horses, and other critters our civilization has grown up with.
Change in species is a fact. But does the change really have to limit itself to an everlasting bar against returning to old forms? I can't believe all biologists really believed this. Otherwise, why would anyone have attempted to preserve Przewalski's horse? The population had dwindled to a small number of zoo-kept animals by the late 1950s. Supposedly, that was too few to ensure genetic diversity, and yet today we simple lay people are told that Przewalski's horse has a pretty good chance at recovering in the wild, even to the point of building up a new genetic diversity that can sustain itself and (I'm being extrapolative here) eventually lead to sub-speciation (assuming the species lasts another 1-10 million years).
I am reminded of the (super-volcanic) Toba Event (of about 70,000 years ago), in which (supposedly) humanity was nearly wiped out, reduced to a small number (perhaps only a few thousand) of "breeding" individuals. We appear to have made a remarkable recovery since then (along with -- presumably -- many other species that were similarly threatened). These kinds of genetic bottlenecks have probably happened quite often throughout Earth's timeline. In fact, I'm sure they would have been necessary for a lot of speciation events (which, of course, were not real "events" but long periods of genetic isolation).
But there is no going back, it seems. As our environment changes we move forward biologically and there is no going back. Various mass extinction events seem to indicate that even though evolution might swing back to a similar form, function, and lifestyle the actual genes won't return within any specific species. Of course, gene-level adaptation doesn't mean that old habits cannot be reacquired, as the University of Michigan team argues based on their research.
Life can adapt in multiple directions and that may indeed include going back full-circle. Last year I wrote here on Science 2.0 that "Naturality does not return to a prior state, though [a Chronocity Set] S may reassemble itself into something that looks like a prior state (but with different components)". Although I invoked the theory of evolution in that article to help describe the increasing complexity of the universe (as it changes over time) I was not aware of Dollo's Law. Now, having read a single article that fails to encapsulate the entire debate, I cannot help but notice a strong similarity to my proposal, that change is essentially unidirectional but that change may nonetheless lead to something that looks and feels (but is not) a reversion.
If the biologists could find a pre-parasitic ancestor of today's non-parasitic mites, would they be able to establish a 1-to-1 genetic correlation (thus showing that evolution did indeed reverse itself) in the DNA? If such a comparison could be made, and the result confirmed, then my idealistic comparison is not really valid. If, however, we could show that the probability of return to a precise biological (genetic) state is zero then that would provide us with a clear example of a Morphing Set in terms of biology.
Is the genetic code of life a transformational puzzle or a transitional one? If it is transformational then any organism no matter how complex or simple could evolve to take the precise genetic form of its ancestors. If it is transitional then no matter how similar an evolved function is to an ancestral state there must be some distinct difference. I see it as an Either/Or question because in a Transformational Genetic Universe the transformations alone would suffice; however, in a Transitional Genetic Universe life would be forced to find new solutions to old problems.
My theory of Naturality cannot exist in a Transformational Universe. And that leads me in my lay capacity to wonder if the Universe is Transformational or Transitional. As I now understand it (thanks to a paper Hank pointed me to) the space between things is constantly expanding. The greater the distance between any two things the faster all the space in-between them expands. In a Transitional Universe that expansion can change but I don't think it can be reverted. However, in a Transformational Universe that expansion might indeed revert.
By "revert" I don't mean simple "reverse". A reversal of expansion is a change in velocity. A reversion is more like an undoing of what has been done. We say that Time is unidirectional but science fiction (or maybe fantasy) is filled with stories about multi-directional time. If we can show that Time is more than just unidirectional (in our own Universe) then I suppose that would mean that our Universe is Transformational, and the implications of such a fundamental principle are profound. We would have to redefine our (to me somewhat vague) ideas about infinity.
In fact, another recent article about the oldest known star in the universe appears on the surface to raise a question about our estimates of the age of the universe (although I am sure the question is really about our ability to measure age within Space and Time). Science would enter a very interesting time if it could be shown there is indeed a star older than the known age of the universe, would it not? That might be proof that the Universe is Transformational and that we need to redefine how we analyze and discuss Time.
If, as according to my previous proposition, Complexity Increases Over Time, then we must be in a Transitional Universe -- whereas if Complexity Reorganizes Itself Over Time, then we must be in a Transformational Universe. I don't see how we can have it both ways.
Dollo's Law precedes my thoughts on Change and Changefulness. He might have been on to something much larger than what he had in mind, even if it is all just about probabilities.