At Phys.Org there is an article describing the work of some scientists with the “chordate” Oikopleura dioica which does not have the genes for Retinoic Acid (RA), which all other chordates have. RA is a form of Vitamin A and is needed in the development of the heart.
In this organism, the heart develops but without the presence of RA. How can this happen? Answer: “Regressive Evolution.”
All the genes for RA have been lost, and lost in a non-random fashion.
You would be hard-pressed to identify the above organism (a planktonic organism) as a “chordate,” but developmentally, it is one—you know, the chordate “body-plan” allows it to be identified as one. Now, this O. dioica cannot be the LCA of chordates because it lacks RA, so we have “evolution” via loss of genes, a theme that was sounded here not too long ago.
What’s intriguing here, of course, is the “non-random” nature of this “loss.” Nevertheless, what is seen with O.dioica has to be called “evolution” since it cannot have happened before all the other changes that took place in phylum chordata, but after some LCA. But then what do you call this process?
“Regressive evolution” seems to be just the term the doctor ordered. If you want to explain what biology produces, all you need is an ever expanding “dictionary.”
If you add new genes, this is “evolution.” If you lose already present genes, this too is “evolution.” If you add the genes “non-randomly” (transposons), this is Darwinian. If you lose genes “non-randomly,” this too is Darwinian.
Heads I win; tails you lose.