If evolution is to mean "change in population over time," then I emphatically say, "Yes, evolution has occurred."
However, when most people think of the term "evolution," they think of "all life coming from a common ancestor." Nobody has seen this, of course. This study does not show one kind of organism evolving into another different kind of organism.
These lizards had a cecal valve to slow down passage of plant food such that they could ferment the plant material. This structure is not present in the parent population nor in a similar, extinct, population of lizards that used to inhabit the same area. However, the cecal valve is present in other lizards that eat plants.
The head shape and size is definitely natural selection, but not via random mutation. This would be the kind of natural selection occurring in other organisms that exhibit a characteristic that exists on a continuum. For example, if the height of a plant were suddenly critical such that tall plants were selected for in an environment...Yes, the small plants would be selected against, and their percentage within the population would therefore decrease. Same thing here. Larger heads with greater bite forces are required so the smaller heads/smaller bite forces are selected against and therefore decrease in percentage abundance within the population. This is natural selection, but not natural selection of some mutation. This is natural selection of an already existant characteristic in a continuum.
Mitochondrial genetic studies indicated the old and new lizards were genetically identical. Therefore, the gene for the cecal valve must have been present in the original parent population, but not expressed. This is not evolution. This is the selective expression of genes based upon environmental factors.
Again, no new genes were introduced...at least not that I could see by reading this study. Since no new genes were formed, no evolution has occurred. This is yet another wonderful example of the power of natural selection on already present genes!! That in and of itself is awesome!
Thanks for the good read, and the good conversation! It is always stimulating!