Here, extra legs and here Scientific American, page 2 mentions parasites other causes are noted.
It takes no great leap to find potential for attribution here, well outside of aberrantly rapid evolution. Isolated populations will be exposed to different environmental factors, including vegetation, chemicals (natural and manmade, the latter in precipitation or runoff) and quite possibly different pathogens. One population exposed to a chemical (natural or manmade) or a pathogen, be it viral, bacterial, fungal, or parasitic, will naturally successfully breed only those who can adapt to or are resistant to that pathogen or chemical and still produce viable young.
That change could handily occur in just a few generations, not even 8000 years.
The other population, not exposed, might not fare as well if the pathogen or traces of the chemical are transferred in the act of breeding and that population has little or no natural resistance to the pathogen.
A significant difference between the case you are citing and the frogs in the article is that your case is about a phenomenon that appeared within a single community of frogs and the article's case is about a split group that only exhibits the defect when interbred. However, your suggestion that exposure of one group to a pathogenic or parasitic environmental agent might have happened. In that case, it would have contributed to the environmental pressures that, through natural selection, favored genetic drift that was successful at overcoming this change to the environment.
However, while an infected parent might pass along a pathological infection to its offspring, it is unlikely that this would become a heritable entity. Especially if the infectious pathogen prevents viability.
In short, a pathogenic infection is more likely to have been a factor responsible for the genetic drift of one group than to have become a heritable trait itself.
Lamarck lives!
"However, while an infected parent might pass along a pathological infection to its offspring, it is unlikely that this would become a heritable entity. "
Didn't mean to impugn you. Obviously if the acquired trait is passed down it's Lamarckian, so you pass with flying colors, but it's not easy to figure out if SJ agrees with that.
The best I can gather is that he thinks a pathological infection could be a force for Natural Selection, as well it might.
But one has to be real careful with parasites. If the parasite is relatively specific as its host(s) and it's too successful, then it dooms both its host and itself.