The snake example you provide is a single animal that adapts itself to its surroundings. Adaptation is similar, yet very different from evolution. Adaptation happens on a short timescale (as little as seconds, to perhaps years, but within a single life time, without substantial changes in DNA other than, perhaps, epigenetic ones), while evolution may occur over the course of millions of years or more, and likely involving gross changes in appearance and hereditary material. I am not entirely sure what you are exactly after.
A striking example of adaptation is the fish Tilapia, an invasive species of fish (Fig. 1) that is capable of sustaining fresh water as well as sea water environments. The transition from one medium to the other is mediated by gross changes in endocrinology. Your specific snake example on size adaptation seems, however, unsupported based on non-scientific websites at least.
Fig. 1. Nile Tilapia. Source: Sea EX
If you are after examples of swift evolution -There is a nice example of the Waterlily Beetle, Galerucella nymphaeae (Fig. 2), that shows striking host-race formation, i.e., the same species of beetle shows clear adaptations (on an evolutional scale) dependent on which plant it has been found. It has been shown that these beetles extracted from different plants (Nymphaeaceae versus Polygonaceae) showed strong morphological and ecological differentiation to their different host plant families. Beetles from Nymphaeaceae were significantly larger and had disproportionally larger mandibles than beetles from Polygonaceae. It is argued that these differences are adaptations (on an evolutional scale) to the tougher leaves of the Nymphaeaceae, which has been linked to genetic differences. In addition, females showed a strong oviposition preference, and all beetles showed distinct feeding preferences for the host family from which they originated. Moreover, these host preferences resulted in clear differences in survival: survival was 2 to 11 times higher on the natal host family than on the alternative host family.
Fig. 2. Waterlily Beetle. Source: Wikipedia.
Exactly how fast these beetles have adapted I am unsure of, but the interesting thing is that these beetle races are not geographically separated by a natural barrier like a sea, a mountain range etc. Instead, they live side-by-side, but have adapted to a specific host plant.
- Pappers, Evolution in action: Host race formation in Galerucella nymphaeae. PhD thesis (2001), Radboud University, Nijmegen, the Netherlands