Adaptive responses such as this cause me to wonder whether experts in genetics, given the currently advanced state of basic knowledge and electron microscopy (or whatever the currently most advanced microscopy may be), can determine whether the gene that makes the longer legged version of this creature was present in the gene pool all along. This question would apply to any such adaptive change.
Or, a variant: Did random mutation provide the requisite new information, then through normal interbreeding become sufficiently widespread to make possible a large scale change in the population in question?
Or is my question just plain dumb?
Your question is anything but dumb. It is insightful and clearheaded.
We know mutations happen. Most have very little impact for a variety of reasons it would take a couple of chapters of a textbook to explain. Mutations also come in kinds and sizes.
Some changes matter right away. Say, poor development of legs. Critter probably dies young and mutation is taken out of the potpulation.
Another change, let’s say a bit of extra thickness to fur, have no immediate impact. If winters get colder, it would be beneficial, if summers get hotter, it would become a problem and, again, get taken out.
So, you can see that over time beneficial or potentially beneficial mutations tend to increase in the population. If circumstances don’t change, you have a very diverse population. If they do, the best collection of accumulated mutations gets a head start for the next generation. Rinse, repeat.
Eventually (millions of years) they don’t look much like their ancestors at all.
Hope this helps. If anything is not clear, just ask. A lot has been omitted and I may have missed some clarifying points.