Summary
Reactive Evolution is the ability to adapt to certain attacks in order to become more resistant to them, and more successful under the given conditions.
As opposed to Adaptation, Reactive Evolution does not only alter the user to make them better suit their environment, but it also improves them with new abilities, greater resistance, more power, etcetera.
This power is prone to the No Limits Fallacy. However, it can still be quantified by its showings in-verse, and a reasonable limit can be determined based on the displayed feats.
Examples
As with all example lists, this is not comprehensive.