Final answer:
Natural selection repurposing existing structures, behaviors, or genes is called preadaptation or exaptation, highlighting the flexibility and opportunistic nature of adaptive evolution.
Step-by-step explanation:
When natural selection repurposes existing structures, behaviors, or genes, this process is known as preadaptation or exaptation. These terms describe how features that originally evolved because they served one function may later serve a different function. An example of this would be bird feathers, which originally evolved to help with temperature regulation, later becoming useful for flight. This showcases the ability of natural selection to work on the existing variation within a population, leading to adaptive evolution where there is an increase in frequency of beneficial alleles and a decrease in deleterious alleles.