Many species of plants and animals adapted to desert conditions probably did not arise there. their success in living in deserts could be due to structures that originally had one use but became adapted for different functions.

Exaptations

Explanation:

Exaptation explains a shift in the function of a trait during evolution. For example, a trait can evolve because it is used for one particular function, but eventually it may come to be used foranother function. Exaptations are found both in anatomy and behaviour. A classic example are bird feathers: at first they may have evolved for temperature regulation, but subsequently were adapted for flight. Exaptation was said to proposed by Stephen Jay Gould and Elisabeth Vrba as a substitute for what they considered to be a teleologically loaded term 'pre-adaptation'.[

nucleus

would be most reasonable answer tbh