Final answer:
The true statement about preadaptations is that they are traits that originally evolved for one purpose but also happen to suit another, different function. This illustrates evolution's opportunistic nature and the way natural selection can repurpose existing traits for new uses.
Step-by-step explanation:
The correct statement regarding preadaptations is D: A trait that evolved for one function, but that coincidentally is also suited for a new, different function. This concept is an aspect of evolutionary theory where a heritable trait that was selected for one particular use may, by chance, prove useful in a new environment or for a different purpose. This reflects the idea that evolution is not goal-directed; it does not strive to perfect or adapt organisms for future uses, but rather selects traits that provide an advantage under current conditions.
For example, the structure that eventually became a wing might have originally evolved for a different purpose, like temperature regulation or gliding between trees. Only later would this trait have been co-opted for flight, proving to be a preadaptation for this new function. It's important to realize that evolution operates without foresight, and preadaptations are a byproduct of natural selection's influence on existing variation within a population.