When a trait arises due in the course of evolution to copying rather than for purely functional reasons.
“The curious fallings short of what would seem to be perfect design, that are the best evidence for a historical process of descent with modification; they are the best evidence of copying, instead of independent re-inventing, of the the design in question. We can now see better why this such good evidence. The odds against two independent processes arriving at the same region of Design Space are Vast unless the design element in question is obviously right, a forced move in Design Space. Perfection will be independently hit upon again and again, especially if it is obvious. It is the idiosyncratic versions of near-perfection that are dead giveaway of copying. In evolutionary theory, such traits are called homologies: traits that are similar not because they have to be for functional reasons, but because of copying.” (136, Darwin’s Dangerous Idea)
Darwin’s Dangerous Idea