I mean, in general, yes that is true in biology. The energy required to maintain bodily structures mean that they tend to be relevant to evolutionary fitness.

I mean, in general, yes that is true in biology. The energy required to maintain bodily structures mean that they tend to be relevant to evolutionary fitness. There are very few structures that are entirely irrelevant, even if the specific difference in form between different populations may be the result of genetic drift. That said, I think your example would work by replacing it with "sexual selection." Don't know why bird feathers have complex micro-structures allowing them to reflect UV light? Let's just call it "sexual selection."