Creationists, and even some evolutionists, define “vestigial organ” as a useless leftover of evolutionary history. There is, however, a problem with calling an organ “useless” and using it as evidence of evolution: it’s very difficult to know if an organ really is useless. There may be a few cases where it seems likely that the organ or gene in question really is useless. Even so, “vestigial” is better defined as an organ that no longer performs its primary function. Organs like this can provide powerful evidence for evolution. As Jerry Coyne said, “Wouldn’t it be odd if a creator helped an ostrich balance itself by giving it appendages that just happen to look exactly like reduced wings, and which are constructed in exactly the same way as wings used for flying?” (Why Evolution is True, p.58). It makes much more sense to view the wings of flightless birds as leftovers inherited from their flying ancestors, leftovers which evolved another function. Here’s another example: There is a species of whiptail lizard that reproduces asexually– the entire population is female (See number 8 in Livescience’s Top 10 Vestigial Organs). Even so, the lizards still engage in sexual behaviors from time to time, even though sexual behavior obviously doesn’t serve its primary function of creating babies in an asexual species. Even if such behavior may have a function, it’s clear that the presence of such behavior only makes sense if the whiptail lizards are descended from a sexual species (which they would have to be, since most of their near-relatives are sexual).
The same thing can be said about numerous other organs in other species: tiny leg bones in whales, tail bones and body hair in humans, cave fish and salamanders with eyes covered by scales/skin,”broken genes,” and so on. My favorite is the extensor coccygis, a muscle used to move the tail that is still found in a few human beings. Darwin had pointed out earless sheep have been bred, and that these earless sheep still have a stump where those ears used to exist. Likewise, there are breeds of hornless cattle who occasionally regrow diminished horns. When one form evolves from another, we expect there will be bits and pieces of organs that the ancestor had, because that’s what we’ve observed with artificial breeding (See the section on “Rudimentary Organs” in Origin of Species).
Vestigial Organs are almost one hundred percent expected if evolution took place, but not necessarily expected if creationism is true.