Vestigial structures are structures that have supposedly lost most or all of their ancestral function in any given species. Structures are called vestigial on comparison with homologous structures in related species. Vestigiality occurs by ordinary evolutionary processes. Normally, it occurs by the loss of functions of a feature that is no longer subject to positive selection pressures once it loses its value in an altered environment. But it can also become harmful instead of useless. For example, once an island-dwelling species loses its capability to fly, it can be harmed much more easily than before. In humans, a couple of vestigial structures include the appendix and the coccyx or tailbone.