One recurring criticism of natural selection is that it can't generate a new structure or species; only modify existing ones. Many structures in their early evolutionary stages couldn't have performed roles that the fully formed structures do, so it was unclear how natural selection favored them. For example, half a wing or a rudiment of a feather for a bird that flies. To refute this criticism, Darwin proposed exaptation, or the utility of a structure for a role not part of the original evolutionary function. For example, rudimentary feathers could have been used for thermoregulation, and only later developed aerodynamic properties.
Work Step by Step
A criticism is that natural selection can't generate new structures/species, which Darwin refuted with exaptation.