In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary loss. Here the lack of a structure is a derived trait.
For the sake of precision, the term "derived" is preferred to "advanced," a term which may inaccurately imply superiority. Simplicity is often secondarily derived. For example, the absence of mitochondria in the anaerobic protist Entamoeba histolytica is a result of their secondary loss, and when considered in the context of eukaryotes as a whole, is a derived trait. Likewise, the primitive character state for birds (i.e. the state possessed by their last common ancestor) is flight, which was secondarily lost by penguins and dodos.
Whether or not a trait is considered derived depends on the group in question. For example, among the (crown group) tetrapods, having five fingers is the primitive trait - as their last common ancestor bore a five-digit hand. However, amongst the vertebrates, five fingers is a derived trait, as the last common ancestor to the vertebrates did not even bear fingers.
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