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synapomorphy
In cladistics, a synapomorphy or synapomorphic character state is a trait that is ancestral and inferred to have been present in their most distant common ancestor A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question is assumed to have originated in their oldest common ancestor. The word "synapomorphy," coined by German entomologist Willi Hennig, is derived from the Greek words ', ''syn'' = with, in company with, together with; ', ''apo'' = away from; and ', ''morphe'' = shape.
==Primative trait==
For the sake of precision, the term "primative" is preferred to "simple". 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.〔(【引用サイトリンク】year= 2000 )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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