Homological trait in phylogenetics
While phenograms are formed on the basis of all the traits that can be differentiated in the members of the studied species, a scheme of cladogenesis, which is intended to express not similarity but rather relatedness of species, must be expressed solely on the basis of certain subsets of these traits. For example, it is quite obvious that relationships between species cannot be derived on the basis of shared traits, formed in the individual species during evolution independently of one another, i.e. traits that were not present in the closest common ancestor of the compared species. Such a trait is termed homoplasy. Homology is the opposite of homoplasy; this is a common trait of two or more species that these species inherited from their closest common ancestor.
The definition of a homological trait is, in a certain sense, relative. On the one hand, the wings of birds and the wings of bats are homoplasies, as no exclusive common ancestor of birds and bats had wings. However, wings evolved only once in the line of birds and the line of bats. The presence of wings in birds and bats is thus homoplasy from the viewpoint of the two groups together and is a homological trait from the viewpoint of each group separately. The situation is further complicated by the fact that, in both birds and bats, wings were formed from the front limbs of vertebrates and the common ancestor of these two groups had front limbs. Thus, the presence of front limbs must also be considered to be a homological trait that birds and mammals inherited from their common ancestor.