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The PhyloCode
Article 14. Synonymy
14.1. Synonyms are names that are spelled differently but refer to the same taxon. In this code, synonyms must be established and may be homodefinitional (based on the same definition) or heterodefinitional (based on different definitions). The criteria for determining whether definitions are different are described in Art. 13.2, including Notes 13.2.113.2.3.
Note 14.1.1. Homodefinitional synonyms are synonyms regardless of the phylogenetic context in which the names are applied. However, in the case of names with different definitions, the phylogenetic context determines whether the names are heterodefinitional synonyms or not synonymous.
Example 1. Suppose that Hypothetica were defined as the least inclusive clade containing species A and B, and Cladia were defined as the least inclusive clade containing species C and B. In the context of any hypothesized phylogeny in which A shares a more recent common ancestor with C than either does with B, Hypothetica and Cladia would be heterodefinitional synonyms. However, in the context of an alternative hypothesis that A and B are more closely related to each other than either is to C, Hypothetica and Cladia would not be synonymous.
Note 14.1.2. Node-based, apomorphy-based, and branch-based definitions (Note 9.3.1) usually designate different clades, although they may be nested clades that differ only slightly in inclusiveness. Therefore names based on two or more of these different kinds of definitions usually are not synonyms. (In theory, it is possible for different types of definitions to designate the same clade. For example, in cases in which doubling of the chromosomes (autopolyploidy) causes speciation, the apomorphic chromosome number arises simultaneously with the splitting of a lineage. In such cases, an apomorphy-based definition that uses this chromosome number as a specifier will refer to the same clade as a branch-based definition that uses the species in which the chromosome doubling occurred, or one of its descendants, as the internal specifier.)
14.2. If there are two or more synonyms for a clade, the accepted name for that clade is the earliest acceptable one that applies to it, except in cases of conservation.
14.3. When two or more synonyms have the same publication date (Art. 5), the one that was registered first (and therefore has the lowest registration number) takes precedence.