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A language family is a group of languages related through descent from a common ancestor, called the proto-language of that family. The term 'family' reflects the tree model of language origination in historical linguistics, which makes use of a metaphor comparing languages to people in a biological family tree, or in a subsequent modification, to species in a phylogenetic tree of evolutionary taxonomy. No actual biological relationship between speakers is implied by the metaphor. Estimates of the number of living languages vary from 5,000 to 8,000, depending on the precision of one's definition of "language", and in particular on how one classifies dialects. The 2013 edition of Ethnologue catalogs just over 7,000 living human languages. A "living language" is simply one that is used as the primary form of communication of a group of people. There are also many dead and extinct languages, as well as some that are still insufficiently studied to be classified, or even unknown outside their respective speech communities. Membership of languages in a language family is established by comparative linguistics. Sister languages are said to have a "genetic" or "genealogical" relationship. The latter is older, but has been revived in recent years to better distinguish the relationships between languages from the genetic relationships between people. The evidence of linguistic relationship is found in observable shared characteristics that are not attributed to contact or borrowing. Genealogically related languages present shared retentions, that is, features of the proto-language (or reflexes of such features) that cannot be explained by chance or borrowing (convergence). Membership in a branch or group within a language family is established by shared innovations, that is, common features of those languages that are not found in the common ancestor of the entire family. For example, Germanic languages are "Germanic" in that they share vocabulary and grammatical features that are not believed to have been present in the Proto-Indo-European language. These features are believed to be innovations that took place in Proto-Germanic, a descendant of Proto-Indo-European that was the source of all Germanic languages. ==Structure of a family== Language families can be divided into smaller phylogenetic units, conventionally referred to as ''branches'' of the family because the history of a language family is often represented as a tree diagram. A family is a monophyletic unit; that is, all its members derive from a common ancestor, and all attested descendants of that ancestor are included in the family. (In this way, the term ''family'' is analogous to the biological term ''clade''.) Some taxonomists restrict the term ''family'' to a certain level, but there is little consensus in how to do so. Those who affix such labels also subdivide branches into ''groups'', and groups into ''complexes''. A top-level (largest) family is often called a ''phylum'' or ''stock''. The term ''macrofamily'' or ''superfamily'' is sometimes applied to proposed groupings of language families whose status as phylogenetic units is generally considered to be unsubstantiated by accepted historical linguistic methods. For example, the Celtic, Germanic, Slavic, Romance, and Indo-Iranian language families are branches of a larger Indo-European language family. There is a remarkably similar pattern shown by the linguistic tree and the genetic tree of human ancestry that was verified statistically. Languages interpreted in terms of the putative phylogenetic tree of human languages are transmitted to a great extent vertically (i.e. by ancestry) as opposed to horizontally (i.e. by spatial diffusion). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Language family」の詳細全文を読む スポンサード リンク
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