A language family is a group of languages related through descent from a common ancestral language or parental language, 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. Linguists therefore describe the daughter languages within a language family as being genetically related. According to Ethnologue the 7,111 living human languages are distributed in 141 different language families. A "living language" is simply one that is currently used as the primary form of communication of a group of people. There are also many dead languages, or languages which have no native speakers living, and extinct languages, which have no native speakers and no descendant languages. Finally, there are some languages that are insufficiently studied to be classified, and probably some which are not even known to exist outside their respective speech communities. Membership of languages in a language family is established by research in comparative linguistics. Sister languages are said to have a "genetic" or "genealogical" relationship. The latter term is older.[obsolete source] Speakers of a language family belong to a common speech community. The divergence of a proto-language into daughter languages typically occurs through geographical separation, with the original speech community gradually evolving into distinct linguistic units. Individuals belonging to other speech communities may also adopt languages from a different language family through the language shift process. 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.