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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' comes from 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.
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 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, the 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).
Some closely knit language families, and many branches within larger families, take the form of dialect continua, in which there are no clear-cut borders that make it possible to unequivocally identify, define, or count individual languages within the family. However, when the differences between the speech of different regions at the extremes of the continuum are so great that there is no mutual intelligibility between them, the continuum cannot meaningfully be seen as a single language. A speech variety may also be considered either a language or a dialect depending on social or political considerations. Thus different sources give sometimes wildly different accounts of the number of languages within a family. Classifications of the Japonic family, for example, range from one language (a language isolate) to nearly twenty.
The common ancestor of a language family is seldom known directly, since most languages have a relatively short recorded history. However, it is possible to recover many features of a proto-language by applying the comparative method—a reconstructive procedure worked out by 19th century linguist August Schleicher. This can demonstrate the validity of many of the proposed families in the list of language families. For example, the reconstructible common ancestor of the Indo-European language family is called Proto-Indo-European. Proto-Indo-European is not attested by written records, since it was conjectured to be spoken before the invention of writing.
Sometimes, however, a proto-language can be identified with a historically known language. For instance, dialects of Old Norse are the proto-language of Norwegian, Swedish, Danish, Faroese and Icelandic. Likewise, the Appendix Probi depicts Proto-Romance, a language almost unattested due to the prestige of Classical Latin, a highly stylised literary register not representative of the speech of ordinary people.
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Most of the world's languages are known to belong to language families. Those that have no known relatives (or for which family relationships are only tentatively proposed) are called language isolates, which can be thought of as minimal language families. An example is Basque. In general, it is assumed that language isolates have relatives, but at a time depth too great for linguistic comparison to recover them.
Languages that cannot be reliably classified into any family are known as language isolates. A language isolated in its own branch within a family, such as Armenian within Indo-European, is often also called an isolate, but the meaning of isolate in such cases is usually clarified. For instance, Armenian may be referred to as an Indo-European isolate. By contrast, so far as is known, the Basque language is an absolute isolate: It has not been shown to be related to any other language despite numerous attempts, though it has been influenced by neighboring Romance languages. A language may be said to be an isolate currently but not historically if related but now extinct relatives are attested. The Aquitanian language, spoken in Roman times, may have been an ancestor of Basque, but it could also have been a sister language to its ancestor. In the latter case, it would make Basque and Aquitanian form a small family together (ancestors are generally not considered to be distinct languages for this purpose).
Shared innovations, acquired by borrowing or other means, are not considered genetic and have no bearing with the language family concept. It has been asserted, for example, that many of the more striking features shared by Italic languages (Latin, Oscan, Umbrian, etc.) might well be "areal features". However, very similar-looking alterations in the systems of long vowels in the West Germanic languages greatly postdate any possible notion of a proto-language innovation (and cannot readily be regarded as "areal", either, since English and continental West Germanic were not a linguistic area). In a similar vein, there are many similar unique innovations in Germanic, Baltic and Slavic that are far more likely to be areal features than traceable to a common proto-language. But legitimate uncertainty about whether shared innovations are areal features, coincidence, or inheritance from a common ancestor, leads to disagreement over the proper subdivisions of any large language family.
A sprachbund is a geographic area having several languages that feature common linguistic structures. The similarities between those languages are caused by language contact, not by chance or common origin, and are not recognized as criteria that define a language family. An example of a sprachbund would be the Indian Subcontinent.
The concept of language families is based on the historical observation that languages develop dialects, which over time may diverge into distinct languages. However, linguistic ancestry is less clear-cut than familiar biological ancestry, in which species do not crossbreed. It is more like the evolution of microbes, with extensive lateral gene transfer: Quite distantly related languages may affect each other through language contact, which in extreme cases may lead to languages with no single ancestor, whether they be creoles or mixed languages. In addition, a number of sign languages have developed in isolation and appear to have no relatives at all. Nonetheless, such cases are relatively rare and most well-attested languages can be unambiguously classified as belonging to one language family or another.