Language families.
The "genetic" relatedness of languages and their grouping into "families" is a metaphor taken from biology. The family tree model fails to account for the mutual influence of languages; hence the "wave" theory. Family relationships are set up by systematically recurring phonetic differences, rather than similarities. The 4,000 languages are grouped in about 50 language families:linguistics tries to find their common features, called "language universals".
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