Genetic relations among humans and groups of humans have to do with the biological transmission of genes from generation to generation. Genetic relations among languages, however, are not biologically based, but are defined by cultural transmission from generation to generation. That is, languages are learned, not inherited via the genes.
All languages change during the course of time, and the longer the time period the greater the changes. When a language is culturally transmitted by speakers to their offspring over many generations, it can become so different that it is given a different name. Thus, for example, the Latin spoken in parts of the Iberian peninsula changed over time and became Spanish; Proto-Germanic became Anglo-Saxon/Old English in England; Proto-Slavic became Old Church Slavonic, etc.
A language whose speakers lose contact with one another can eventually evolve into numerous distinct languages. This is what happened with Proto-Indo-European, Latin, Proto-Germanic, and countless other languages spoken at various earlier periods in human history. Latin became Spanish, Portuguese, Italian, French, and a number of other languages spoken by groups currently without national status (e.g. Catalan, Galician, Provencal, Surselvan, and other lesser known Romance languages). Proto-Germanic became English, Frisian, Dutch, German, Icelandic, Danish, Swedish, Norwegian, and a few other languages, We call such groups of languages related languages, because they are related to each other by virtue of paths of parent-to-offspring cultural transmission that trace back to a common source language.
Cultural transmission that happens between groups of adults, in which one group takes some of the words of the other group's language, is a different kind of relation called language contact. The term "related languages" is not used for this kind of situation.
Metaphorically, we can refer to the relation defined by a parent-to-child chain of language transmission as genetic relationship of languages. The source language can be called the "ancestor language" or the "mother language", and the later languages deriving from it are called the "descendant languages" or the "daughter languages". Daughter languages are descended from the mother language. If there is only cultural contact as described above, the relationship is not one of genetic descent.
Genetically related languages can be closely related, or more distantly related, depending on how directly they trace back to a common source. Degree of relatedness can be represented by setting up a genetic classification of languages, shown in the form of a family tree in which daughter languages are plotted in relation to their mother languages, with direct connections representing closer relationship and indirect connections representing more distant relations. For example, English and Dutch are closely related languages and would be right next to each other on the Indo-European family tree, because they are both directly descended from a common ancestor language (Proto-Low Germanic) that itself came from Proto-Germanic. English and Latin are also related, because they both trace back to the ancient Proto-Indo-European language, but they are much more distantly related because more time has passed between the source and the daughter languages and there were several language splits that happened in the meantime. The intermediate splits define groupings of more closely related languages like the Germanic languages and the Romance languages.
Genetic relations among languages are uncovered by observation of language data (words, and if available, grammatical endings) and systematic logical inference. A pattern of similarities across many languages in many basic words and endings, which is too widespread across the words to be due to chance or sound symbolism, and in which loanwords have been ruled out (based on non-basicness and other features typical of loanwords), must be due to genetic relationship. As the great Indo-Europeanist and Celticist Calvert Watkins wrote:
It has been rightly said that the comparatist has one fact and one hypothesis. The one fact is that certain languages present similarities among themselves so numerous and so precise that they cannot be attributed to chance and of such a kind that they cannot be explained as borrowings or as universal features. The one hypothesis is that these languages must then be the result of descent from a common original.--Calvert Watkins, "Indo-European and the Indo-Europeans"
The family tree metaphor for language relationship is useful because it captures some similarities between language transmission and transmission of genes. Both involve transmission from generation to generation, but in one case it is cultural (the languages), the other biological (the genes).
One problem with the family tree metaphor is that people often logically confuse language relationship with biological relationship. They apparently do this because sometimes biologically related groups of humans speak linguistically related languages, and perhaps because both types of relations share a similar 'branching tree' structure involving degrees of closeness of relation. Also, they fail to think about how language is transmitted, and about the obvious counterexamples to the false general principle 'people with the same language ancestry must have the same biological ancestry' and vice versa. The generalization is obviously false, since a language can always be adopted by a people whose ancestors spoke other languages. For example, most, if not all, American Indians in the U.S. speak English. Most people in the U.S., in fact, speak English, with the exception of recent immigrants. Conversely, it is not true that people with the same biological ancestry necessarily speak the same language. Do you speak the language of your biological ancestors? Most Americans do not.
The problem of confusion of biological vs. linguistic ancestry and genetic relations can easily be avoided if we just recall that all culturally transmitted artifacts, technologies, and institutions are learned, not innate, including human languages; and that the family tree metaphor of language relationships is just that, a metaphor.
© Suzanne Kemmer