1. Historical-comparative linguistics - studies language drift, the gradual, changes that slowly build up over time to create new dialects and eventually new languages.
Genetic linguistics - studies which languages are related
language family - diverged from a common source
mother language and daughter languages
proto-language - the source (mother) of a language family that can be reconstructed by comparing the sounds of the daughter languages
language isolate - language not demonstrably related to any other
Basque (Spain), Ket (Siberia), Ainu (Japan), Haida (British Columbia)
lumpers (deep genetic comparisons) - Joseph Greenberg (Stanford)
Mother Tongue theory - all languages descend from a common source, called "Proto-World," probably spoken in East Africa about 40,000 years ago.
splitters (comparisons of only less than five to seven thousand years old - posit several hundred language families
2. Language contact studies - how language contact with other languages cause changes - usually considered a part of sociolinguistics. But sometimes the mixing is so intense that a new language results, with multiple parents instead of a single parent:
pidgin - a makeshift language that is no one's native language; used as a social register during inter-ethnic communcation.
creole – results when a pidgin is learned natively by children, thereby creating a new language.
A creolized language, or abrupt creole results when a large group of adults abruptly shifts to a new language, learning it with massive interference from their original language; the next generation then learns this mix as their native tongue, thereby abruptly creating a new language or new dialect.
Unlike other languages, pidgins, creoles, and creolized languages all have more than one "mother language"
3. Ecological linguistics - how such factors as geography, climate, plant and animal domestication, food economy and disease patterns influence language spread, extinction and mixing