This is interesting:
Conquest by a minority, including Renfrew's elite dominance and system-collapse models.
Under conquest, a people, usually with strong social hierarchy and military organization, takes power in a country and imposes its language and usually much of its global cultural inheritance, retaining for itself positions of power and control of wealth. Conquerors, if well-organized, can be a small minority. Two such cases are the previously cited examples of Turkey and Hungary, which are well-known historically and are further studied in chapter 5. In both cases the genetic traces of the invaders are, at best, extremely modest since they were not sufficiently numerous to influence strongly the genetic pool of the previous inhabitants.
Conquest does not always involve language replacement.
Several barbarian invasions after the fall of the Roman empire did not have a marked effect on local languages, although in some cases the original barbarians' dialect has been conserved to these days in certain small areas.
In Renfrew's terminology, system collapse, generating a power vacuum, may result in unusual circumstances, giving a chance to certain minorites to take control and impose their language. Two examples cited by Renfrew are the fall of the Roman Empire in Britain, after which Anglo-Saxon mercenaries, perhaps with the help of kin from abroad, acquired power; and the fall of the Myan civilisation around the tenth century A.D., about which much less is known. As acknowledged by Renfrew, this mechanism could be considered a special case of elite dominance.
Unlike Renfrew, who has chosen not to consider genetic aspects of these phenomena, we are interested in joint history. In the demographic-subsistence model, there is clearly replacement of both languages and, at least partially, also of genes. Most elite-dominance situations are likely to leave the genes largely or relatively intact.
— Cavalli-Sforza et al., The History and Geography of Human Genes, ISBN 0691087504, p. 102
How often has this happened in history? It would be interesting if you could give examples and cite sources, preferably academic sources, of when and how language replacement occurred in history, and what was the cause.
I'll start with an early historical case in point:
Parpola, Simo (2004). "National and Ethnic Identity in the Neo-Assyrian Empire and Assyrian Identity in Post-Empire Times". Journal of Assyrian Academic Studies Vol. 18 (No. 2).
In this study, Simo Parpola argues that there was a language shift in the Neo-Assyrian Empire from Akkadian to Aramaic. He calls this, "The Aramaization of Assyria". For a more general discussion about how this has affected the modern Assyrian Neo-Aramaic language, discuss it in this thread, because the Assyrian language isn't the focus of this specific topic.
I've also read some hypothetical speculation by Svante Päboo, that ethnic Finns used to speak an Indo-European language, before they shifted to an Uralic language, which became the modern Finnish, Finno-Ugric language:
Similarly, studies of mtDNA have identified large genetic distances between the Saami and other Europeans, including the Finns (Sajantila and Pääbo 1995; Sajantila et al. 1995). Likewise, Lahermo et al. (1996) found no overlap between Saami and the remaining European mtDNA patterns and concluded that the Saami and the Finns must have different genetic histories. One alternative hypothesis to explain the presence of genetic differences and language similarities in the Finns and the Saami involves a language shift by the Finns from Indo-European to Finno-Ugric
(Sajantila and Pääbo 1995).
Note to PeterThaGreat: let's not make this a bashing Finns thread, I just used it as an example for the sake of argument and discussion.
If you know more examples, preferably with good sources, let's discuss this.