The Byzantine defeat at Manzikert in 1071 did not itself lose Anatolia, but the civil wars that followed opened the plateau to Turkmen groups who settled it over the next generation, and by the 1080s a Seljuk sultanate was established in the interior. What happened to the existing Christian population is one of the sharper disputes in medieval history. Speros Vryonis's 'The Decline of Medieval Hellenism in Asia Minor' (1971) argued for large-scale disruption, flight and the destruction of the Christian ecclesiastical structure, producing eventual Islamisation. Claude Cahen and, more recently, A. C. S. Peacock and Alexander Beihammer have emphasised continuity, negotiated accommodation, intermarriage and gradual conversion, with Christian communities persisting for centuries. Both readings work from the same thin body of chronicles; no demographic reconstruction is possible. The entry records a securely documented transfer of sovereignty over a very large territory accompanied by undocumented but real displacement.
Seljuk and independent Turkmen groups occupied the Anatolian plateau after the Byzantine defeat at Manzikert, displacing or absorbing the Greek- and Armenian-speaking Christian population over the following century
There is no census. McEvedy and Jones put Asia Minor at roughly 7 million around 1000 CE, and Russell's estimates for the Byzantine provinces are of the same order; some Byzantinists argue for 8-10 million at the eleventh-century peak. The incoming Turkmen population is generally reckoned much smaller, in the low hundreds of thousands, so the demographic change in the following centuries was mainly conversion and language shift rather than replacement.
No usable figures. The chronicles describe raids, sieges and famine over a century of intermittent warfare without totals, and the scholarly dispute is about the character of the change rather than a measurable death toll.
Turkey, Bulgaria, Armenia