Mainland, Island Greeks Genetically Diverged in Middle Ages

Genetic analysis proves that following thousands of years of conquests and migrations, peoples living around the Mediterranean today share common ancestors, with one surprising outlier: Greece.
Elizabeth Sloane / Ruth Schuster
Source - http://www.haaretz.com/archaeology/1.798089

Santorini, a Greek island, whose people are genetically more similar to southern Italians than to mainland Greeks. Sturt Manning/Bloomberg
The peoples living today around the Mediterranean Sea are all related, after thousands of years of intermittently attacking and loving each other, a new genetic study has unsurprisingly shown. However, the data from the international team of scientists found a startling exception: mainland Greeks, who seem to be genetically closer to Albanians than to their brethren in the Greek islands.
Wars, population movements led to mixing, with the pot being stirred every time the peoples living around the sea were invaded (or were invaded) by somebody else. Which happened a lot, thanks to from aggressive colonization such as by Greece and Rome in classic history, or passive migration.
One upshot is that the ancient populations around the Mediterranean became increasingly genetically variable, and tracking down any specific group's origin became increasingly difficult.
"Present-day population diversity is composed by multiple genetic layers, which make the deciphering of different ancestral and historical contributes particularly challenging," write the scientists behind a new study, that looked at 23 populations.
Different genes for different Greeks
The scientists did find the expected degree of genetic continuity extending from Sicily to Cyprus.
The scientists were not expecting to find that the people in the Greek islands appear genetically closer to southern Italians than to the people in continental Greece.  
Meanwhile, the mainland Greeks, including the Peloponnese in southern Greece, had become slightly differentiated. They clustered with populations from the southern Balkans, including Kosovo and Albania.

All the tested populations from Southern Italy, Sicily, mainland Greece, and the Greek islands including Cyprus share a genetic component inherited from early Neolithic farmers, say the researchers. They also have a smattering of traits originating in the Levant and Near East.
Why would the mainland Greeks have gained such strong genetic affiliation with the Balkans? Population expansions during the Middle Ages, suggest the scientists, noting the Slavic migrations.
In other words, the mainland Greeks and southern Balkan peoples would at an earlier time also been part of what the team calls the "Mediterranean genetic continuum". But their genetics were augmented with the Slavic component.
In yet other words, if the data interpretation is correct, the Slavic genetic addition to the mainland Greeks was historic, not prehistoric, and so was the genetic divergence between the Greek islanders and the mainland Greeks.
It's Greek to me
Another question that the genetic study addresses is the origin of the -day Greek-speaking communities in Southern Italy. Hellenic (Magna Graecia) and Byzantine colonization is one possibility, but the preponderance of historical and linguistic evidence agree that these groups are a remnant of a wider Greek-speaking area. Throughout the area, Greek was widespread before Latin.
Historical and linguistic data suggest that southern Italy has been characterized by "a pervasive multilingualism" at least from antiquity, write the authors, and conclude: "both cultural transmission and genetic admixture may have played an important role in the formative process of these groups since the very beginning.