Spain and Portugal have a history of fervent Catholicism, but almost a third of the population now turns out to have a non-Christian genetic heritage. About 20 percent of the current population of the Iberian Peninsula has Sephardic Jewish ancestry, and 11 percent bear Moorish DNA signatures, a team of geneticists reports.
The genetic signatures reflect the forced conversions to Christianity in the 14th and 15th centuries after Christian armies wrested Spain back from Muslim control.
The new finding bears on two very different views of Spanish history: One holds that Spanish civilization is Catholic and all other influences are foreign, the other that Spain has been enriched by drawing from all three of its historical cultures - Catholic, Jewish and Muslim.
The genetic study, based on an analysis of Y chromosomes, was conducted by a team of biologists led by Mark Jobling of the University of Leicester in England and Francesc Calafell of the Pompeu Fabra University in Barcelona.
The biologists developed a Y chromosome signature for Sephardic men by studying Sephardic Jewish communities in places where Jews migrated after being expelled from Spain in the years from 1492 to 1496.
They also characterized the Y chromosomes of the Arab and Berber army that invaded Spain in 711 A.D. from data on people now living in Morocco and Western Sahara.
After a period of forbearance under the Arab Umayyad dynasty, Spain entered a long period of religious intolerance, with its Muslim Berber dynasties forcing both Christians and Jews to convert to Islam, and the victorious Christians then expelling Jews and Muslims or forcing both to convert.
The genetic study, reported online Thursday in the American Journal of Human Genetics, indicates there was a high level of conversion among Jews.
Jonathan Ray, a professor of Jewish studies at Georgetown University, said that a high proportion of people with Sephardic ancestry was to be expected.
"Jews formed a very large part of the urban population up until the great conversions," he said.
The genetic analysis is "very compelling," said Jane Gerber, an expert on Sephardic history at the City University of New York, and weighs against scholars who have argued that there were very few Jewish conversions to Christianity.
Ray raised the question of what the DNA evidence might mean on a personal level. "If four generations on I have no knowledge of my genetic past," Ray said, "how does that affect my understanding of my own religious association?"
The issue is one that has confronted Calafell, an author of the study. His own Y chromosome is probably of Sephardic ancestry - the test is not definitive for individuals - and his surname is from a town in Catalonia; Jews undergoing conversion often took surnames from place names.
Jews first settled in Spain during the early years of the Roman empire. Sephardic Jews bear that name because the Hebrew word for Spain is Sepharad.