King Charles II of Spain was physically and mentally disabled, infertile — and extremely inbred. When he died in 1700, aged 38, so did the male line of the Spanish Habsburg royal family, as famous for their pointed jaws as for their extreme consanguinity. A provocative analysis now suggests that the Habsburg royal family might have evolved under natural selection over three centuries to blunt the worst effects of inbreeding. Evolutionary theory predicts such a 'purging' process, and researchers have documented the effect in animals and plants. But evidence among humans is scant — in part because of the dearth of data on inbred families spanning many generations.
Royal families such as the Habsburgs are an ideal place to look, says Francisco Ceballos, a geneticist at the University of Santiago de Compostela in Spain, who led the research. He and colleague Gonzalo Álvarez used written records to track the marriages, births and deaths of 4,000 individuals across more than 20 generations. “The royal dynasties of Europe are a lab of inbreeding for human populations,” says Ceballos. The team's study is published this month in Heredity1. The Habsburg pedigree resembles the organizational flowchart of a dysfunctional government agency. Inbred marriages, such as those between first cousins or between uncles and nieces, were the rule rather than exception. Such pairings, which include the marriage between Philip II of Spain — Charles II's great-grandfather — and his niece Anna of Austria, were used to keep titles in the family and to forge political alliances.