Off with their heads
The kings and queens of England may have murdered their closest kin to secure the throne, but they still stayed true to a basic rule of evolutionary biology.
The history of Britain between the 14th and 16th centuries is full of epic tales of jealousy, intrigue and murder. To ensure their right to the throne remained unchallenged, members of the royal family frequently murdered their closest relatives. Both Henry IV and Henry VIII killed five cousins each.
John McCullough, an evolutionary biologist at Cambridge University, and his colleagues at Indiana State University felt sure the lure of the ultimate prize would force British monarchs to break free from the shackles of their own selfish genes. "It was kill or be killed," he says.
But to their surprise, the researchers found that despite the high death toll, the kings and queens of England all obeyed a golden theory of biology known as Hamilton's rule.
According to the theory, people or animals can pass on their genes by helping their relatives. That means they should be more willing to help brothers or sisters, who share half their genes on average, than more distant relatives who share less.
But relatives also have a nasty habit of competing with you for resources, be they chicks squabbling over food, or heirs fighting for the throne. And this is where the flip side of Hamilton's rule comes in.
If you kill two brothers, say, you've essentially wiped out your genetic identity rather than preserving it for future generations, says McCullough. "Then you have really damaged yourself in an evolutionary sense. So you can eliminate relatives but only up to a certain level."
But it turns out no king or queen, from Edward III who succeeded to the throne in 1327 to Elizabeth I who died in 1603, killed enough relatives to wipe out the equivalent of their own genetic inheritance (see graphic).
Edward IV, who reigned in the late 15th century, was the worst offender, executing his brother, George, Duke of Clarence, and five cousins, including Henry VI and Edward, Prince of Wales. Yet even Edward IV's victims shared only two-thirds of his genes in total.
"I was astounded. We thought that at least two or three would violate [Hamilton's rule] because some very close relatives were killed," says McCullough. "They had no theory of genetics at the time so they were simply operating under their own set of rules," he says. "As it turns out, it is in accord with scientific expectations."