The Black Death epidemic is estimated to have killed as much as 50 percent of the population of Europe. Three centuries later, a fifth of the population of London died in the Great Plague.
However, these diseases didn't kill randomly, Hoelzel explained.
"The plague killed some people while others remained resistant," he said.
Entire villages of related families were often wiped out, perhaps removing an entire genetic lineage in one fell swoop.
Eske Willerslev, a specialist in ancient DNA from the University of Copenhagen, said he is surprised by the findings but agrees that the historic epidemics may explain the loss in diversity.
"If there were only small numbers of people with each DNA type, then the plague could have been enough to make many of these DNA types disappear," he said.
Since the epidemics, it appears that England hasn't been able to make up the loss to the gene pool, despite the high rate of immigration into the country over the past 200 years.
"Enough diversity was lost to not be fully compensated by the recent immigration effect," Hoelzel said.
(Read related story: "British Have Changed Little Since Ice Age, Gene Study Says" [July 19, 2005].)
Not everyone is convinced by the new findings.
Mark Thomas, a geneticist at University College London, thinks the reduction in diversity can be explained in a more mundane way.
"Ancient DNA tends to elevate diversity, because the way DNA is damaged over time tends to mimic the mutations that lead to diversity," he said.
The way that DNA degrades after a person's death can make ancient DNA appear to have more variation than modern DNA, he explained.
Willerslev, the expert in ancient DNA agreed, saying, "DNA damage, an artifact of the data, is the other obvious explanation for this decrease in diversity."
Hoelzel countered that DNA damage couldn't explain the changes his team observed.
"We undertook multiple controls to ensure that DNA contamination and post-mortem change could not explain the change in diversity," he said.
Meanwhile, Thomas said he also doubts that the Black Death and the Great Plague would have caused enough reduction in population to explain the drop in diversity.
"The population reduction would have had to be extreme in absolute, rather than relative, numbers to cause the loss of diversity claimed," Thomas said.
Hoelzel responded that the loss was not simply the result of the sheer numbers killed by the plagues, but rather was a function of the particular genetic lines that disappeared during the epidemics.
"A typical population bottleneck [an extreme reduction in numbers] couldn't explain the loss—too many [people] are known to have survived," he said.
"It would need instead to be related to the differential survival of families, or natural selection, but either mechanism could explain the loss observed."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES