In Bedbugs, Scientists See a Model of Evolution

In the closing sentence of “The Origin of Species,” Charles Darwin marvels at the process of evolution, observing how “from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Few people would describe bedbugs as most beautiful or most wonderful. Yet this blood-feeding pest may represent an exceptional chance to observe the emergence of Darwin’s “endless forms”: New research indicates that some bedbugs are well on their way to becoming a new species.

“For something that is so hated by so many people, it might just be a perfect model organism for evolutionary questions,” said Warren Booth, a biologist at the University of Tulsa and a co-author of the new study, published in Molecular Ecology.

Scientists have been very slow to appreciate the biology of bedbugs despite the fact that the insects have infiltrated human shelters for thousands of years. That’s because the insects practically vanished at the dawn of modern biology in the 1940s, thanks to the widespread use of DDT.

Bedbugs have returned with a vengeance in recent years, partly because they have evolved resistance to pesticides, and scientists are struggling to learn more about these pests. It’s a much bigger challenge than examining, say, monarch butterflies.

“It’s very hard to study in them in the wild, because often people don’t want you to use their house as a laboratory,” said Toby Fountain, a postdoctoral researcher at the University of Helsinki. “They just want to get rid of them as quickly as possible.”

Dr. Fountain, Dr. Booth and other bedbug experts study the insects by collaborating with exterminators, who preserve some specimens in alcohol and ship them to the scientists. The researchers extract the DNA, finding clues to the bedbug’s natural history.

Variations in their genes reveal a lot about how the insects move around the world. As it turns out, their DNA even offers clues as to how bedbugs became such annoyances in the first place.

For the new research, Dr. Booth collaborated with Ondrej Balvin, a bedbug researcher at Charles University in Prague who collected bedbugs from across Central Europe with his Czech colleagues.

The common bedbug, Cimex lectularius, feeds not only on humans but on other animals, especially bats. So as well as collecting human-feeders, the researchers gathered bedbugs from bat roosts in houses, churches and castles.

Little was known about the bugs that depend on bats for their meals. “The big thing that this paper adds is the bat side,” said Dr. Fountain, who was not involved in the new study.

Dr. Booth compared DNA sequences from 214 bedbugs. Those that live with bats, he found, were genetically quite distinct from those living with humans.

“The pattern was so stark, I’d never seen anything like it,” Dr. Booth said.

The results support a hypothesis that Dr. Balvin and other researchers have put forward to explain how bedbugs started making life unpleasant for humans. They argue that Cimex lectularius started out living in caves, feeding on bats. When early humans showed up in the caves, some of the bedbugs turned their attention to their new hosts.

“This paper shows that that is true,” Dr. Booth said.

When humans left caves and began building dwellings, they brought their new admirers along. But humans represented a new challenge for the insects, requiring new adaptations.

For one thing, we sleep at night, not in the daytime, which meant that the bedbugs had to shift their schedule. Dr. Balvin and his colleagues also have found that bedbugs that feed on humans have longer, thinner legs than those on bats, perhaps because bedbugs that feed on people no longer need to cling to bats hanging from cave roofs.

The insects also evolved adaptations for feeding on human blood. Researchers at Masaryk University in the Czech Republic have found that bedbugs adapted to feeding on humans have a shorter life span if they drink only bat blood.

Once humans started putting up buildings, bats began building roosts in them. Dr. Booth’s new results suggest that the bats brought their own bedbugs, too.

Yet for thousands of years, the two populations of bedbugs have shared the homes but haven’t interbred — although they can, for now. Bedbugs that feed on humans still belong to Cimex lectularius, Dr. Fountain said. But the two populations are diverging.

For instance, many bedbugs that feed on humans carry a genetic variation that makes them resistant to pesticides, Dr. Booth found. The bedbugs on bats still carry a version of the gene susceptible to pesticides.

Dr. Fountain said that the scenario laid out by Dr. Booth and his colleagues is consistent with their evidence, but that it needed to be confirmed with a bigger survey of bedbugs from around the world.

“The cool thing is we have the tools to be able to do it, which we didn’t have two or three years ago,” Dr. Fountain said.

If bedbugs continue to infest our buildings, they will diverge even further from their bat-feeding cousins. At some point, they will become a species of their own, adding one more branch to Darwin’s tree of life.

“It’s right on the cusp,” Dr. Booth said.