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Speciation: For head and body lice, it never really happened – as was once believed

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head louse left, body louse right/CDC

From “Head and Body Lice Appear to Be the Same Species, Genetic Study Finds” (ScienceDaily, Apr. 9, 2012), we learn,

Scientists have long debated whether human head and body lice are the same or different species. The head louse (Pediculus humanus capitis) is a persistent nuisance, clinging to and laying its eggs in the hair, digging its mouthparts into the scalp and feeding on blood several times a day. The body louse (Pediculus humanus humanus) tends to be larger than its cranial counterpart, and is a more dangerous parasite. It lays its eggs on clothing, takes bigger blood meals, and can transmit relapsing fever, trench fever and epidemic typhus to its human host.

Previous studies have found that even when they are both present on the same host, head and body lice don’t stray into each other’s territories. They don’t breed with one another in the wild, but they have been shown to successfully reproduce under specific laboratory conditions. The presence of head lice has little to do with human hygiene, but body lice seem to appear out of nowhere when hygiene suffers — in times of war or economic hardship, for example.

“The differences in their sequences were so minor that if we didn’t know they were separate groups, we would have considered them the same species,” he said.

It’s not clear that they don;t consider them the same species nowe, as a matter of fact.

“As body lice transmit diseases and head lice don’t, this system provides a unique opportunity to understand subtle changes that allow body lice to transmit human diseases,” said graduate student Brett Olds, who conducted the genetic analysis.

What this story points to, not that the release spells it out, is that speciation can actually be quite difficult, and may never really finally happen – contrary to the expressed homes and beliefs of many evolutionary biologists. One difficulty in this particular case is that the two types of louse remain in proximity. So, though they don’t usually mate, they can, as the lab experiments show – doubtless retarding speciation indefinitely.


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