many rivers to cross

Update: Dr. Mesibov was kind enough to review my amateur analysis and has filled in some guesses I'd had to make with hard data. Please see his comment attached to this posting. As the primary point of my note here — the role of hyperbole in popular science writing — seems uncontested I am leaving the post as it was until I can better incorporate his well received corrections.


Someone asked about a story posted in the Science News Blog with the enticing title Mysterious Border Between Two Millipede Species Runs for Over 140 Miles. I've never been able to understand why headline writers need to spice up what is actually interesting science as if someone who is going to read about carrion eating millipedes really needs to be titillated into paying attention. A quick search of the Science News Blog article title shows that the story has already been picked up by multiple outlets.


The article summarizes a paper describing a methodology for gathering data. Although the observation by Dr. Mesibov is of the form “that's a slightly surprising result, I wonder why?” the Science News, like too many popular science accounts tries to sex up a rather mundane story (which is kinda funny for a class of animals that are most easily distinguished by microscopic examination of their penises).

Assuming Mesibov's data are correct (and Mesibov certainly knows his Tasmanian millipedes) it is just a notably  long measurement of the well known phenomenon of parapatric speciation.

As usually happens when a parent species splits into two, hybridization between the species is severely penalized by selection because the mixed genes don't work together as well as those of either species separately. A common example is the human guided hybridization of horses and donkeys to produce mules which are almost always sterile (where sterility is the ultimate penalty in natural selection). Humans have to cause the interbreeding because although horses and donkeys share a common ancestor and are so closely related that they can produce viable offpring they don't — of their own free will — interbreed. In the course of their speciation, selection retained preferences in each that made them want to mate with their own and not across species lines.

I expect Mesibov's preliminary discovery is a result of a few factors: The total range of T. compitale is small (125 hectares, or about the size of a US single family farm) so the ability to gather specimens is particularly good. Contrastingly, although a zone of parapatriation of 100m may sound narrow to us, to a 15mm millipede that's a long distance. 

There isn't a ton of data on home ranges of small earth dwellers because it's kinda difficult to catch-mark-release-repeat with bugs the size of a fingernail. Picking the beetle Abax parallelepipedus as a reasonable arthropod stand-in (biased by ecology to likely having a larger home range than tasmaniosomans) our friend the Black Ground Beetle never makes it much father than 10m from where it hatched over the course of its entire life. So, if you are a Tasmanian millipede with extremely goal directed ancestors, it could take ten generations or more to cross that 100m boundary zone. This multi-generational journey would be rather difficult because somewhere in the middle you're going to have one hell of a time finding a mate.

Is it interesting? Sure. Is it “mysterious”? No. “As yet unknown” is a more accurate description of the phenomenon that Dr. Mesibov reported.