[Smokin' Joe]

While northern and southern lineages could breed successfully, they apparently had diverged enough during their million-year separation that offspring of southern females and northern males failed to develop beyond the tadpole stage. Though crosses involving northern females and southern males successfully produced frogs, the offspring developed more slowly than the offspring of pairs of northern frogs.

Do the southern frogs (or the northern ones for that matter) have some disease or parasite which could cause this effect? Something like what caused all the three-legged frogs a few years back?

[staterightsfirst]

No disease and no parasite required - over time, the genomes just diverge enough that combining the two doesn't work anymore.

They think this has to do with mitochondrial DNA, which is passed on exclusively by the mother (hence why northern female + southern male = OK but slow and southern female + northern male = no development).

Mitochondrial DNA is complimentary to the regular DNA - so say the original ancestor needs genes A and B to survive, mito. DNA produces A, and regular DNA supplies B.

Then, the southern frog mito DNA mutates slowly such that A becomes gene C - but that's okay, because B in regular DNA changes along with it to D such that they work even better than before - so frogs with DC were more successful than AB. Over time, DC becomes more and more common, because they work better.

However, northern still use AB - so when a southern female and a nothern male mate, the southern female contributes mitochondrial gene C, and the northern male contributes gene B - but B and C don't work together at all anymore and no offspring even develops!

Hope this helps...