Today’s password is coevolution.
First of all, here is a pair of phylogenies. The one on the left shows the relationships of a bunch of dove species, and the one on the right shows a bunch of lice that parasitize the doves. The lines connecting doves and lice indicate lice that parasitize each dove species..
Notice that parts of the tree mirror each other. We see that closely related dove species are infected by closely related lice. But that tree is nowhere as clean as this one:
On the left is an aphid phylogeny, and the tree on the right is from their obligate symbionts (speaking of aphids, I do plan on blogging about this). That’s some tight concordance. As the aphid species diverge, their symbionts diverge in parallel. That’s coevolution.
Wade MJ. 2007. The co-evolutionary genetics of ecological communities. Nat Rev Genet 8: 185-195. doi: 10.1038/nrg2031