Argentinian ants

Once upon a time (as the story so often goes), there was a small colony of ants.  These ants lived in a somewhat unusual place, at least as far as ants go.  It didn't seem that unusual at first glance, perhaps-- simply a little chunk of floodplain in Argentina where a few rivers converged.  But that flooding meant that the ants were regularly washed out of their nests.

One side effect of this regular displacement is that the weaker, slower ants were weeded out fairly rapidly.  The rest relatively quickly evolved a somewhat more aggressive approach to their neighboring colonies.  You see, normally, ants aren't completely xenophobic.  They'll capture enemy ants, add them to their colony, sometimes mate with them, etc.  This means that the genetic makeup of the colony will slowly drift over time, which is generally a good thing for the species.  However, these Argentinian ants, bred by the rising waters for exceptional fierceness, simply don't do that.  When they meet an enemy ant?  They kill it.  To death.

All of which would be a minor entomological curiosity, except that sometime, back in the 1800s, a few of these ants made their way onto a ship, and were carried up from Argentina to New Orleans, where they started a new colony.  And started killing.  And spreading.  And remember, no fraternizing with the enemy!  So (and here's where it starts getting really interesting) these newly spreading colonies maintained the same basic genetic makeup as the original.  If an Argentinian ant from the motherland meets one from Louisiana?  They recognize each other, and coexist.  Genetic purity FTW.

And now, since they're so very aggressive, they've spread over pretty much the entire globe.  There are Argentinian ants everywhere, all more or less part of the same supercolony.

Primary source: http://www.radiolab.org/blogs/radiolab-blog/2012/jul/30/ants/