Eels are like that.
They converge on the Sargasso Sea from all over Europe and North Africa - but rather than mingle, they tend to breed with other eels from their home region, researchers have discovered 1 . And this idea was supported by data from proteins and DNA that showed no strong differences between populations from different areas. The length of microsatellites varies very widely between individuals, as the cell's machinery often loses count when copying and aligning such monotonous sequence. This makes them ideal for genetic fingerprinting. Microsatellites from 611 eels migrating out of rivers that flow into the Mediterranean, Atlantic, Baltic and North Sea pinpointed where the eel had come from.