Last night the BBC taught me a new technical word. Way back in the 30's a lot of work was done catching elvers (very young eels) all over the Atlantic, and making a plot of size of elver at each location. This led to a number of concentric circles with common center in the Sargasso Sea.
The interpretation was that all Atlantic eels were spawned in the Sargasso Sea. And it was assumed that all eels in Europe,the Mediterranean, and North America were of one breed. Now DNA testing has shown that the breeding is not "panmictic" meaning random. There are DNA differences for each of the three groups. What was not explained is how each of the three groups know where their proper destination is.Two other technical words not mentioned: fish like salmon that go upstream to spawn are called anadromous. The eels on the other hand spend most of their life in fresh water, and go downstream to find spawning grounds and so are called catadromous.
I do think I will have a long wait before I get a chance to use "panmictic" in a conversation