Because Darwin.
From Science Nordic:
Both groups breed in the same area in the Caribbean, the Sargasso Sea, but typically you would expect a long-term physical separation of the two, to allow them to develop in different directions. This makes them a great mystery of evolution.
Now scientists have solved this puzzle. It turns out that the eels became two species 3.5 million years ago, when Panama arose from the sea, separating the Atlantic and Pacific Oceans. One group of eels were led to Europe by the Gulf Stream, where a new species emerged
…
The eel’s life story is difficult to understand from an evolutionary perspective.
When they say “an evolutionary perspective,” they mean a Darwinian perspective, of course.
Take an eel in a Danish stream — when it is time to mate, they do not choose the easy option and find an eel in the same river to breed with.
Instead, they embark on a perilous 5,000 kilometre journey to spawn in the Sargasso Sea.
The same goes for eels from all across Europe, and in this way they comprise a single population of European eels that all go to breed on the other side of the Atlantic.
After the breeding season is over, the new eel larvae float all the way back to Europe, following the Gulf Stream and other ocean currents. A few years later, they settle in coastal waters, rivers, and lakes. They can be found in Iceland and Norway in the north, all the way down to Morocco in the south, and this epic life cycle begins all over again.
But the Sargasso Sea is not the reserve of European eels — American eels also breed there, and the two breeding areas overlap. Sometimes the two species also breed with each other, which has been difficult to explain.
Unless they aren’t really two separate species, just two populations.
The genomes were virtually the same, except for a couple of differences in a few very specific parts — those affecting growth and metabolism. More.
But remember, Darwin’s followers need separate species the way a crack addict needs his fix.
Having listened to this kind of thing for many years now, I suggest ignoring it all unless you have an exam to write. And then just front the house line for a pass.
Alchemy was probably a lot like this. One of these days, we’ll have real information.
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Hey, eels climb rock wall to get back home: