not all things are an adaptation to evolutionary pressures
sometimes there are just single nucleotide polymorphisms that change things.
Yeah, if it hinders it goes, if it benefits it prevails, but sometimes things are just neutral
A second spleen is probably just a developmental defect and not genetic in any way.
Also, that’s not how evolution works. “Evolutionary pressure” doesn’t cause spontaneous gene changes to appear. Genes naturally drift and mutate, and then selection pressures might eventually favor one set of genes over another.
Well it functions in both blood filtration and for blood/immune cell storage, both of which are critical for staying alive and reproduction. But when it comes to evolution, things aren’t always straight forward. Maybe there was a entirely different function which had a bigger positive impact on overall fitness and since the spleen situation didn’t decrease fitness, it too was past along.
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