…huh. Y’know, I never noticed that before. That’s… odd. I think the reason the writers say this about Seadra in particular is because they want to hint at the gradual awakening of Horsea’s Dragon-type abilities as it evolves (note that this line first shows up in the second generation, when Kingdra was introduced). That doesn’t mean it makes sense, of course, but I think that’s what they’re trying to get at. They also note that this discovery quickly became “a hot topic” so they’re clearly aware that what they’re describing is an odd thing. Animals… shouldn’t gain extra genes partway through their life cycle. Bacteria can do it; a lot of bacteria have specialised enzymes that allow them to splice bits of DNA from other bacteria into their own, but animals can’t really do that because they have billions or trillions of copies of their DNA spread out over their cells. If an animal undergoes metamorphosis, all the genes that do everything the adult form needs should be there from birth; they just require particular stimuli to switch them on. Now that could be what this really means – that scientists identified a gene that wasn’t being expressed in Horsea but was in Seadra – but that doesn’t seem like it would be worthy of comment. So is it possible that something actually adds a whole bunch of extra base pairs to one of Horsea’s chromosomes when it evolves…? Hell if I know. If I had to speculate, I’d guess that there’s some symbiotic bacterium-like organism, possibly related to Pokérus (call it a midichlorian if you like), that goes through the body subtly altering the DNA of cells it encounters, and when the number of altered cells reaches a certain tipping point, the process dramatically accelerates and evolution happens. This is a total guess, based on real-world phenomena I happen to be vaguely familiar with, but if I were a Pokémon Professor I’d start with a hypothesis along those lines.