Something that I was just wondering about, is there a way for a Pokemon that evolves through trading to evolve another way? Or was this method of evolution only discovered after humans started catching and trading certain pokemon species? Like has there ever been any cases of a Pokemon to appear in the wild that wasn’t abandoned but is the final stage of say the Gasty line? And how does trading a Pokemon even cause it to evolve? What’s the science behind it?

Certainly, anonymous! In fact, trade-based evolution is only the easiest and most recently discovered method for specific species, and it’s frequently the most humane, all of which is why it’s often recommended by both the pokédex and modern-day researchers. So yes, to answer one of your questions, trading as a method has only existed as long as the trade machine has.

However, all of the species that evolve via trading are fully capable of evolving on their own. It’s just that their evolution requirement without trading would be age. There are, for example, wild gengar and steelix in Sinnoh, but both of these are frequently decades if not centuries old. What this means specifically depends on the species. For example, in steelix’s case, the reason why it takes so long is because onix need time to devour enough minerals to form the metal coat needed for evolution, as well as enough time to allow pressure and heat from the earth itself to compress said metal coat into a form of armor. Haunter, meanwhile, evolve by age because they need time to master the shadow abilities their evolved forms are most famous for. In some cases, such as scyther and kadabra, it’s rare for a specimen to survive long enough to achieve evolution, which is why some species of trade-based pokémon are extremely rare to nonexistent in the wild (as their evolved forms, anyway). Hence why trading is considered a more humane path to take according to researchers. Overall, though, how long it takes entirely depends on the species. As I’ve mentioned just now, onix and haunter may take centuries to evolve, but pokémon with shorter lifespans may take only decades.

That being said, trade evolution is not a well-understood subject, and for the most part, its discovery in the first place was purely accidental. My personal theory is that there are two factors involved. First, when transferred (stored within a poké ball, uploaded into the storage system, sent via trade), your pokémon are actually converted into data via a very complicated process. This data packet not only contains everything about the pokémon itself but also information regarding its trainer, where the pokémon was caught, and so forth. Due to the structure of this data, it’s impossible for any ordinary human to edit it, but the trading machine was designed to do this automatically. Think of it like an automated metadata editor, basically.

However, in certain cases, the code for a pokémon’s current trainer may be linked to other lines of code within the data packet that make up the pokémon—specifically its evolutionary switch. So, when the current trainer information is modified, that switch will be flipped from “off” (as in, this pokémon is not ready to evolve) to “on” (this pokémon is ready to evolve). That, combined with the second factor—in this case, the fact that the added electricity from the trading machine itself could provide the energy needed for evolution—may result in the pokémon being set to evolve upon taking a physical form.

Of course, there is one other factor involved in all of this, and that’s the held item. Everstone “overwrites” evolutionary data in all cases and will thus force that switch to stay in the “off” position. Likewise, some pokémon’s evolutionary switch—in particular, pokémon such as onix, scyther, poliwhirl, and all others that require a held item to evolve—will not be fully complete unless the added data from the evolutionary item is present.

In short, a lot of it has to do with data or age, and evolution in general is an extremely complex but highly fascinating process.