A/N: This is meant to be an intellectual look at the science of Pokémon evolution. This idea was burning in my mind, so I had to write it. Just imagine this as a lecture in a Pokémon college or an excerpt from a text book.
Pokémon evolution
It is a strange world we live in, the Pokémon world. We humans over the ages have lived side by side with these fantastic beasts. And now, with the help of improved technology and generations of research, most Pokémon evolutions can be explained.
Firstly in Pokémon evolution, the subject that comes up is mainly, "how?". Science can now show that Pokémon evolution in a result of age and fighting experience. Humans, as they mature, create hormones and other chemicals to stimulate growth. A new born human is the fastest known case of this, gaining pounds from mere days of nutrition, sleep and exercise. A human grows slowly and constantly until old age, where bones and muscles shrink and loose vitality.
In a Pokémon's case, they have enormous growth spurts (even faster than a human child), changing their entire form in a flash of white light. A classic example of Pokémon evolution is when a rare starter Pokémon such as Torchic grows to level 16 (though we are still unsure of the way a Pokémon gains experience and grows consecutive levels). The Torchic then glows white and it transforms into a Combusken. This is a drastic change in body structure and is impossible for any human body to copy it. For a Pokémon, a Torchic may represent a child, and a Combusken the equivalent to an adolescent. It seems inconceivable that the body would be able to mature that fast and without any whiplashes or recoils. It seems that Pokémon bodies were able to withstand such a rapid change from the beginning of their existence.
Also of interest of the Torchic growing into a Combusken, there is a noticeable change in body and bone structure. X-rays have shown that a Torchic had no prior bone structure that looks like it would become arms of a Combusken, and yet all Combuskens have three digit arms. This quick appearance of new bones seemingly appearing from nowhere is still a mystery that scientists are working on to this day. Also of notice is the transition of Mudkip, Snivy, Tepig, and Oshawott into their higher stages. What is curious is the change from being a quadruped to a biped; a Mudkip grows into a Marshtomp and suddenly gains the ability to walk on its hind legs, as is the case with Tepig. Snivy loses its legs and arms entirely, and the bones disappear in Serperior's x-rays. Oshawott falls the opposite way of Mudkip, falling back to four feet instead of staying bipedal. But this is not the most curious of Pokémon evolutions.
Along with the appearance and disappearance of bone structures in the quick evolution transition, there are far stranger topics of Pokémon growth. They are; old un-evolved Pokémon, Special evolutions, Legendary Pokémon, and Non-evolving Pokémon.
Old un-evolved Pokémon
There have been many experiments done by Pokémon researchers and scientists to discover the truth about Pokémon evolution. One of which focuses on older Pokémon that can evolve into their older forms but haven't. Rattatas were the main subject in this.
In several runs, Rattatas were raised to their appropriate age where most would grow into Raticates. Four total groups were raised equally, but there were three selected groups where the Rattatas were raised, but forbidden to evolve. One group (Group A) was all given a strange item called an Everstone, which suppresses the chemicals and hormones in the Pokémon from triggering the quick growth spurt. Another group (Group B) of the three was constantly startled into shock. This seemed to work, although the Rattatas seemed more apprehensive and jumpy after multiple loud sounds were emitted when one began to glow white. The third group (Group C) of control group Rattatas were pampered and never given battle experience. The other group (Group D) was allowed to evolve normally.
Group D all evolved into normal, healthy Raticates. After a time frame of a month, a third of the other three groups were allowed to become Raticates. Raticates from Group A grew into very strong Pokémon. Even though the Raticates from group D were allowed normal evolution, the control Group A, when removed of their Everstones, grew into more impressive Raticates than Group D. The third of Raticates from Group B were equivalent to Group D specimen, but more wary of loud noises that had disturbed them from evolution. Group C yielded lazy Raticates.
Another test allowed another month to pass before another third of Groups A-C were allowed evolution. Compared to Group D, the Pokémon that showed the most promise was Group A. even though Group A was the fittest, they lacked the move-sets and power a Raticate from Group D learned through life.
There have been similar studies showing that evolved Pokémon at their first opportunity is the best, but other research has countered it, showing that evolving Pokémon later is a better idea. Ideas are still at an impasse to promote a conclusion.
In these fields, it is entirely possible to have an elderly Caterpie or any other kind of 'first stage' Pokémon. Whether it is good for the Pokémon or not, it is still to be proven. It is even possible that Pokémon themselves can control their evolution to a point. Several cases have shown Pokémon to refuse to grow, even when their experience and age are ripe.
Contrariwise, it is possible for younger Pokémon to evolve before their age and experience is high enough. Most cases in early evolution is a high amount of fear, stress or adrenaline that tricks the Pokémon's body into thinking the time for the growth spurt is close at hand. In some cases, it is desperation that drives a Pokémon to grow, to please its trainer or win a match with an extra boost of power.
Special evolutions
It this perspective of Pokémon evolution, there are many different cases of special evolution. 'Special Evolution' refers to a different way of a Pokémon to grow into its next stage with or without the aid of battling, age, and experience. There are Stone evolutions, Friendship evolutions, Statistical Evolutions, Locational evolutions, Appearance Evolution, Item Evolution, Move Evolution, Trade Evolution, and Dual Pokémon Evolution. Each subject will be touched on briefly.
Stone evolutions
There are 9 varieties of evolution stones in where a Pokémon need only to touch the stone to trigger the growth into the next stage of life. They range from Thunderstone to Waterstone, to Moon stone to Shiny stone, and all in between. It is said that the stone contains certain chemicals needed to trigger evolution that a Pokémon needing a stone does not have. It is speculated that Vulpixes and Pikachus of the past did not need to rely on Firestones and Thunderstones respectively, but these species nowadays have lost the needed chemicals and reactants that a Firestone possesses. It is even researched on how these stones got such needed chemicals in the first place.
Friendship Evolutions
Many of the fairy Pokémon have an evolution that is caused by pure emotion; adoration and affection to its trainer or even another Pokémon. The spectrum of Pokémon that require friendship is split between baby Pokémon (i.e., Pichu, Cleffa, etc.) and final evolution Pokémon (i.e., Crobat, Blissey, etc.).
This evolution is hard to predict when it will happen and can vary according to Pokémon personality and trainer treatment. Many things that help in speeding up the Pokémon's affection is keeping the said Pokémon close, feeding them special treats, and giving them soothing items, such as the Soothe Bell. Many people have devoted their life to seeing the signs of Pokémon and their level of affection toward their trainers. It is predicted that the Pokémon that can evolve by love require a high level of serotonin to trigger evolution.
Statistical Evolutions
The only known case of this special evolution is Tyrouge and its evolution into any of Hitmontop, Hitmonchan, or Hitmonlee. If the Tyrouge's attack is higher than its defense, it becomes a kicking Pokémon, and if those stats are reversed, it grows into the punching Pokémon. If the attack and defense stats are the same, it becomes a Hitmontop. Not enough research has been devoted to this phenomena to go into detail on this occurrence.
Location Evolutions
There are many cases of evolution that can only occur in a specified area. A female Burmy with a cloak of leaves will evolve into a plant cloak Wormadam unless the cloak is changed before evolution. Once the Burmy has evolved, its cloak cannot be exchanged. A Burmy's cloak will be changed accoding to where it last fought.
Leafeon, Glaceon, Magnezone, Probopass, are only brought forth when the lower stage of the Pokémon is brought to a certain place. The theory on how this happens is relatively the same as the stone evolutions, as there is a chemical in the ground that a Pokémon needs to change its form.
Appearance Evolution
This only applies to Feebas, a rare, ugly fish. It can evolve into a snake-like Milotic in the Sinnoh and Hoenn regions by increasing its beauty with Pokeblocks or Poffins. It is guessed that increasing Feebas' beauty makes it believe itself to be beautiful and change to embody its idea.
An alternative method was discovered in Unova to give a Feebas a prism scale and trade it.
Item Evolution
Item evolution itself is a short topic, relating only to Weavile and Gliscor, but when paired with Trade Evolution and breeding is quite extensive. Sneasel and Gligar need a Razor Claw and Razor Fang respectively and need to hold these items to evolve. Held items that affect Pokémon evolution during trade include Metal Coat, King's rock, Dragon Scale, and others. This Item Evolution is different from Stone Evolution because Item Evolution requires time spent with the item, instead of instant contact.
Items held while Pokémon breeding can also affect what the baby Pokémon is. A Wobbuffet is born from a female Wobbuffet holding no items, but a Wynaut is born when the female Wobbuffet is holding a Lax Incense item. This was a strange occurrence when it was first discovered, but has been eagerly accepted currently.
Move evolution
Evolution by different Pokémon learning a move is one of the strangest phenomena that science was stumbled upon yet. The fact that an Aipom can grow a second tail due to learning a previously un-learnable move is completely beyond imagination. Research is being committed to studying how a previously un-evolving Pokémon can evolve with a move 'Double Hit'.
Trade evolution
Trade Evolution has been around for a long time, though not fully understood. Some Pokémon like Kababra and Machoke evolve when in the company of a new and foreign trainer. It is the thought that certain chemicals are released into a Pokémon that makes it try to feel able to fit in with a new trainer, and can evolve with time with the new trainer. It is even discovered the traded Pokémon have gained the ability to gain more experience in the company of a different trainer than its original.
In the case of Unova, an Escavalier and Accelgor are only evolved when a Karrablast is traded with a Shelmet. It is thought that Karrablast somehow steals the Shelmet's shell during the trade and becomes a heavily armored Escavalier, and Shelmet becomes a shell-less Accelgor.
Dual Pokémon Evolution
Dual Pokémon evolution has been classified as a classic evolution. Even so, the modern evolution of Mantyke into Mantine requires an additional Pokémon to be present (Remoraid).
This is classic evolution because it affects many of the original Pokémon. Originally, Slowpoke was only thought to evolve when a Shelder bit onto its tail and transformed the two Pokémon into a Slowbro. A Magneton was formed by three Magnemite and a Metang was formed by two Beldum, and a Metagross by two Metang. However, these examples of dual Pokémon evolution can be overlooked. A Slowpoke becomes a Slowbro at level 37, and it has been shown that no Shelder is needed for a Slowpoke to evolve. But just because it isn't necessary doesn't mean it's the best. A Slowbro that had a real Shelder clamp on its tail was shown by stats and tests that it was superior to a Slowbro that had no Shelder encounter. The fact is that a real Shelder instead of a hollow shell (hollow shells apprea on Slowbros that evolve through levels alone) actually aids in fighting with a Slowbro, and acts as two Pokémon would in a battle. The same for Metagross, and Magneton. A Metagross is actually two Metangs or four Beldums joined tighter, making it a super intellectual Pokémon with four brains.
Out of all these types of Special Evolutions, there is still much to be discovered about our strange, powerful Pokémon friends.
Legendary Pokémon
On the subject of legendary Pokémon, there is a strange conception. If Pokémon grow into different appearances and types as they grow, does that mean Legendary Pokémon and non-evolving Pokémon are inferior or pygmy?
Legendary Pokémon have never evolved into a different form. It is slightly in their essence to be separate from other Pokémon and not to change form by means of evolution. It is thought that Legendary Pokémon are already powerful enough that they do not need the stat-enhancing effects of evolution.
Each legendary Pokémon is said to represent or protect something, and are very keen on staying away from humans. This fact had stopped many teams of researchers from learning more about these powerful beings.
Non-evolving Pokémon
There is a division of Pokémon that do not evolve; at least not documented yet of late (Sneasel was thought to not be able to evolve until the Razor Claw was discovered). Of late, here is the list of Pokémon that do not evolve (excluding Legendary Pokémon): Farfetch'd, Kangaskhan, Pinsir, Heracross, Tauros, Lapras, Ditto, Aerodactyl, Unown, Girafarig, Dunsparce, Quilfish, Shuckle, Corsola, Skarmory, Smeargle, Miltank, Sableye, Mawile, Minun, Plusle, Volbeat, Illumise, Spinda, Zangoose, Seviper, Lunatone, Solrock, Castform, Keckleon, Tropius, Absol, Relacanth, Luvdisc, Pachirisu, Chatot, Spiritomb, Carnivine, Rotom, Audino, Throh, Sawk, Basculin, Maractus, Sigilyph, Emolga, Alomomola, Cryogonal, Stunfisk, Druddigon, Bouffalant, Heatmor, and Durrant.
Again as it was for legendary Pokémon, questions arose to see if these 53 species were unable to evolve and are considered weaker or pygmy versions of past Pokémon that could evolve but lost the genetics to do so. Some Pokémon are weaker than the average first staged Pokémon (i.e., Smeargle), but some are quite formidable in their strength (i.e., Druddigon), so the theories are divided.
The ability to grow abnormally fast is a trait shared by many Pokémon, but some species grow fine without it. The species that do not have defined stages of child, adolescent, and adult can still be at that stage in live.
Non-evolving Pokémon are a bit more like humans in the growing aspect, seeing as they grow in size slowly instead of the evolution jump.
As more generations come, we might find more ways to evolve these Pokémon, or we may never know.