In Pokemon, each individual Pokemon “evolves” into a new stage of its evolution. A lot people claim that this can be explained using mere metamorphosis, but there are no mammals that go through such complex metamorphosis.
Using Earth-like biology how can I explain how so many animal kinds (from canines and felines to giant snakes and land sharks) can undergo massive and limb growing metamorphosis? Why would evolution favor a creature changing its form so radically?
Biological Baggage (emphasis on the logical)
Some Pokemon, even the mammalian seeming ones, shoot beams of intense cold out of their mouths (Dewgong), lightning out of their cheeks (Pikachu), and have actual burning flames as a part of their bodies (Ponyta). Others appear to be composed of non biological matter (Muk, Porygon).
Given all the laws of biology and physics they seem to break, a little thing like changing form, which real animals already do, hardly seems worth quibbling about.
But let's assume the evolving creatures in your world do not have these unusual abilities, they are just normal animals that undergo vast physical changes.
Metamorphosis
As Fayth85 mentioned, insects undergo fairly advanced changes as they go from larva to pupa to adult.
She /he also mentioned birds. Look at this picture of the Canada Goose. It is not just a size difference. The chicks have a much shorter proportional neck and different color and texture of feathers.
Not mentioned however are frogs. The change from a purely aquatic tadpole, to a legged, amphibious animal is fair drastic.
But none of those are mammals. However Humans are. During gestation, before certain hormones take effect, all fetuses appear physically female. They are not, technically the still forming reproductive organs are neither. But in terms of body shape they are, and males undergo a moderate change. Then again, when Humans enter puberty, they go another set of physical changes; growing additional hair, change in voice for male, breasts for females. Which bring us to topic two.
Sexual Dimorphism
Even within a given species, the male and the female can have dramatic differences. Male lions are larger, and have a prominent mane that female lack. The male platypus is also generally larger, and also has poisonous spurs on the feet that females lack. Peacocks have huge tail feathers females lack.
There are dozens of other examples one can easily find, but the point here is that difference between individuals in a species need not be limited to age.
The Answer
You want a biological way for your creatures to undergo a sudden metamorphosis. Like an egg hatching or a butterfly leaving the cocoon. In which case I point you towards reptiles.
When a reptile outgrows its skin, the outer layer is shed, revealing a new layer, which is generally much more vibrant, and in some species even a different color (some change colors as they age or based on environment or diet).
Your not-Pokemon could do a similar thing, especially if the change was not dramatic (Pikachu to Raichu rather than Teddiursa to Ursaring). The creature changes on the inside at a slower rate, but once it is ready, casts off its outer layer of skin/fur to reveal the new, 'evolved' form.
Or you could just not go with the 'instant' change, and then you wouldn't even need an additional explanation.
They are artificial creations. Like our current tech, they are designed using modular components, and they accept updates in the field. My Blu-Ray player or dishwasher downloads new firmware; Industrial enginines by IH can have features like total horsepower unlocked or configured by software while in-place.
So consider the “upgrade” to be designed as something like an in-app purchase from the original maker.
With the loss of original maker and the (actual) evolution of the forms as they became feral, the code is a bit mixed up, like with the Dragon’s Teeth of Mirabile by master world builder Janet Kagen. Rather than buy an unlock code, the needed triggers are found by side-effects or evolved access to useful survival traits by the feral form. You might read Kagen's stories to bet a feel for what I have in mind about the DNA.
Being modular, distinct traits like limbs can be recessive or switched off or damaged in a feral remixed offspring. Many features are simple configuration options using common code. Although inspired by legacy bioforms that were unrelated and incompatible, the engineered lifeforms used inter-compatible standardized traits and developmental configuration programming.
Now in ourselves such a change like extra toes would only be effective in the germ line, as development occurs once. But these animals were designed for having upgrades applied. Re-development — metamorphosis — was designed in so changes could be applied to existing phenotypes.
Well, the only real-life comparisons I can offer are in insects. But this is similar to a post:
If you want to go purely with evolution (no "magic did it"), you might want to consider things like longer lifespans, or simply larger beings that go through phases during their growth. There aren't any non-insect examples I can think of, but a halfway example are birds.
http://www.pigeoncontrolresourcecentre.org/html/about-pigeons.html
Because they live 3-5 years on average, it isn't too weird that their metamorphosis goes relatively quickly, but if you take a much larger animal that needs a similar metamorphosis? It could be argued that said changes would take considerably longer.
As for multi staged changes (as in the two higher 'evolutionary' stages of certain Pokemon)? Well, I would argue that because it takes so long, they need to hunt to survive, and therefore evolution favoured a 'multi-stage metamorphosis', to raise survivability of the species.
As I stated in my answer about cancer, DNA is a form of code. This means that certain conditions could trigger some "code".
For instance, when you go from one page to two page mode in Microsoft word is that a "sudden evolution"? Probably not but the analogy holds.
Simply put, the DNA has sections of code that are essentially massive while loops in sequential order. When the conditions for the loop are no longer held, it continues to the next set of code. The cells change what they do in a drastic metamorphic manner. This could be instantaneous or whatever.
As for how they came to be?
Humans made them. I said in this my poke ball explanation. Granted, these humans do it in secret, and therefore, said pokemon are thought to be a product of evolution.
Also, as charizard came to mind I would like to point out a flaw in my concept:
Birds (and therefore probably dragons) get 60% of their lung capacity from within theor wings. This means that when charizards cells begin to migrate during the sudden cellular behavioral change, even 10% of those cells failing to relocate properly could cause the dragon to be deformed, and possible die. It is incredibly likely that pokemon in this universe jave a high likelihood of death and are therefore an "evolutionary abberation" to most experts.
So, a charizard can exist, and it can potentially grow and evolve properly, but 75% of them will likely die. Perhaps this is the in-universe explanation as to why wild charizards do not exist in the games?
Pokemon evolution is not quite analogous to insect metamorphosis. The main advantage of insect metamorphosis is that the children and the adults have a separate food source, preventing competition between generations. However, most Pokemon are similar in shape to their evolved forms and would appear to eat the same foods. Furthermore, evolution is not an essential part of their life cycle; most Pokemon are capable of reproduction without evolving, and evolution is not triggered by age, but by winning fights.
I would therefore guess that Pokemon evolution evolved through kin or group selection; it exists to benefit the species or group more than the individual. Most Pokemon are social (which would explain why they are so easily trained) and their forms reflect their social status; the individuals that are best at fighting evolve and become the leaders and protectors of the group. Evolved Pokemon are larger and need more food, so it benefits the group to have only a small number of "alphas", reducing intra-group competition.
There are some real-life animals that do similar things: Dominant male mandrills have that distinctive blue and red coloration on their face and backsides, dominant female hyenas are larger and develop masculine characteristics, and dominant male clownfish and groupers turn into females. All of these species are highly aggressive and their transformations are thought to be linked to their win/loss ratios in fights.
Evolution as it is depicted in the games and show, where a Pokemon suddenly increases in size and mass, is impossible due to conservation of mass. My guess is that this is an exaggeration of the process; evolution is a change of form accompanied by a rapid growth spurt, but it isn't THAT fast.
I remember in middle school, my biology teacher said that evolution can never happen in one generation, but instead happens over many generations. If I remember correctly it is part of the definition of the word "evolution". However, I have read an novel that I think answers your question as believable as possible.
In the novel, the author plays on the concept that we humans only use 10% of our brain/potential (this concept has already been proven wrong, but still simple enough to have a similar concept to play around with). Similar to TheGreatDuck's answer, the author uses DNA code to layout the human potential of being 5 stages, with most humans living in the first stage their whole life. The story gives an example of a news story of an old lady's lifting up a car in order to save her grandson in the spur of the moment (example he made up in the novel), but the old lady dies shortly after. The rule here is that when humans face near death experience, or in the old lady's case, to protect loved ones, we temporarily unlock our next genetic stage, but will have to endure immense pain shortly after. If one cannot withstand the pain and give in, they die. But if they sustain the pain, they retain their next stage forever.
So there's the author's version of "evolution", where humans "evolve" when facing near death experiences. If that's what you were looking for.
I am unable to provide the novel because it is an online Chinese novel, and is not translated.
