How could animals evolve like they do in Pokémon?

In Pokemon, there are many different creatures. Nearly all of them will, at one point, reach a level of experience where they evolve into a more advanced creature. These evolutions are much more drastic than what human evolution is like. One minute an animal could be an average sized turtle, and the next they are huge and have water cannons on their backs (Wartortle to Blastoise).

How could an evolution like this be possible? Is there any way that science could explain this?

I am more hoping for something that explains a bit less drastic of an evolution than Wartortle to Blastoise, hopefully something more like a small bird turning into a big bird really fast (Pidgey to Pidgeotto) or a bug growing wings (Venonat to Venomoth).

• – grimmsdottir Apr 19 '15 at 14:33
• @grimmsdottir Well, obviously. Pokemon evolution is a lot cooler ;) – michaelpri Apr 19 '15 at 14:39
• They should be called mutamon after mutant and monster. – user6760 Apr 19 '15 at 15:05
• For real evolution in the Pokémon series, I recommend watching this video: youtube.com/watch?v=JF2mRcAoXLQ – Arturo Torres Sánchez Jul 11 '15 at 4:39

Pokémon "evolution" exists and is called metamorphosis

Real world animals do this. However, it's not called evolution because it is not in any way evolution; it's metamorphosis, and rather than "experience", it is food (or sometimes just time) that must be gathered to begin the process.

The caterpillar $\to$ butterfly is pretty much exactly like pokemon evolution (which is probably why this real life metamorphosis inspired Caterpie $\to$ Metapod $\to$ Butterfree).

More examples would be

Tadpole $\to$ Frog
Maggot $\to$ Fly

There are huge numbers of examples of metamorphosis, although caterpillar $\to$ butterfly is one of the most extreme. Generally, they include 4 stages: egg, larva, pupae, adult.

Why is this not evolution

Evolution is change in heritable traits of biological populations over successive generations resulting from a selection pressure in their environment (where "environment" can take a very large definition).

Heritable?

Baby Pokémon are the lowest stage of Pokémon evolution. The concept was introduced in Generation II, along with breeding

http://bulbapedia.bulbagarden.net/wiki/Baby_Pok%C3%A9mon

So we can see that Pokémon evolution is not heritable, baby Pokémon are always born at "the lowest stage of Pokémon evolution".

Selection pressure?

Equally Pokémon evolution always follows a set "plan" (of one of a small number of plans) so the "design" itself cannot be driven by a selection pressure in their environment.

Successive generations?

Finally Pokémon evolution occurs for a single individual so is not over "successive generations"

• Another excellent example is anything with a polyp stage, such as coral or anemones. – Emmett R. Apr 19 '15 at 21:45
• There are also lesser examples of metamorphosis like the winter coat of many mammals and its potentially changing color. Also any time the juvenile is significantly different from the adult (other than size). – ratchet freak Apr 20 '15 at 8:14
• "Press B to cancel evolution" – SaturnsEye Apr 20 '15 at 9:08
• "Equally Pokémon evolution always follows a set "plan" so the change itself cannot be driven by a selection pressure in their environment." - what about something like Eevee, whose entire gimmick is evolving into different things based on what it's been exposed to or how it's been raised? – Pyritie Apr 20 '15 at 9:37
• @Pyritie bee larvae only become queens if fed a certain food. Crocodile gender is decided by the ambient temperature of the eggs. – ratchet freak Apr 20 '15 at 23:42