An aspect of Pokemon that is present no matter what kind of media is being looked at, no matter how old the media is, is that Pokemon have the ability to evolve. This is done through the Pokemon battling other Pokemon and  gaining more experience so that they can get stronger by leveling up to the point where they can eventually evolve and undergo a sudden change in size and shape that increases their strength and gives them more powerful abilities.

Getting one thing out of the way, Pokemon "evolution" is not really "evolution" as it is defined in the dictionary. Evolution, based off of Darwin's findings with Galapagos Island finches, is the process by which species develop and adapt over several thousands of years in order to meet the requirements of the survival of their species. The evolution that happens in Pokemon, on the other hand, more closely resembles metamorphosis. Metamorphosis involves an animal, usually an insect or amphibian, completely changing forms through several stages of transformation spanning anywhere from several weeks to several months. The key here is that metamorphosis takes a fairly long time to happen where the animal will eat and store up energy and resources in order to elicit the transformation. Pokemon, however, evolve in mere seconds. 

One of the biggest difference between metamorphosis and Pokemon evolution is that metamorphosis takes a long time to happen. This means that according to the Conservation of Mass (mentioned on the previous page) all the mass gained during evolution must come from somewhere and it comes from this somewhere very quickly. For example, the Pokemon Mudbray weights 242.5 lbs and upon evolving into Mudsdale, it weights 2028.3 lbs, which is an increase of 1785.8 lbs! In order to obey the Conservation of Mass, this sudden immense change in mass must come from the Pokemon's surroundings. This means that whenever a Pokemon evolves, it would have to absorb all of the elements and materials to make up for the sudden change in mass in the span of a couple of seconds.

Image Credit: 
1. https://secure.polyvoreimg.com/cgi/img-set/cid/91561821/id/gG-NYzgpQb6LqyqYX1tKww/size/y.jpg
2. http://www.pngall.com/wp-content/uploads/2016/05/Caterpillar-PNG-File.png
3. http://img10.deviantart.net/1a3c/i/2012/047/9/4/monarch_butterfly_cocoon_png_by_doloresdevelde-d4pxkp4.png
4. https://img.clipartfox.com/edf1c8ec45c709e7132b535f710cc732_download-png-balloon-image-butterfly-outline-clipart-transparent-background_1100-955.png
5. http://cdn.bulbagarden.net/upload/thumb/5/5d/010Caterpie.png/250px-010Caterpie.png
6. http://cdn.bulbagarden.net/upload/thumb/c/cd/011Metapod.png/250px-011Metapod.png
7. http://cdn.bulbagarden.net/upload/thumb/d/d1/012Butterfree.png/250px-012Butterfree.png
8. http://cdn.bulbagarden.net/upload/1/12/749Mudbray.png
9. http://cdn.bulbagarden.net/upload/f/f7/750Mudsdale.png