The term Eeveelution bugs me. Scratch that. The term Eeveelution makes my blood boil, the very utterance of its syllables causes every brain cell in my body to go into apoptosis. I already have issues with the fact that the in-game process known as Pokémon “evolution” is nothing of the sorts and is actually closer to metamorphosis in most cases. But this word that the Pokémon fandom has adopted for Eevee is even more misleading.
Eevee does not evolve.
In fact Eevee hardly changes at all.
At a very fundamental level, your dear Vaporeon is still an Eevee, granted, with some obvious differences. But these additions to your Pokémon’s physiology are not due to evolution, but to the power of phenotypic plasticity.
All life (as we know it) speaks the same language, the complex yet elegantly simple language of deoxyribonucleic acid, know more commonly as DNA. Its alphabet is composed of only four letters – adenine, guanine, cytosine, and thymine. Together these four nitrogenous bases order themselves in countless sequences to form the instructions for life. First, DNA is transcribed into messenger ribonucleic acid (mRNA). From there, the mRNA is delivered to a sort of protein factory called a ribosome where – to greatly oversimplify things – the mRNA is used to code for various proteins which are what do the real magic. Thus is the way life continues – from DNA to RNA to protein, the Central Dogma of Biology.
However, in some organisms there lay regions of “silent” DNA that are highly methylated, or in other words, are wound up too tightly to be transcribed, and thus remain quiet and unused. But, there are ways for this silent DNA, or heterochromatin, to unravel and be transcribed.
Without altering any of the nucleotides, an organism’s phenotype can change dramatically through epigenetics, the external modification of DNA. Once that silent heterochromatin is demethylated, the DNA can transcribed and the central dogma can continue, the resulting proteins being expressed as a new phenotype.
A great example can be found in bees. All bees essentially are born with the same genotype, with the vast majority destined to become workers as long as no external forces act on them. But a select few during the larval stage are fed a special food called royal jelly, which demethylates the silenced DNA in the bee larvae and allows transcription of the once silent region to begin. This particular gene sequence happens to be especially important, for it allows for the development of ovaries in place of the normal pollen sacs that a worker would grow. The larvae raised on royal jelly unsurprisingly grow to become fertile queens and allow for the continuation of the species.
A similar phenomenon appears to occur in Eevee. Regions of Eevee’s genome remain silent until otherwise activated upon the right external stimuli. For the Generation I species, the phenotype is dependent on exposure to a specific elemental stone – Water Stone yields a Vaporeon phenotype, Fire Stone Flareon, and Thunder Stone Jolteon. In Generation II such epigenetic change is dependent on two factors, friendship and the time of the day. Night gives you an Eevee with an Umbreon phenotype, and day produces an Espeon one. Leafeon and Glaceon work similarly to Gen I, with exposure to a specific rock being the determining factor. And lastly Sylveon, who requires great affection to achieve its phenotype, an act similar to how rat mothers lick their pups in order to activate the genes that boost their immune system.
At the end of the day, it’s still an Eevee, despite what those other fallacious Pokémon “professors” claim, rather, each variation of Eevee is simply displaying a different phenotype triggered by the demethylation and subsequent transcription of once silent regions of DNA. After all, the PokéDex constantly repeats how irregular and plastic Eevee’s genes are. Perhaps those irregularities are due to all the regions of silent heterochromatin lying hidden in Eevee’s genome.