On SR388, the X Parasite threatened to destroy the ecosystem, due to the high rate of reproduction. However, the Chozo stepped in to save the planet, and created Metroids. "Metroid" is the Chozo word for "ultimate warrior." The Metroids kept the X Parasite numbers in check, and became the dominant predators of SR388. (Metroid Fusion)
The natural evolution of a metroid is:
But, sometimes under certain circunstances, the metroid evolve in a different way:
Tallon IV Metroid
The metroid from tallon IV, due to phazon exposure, evolves into a HUNTER METROID, instead of an Alpha Metroid, a form never seen before. The Tallon metroid exposed into RED PHAZON, evolves into a Fision Metroid.
This Tallon Metroid variant has the hability to reproduce using mitosis. This Metroid variant can only be found on the impact crater.
Metroid Prime´s origin is unknown. It could have evolved from a metroid larva exposed to Phazon, this, based on the space pirate log wich mention that Metroid Prime was a small form and now it is growing bigger. Metroid Prime has an exoskeleton. When fighting with Samus Aran, his exoskeleton was heavily damaged, and then his ESSENCE form appeared. While in essence form, Metroid Prime is able to create Tallon Metroids and his variants, make wave attacks, and become invisible to normal sight. Metroid prime in esscence form is invulnerable to all weaponry, only a Phazon based attack can damage him. Samus using Phazon pools created by him, activated her Phazon Beam and killed Prime. Metroid Prime in his last breath, took Samus´s Phazon Suit and evolved into Samus´s nemesis: Dark Samus
Using the phazon suit that he took from Samus, Metroid Prime evolved into Dark Samus, a phazon based clone of the original Samus containing her genetical material. Samus first encounter with her evil twin has on planet Aether, which was recently damaged by a phazon meteorite, similar to the one on Tallon IV. The metorite created a Dark version of Aether. Somehow, in Dark Aether, a lifeform known as The Ing was born, debasting Aether.
The Ing have parastistic properties, and infected all potencial fighting creatures, but not forever, when the host is killed, the Ing escapes. The Metroid didn´t like Ing, and attack them on sight, but, some ings have managed to infect few metroids, making them even more powerful.