Evolutionary arms race

The evolutionary arms race, first mentioned by Nine, is a euphemism for the evolutionary competition between two previously-cooperative species.

Beginnings
It began on a distant planet, eons ago. One species resembled an Earth frog, and one an Earth snake (except the snake had one eye). They originally cooperated to get food and survive, but eventually, the frog evolved to eat the snake while the snake evolved defensive traits against the frog, leading to a cycle in which both species would adapt to and counteract each other's traits in order to survive. The one-eyed snake species eventually evolved into the Dark Ones, and the Frog evolved into the Encyclopods or possibly the other way around.

End of the War
The Dark Ones preyed on all life, driving many species to extinction, while the Encyclopods defended all life and fought against the Dark Ones. Eventually, the life-giving Chi receded, endangering both the Dark Ones and Encyclopods. The Encyclopods were left closest to extinction, leaving a single Encyclopod egg in the form of a Violet Dwarf Star, while all but one Dark One were exterminated by Leo Wong. The last Dark One was saved by Turanga Leela in the year 3009 and attempted to bring about the destruction of the dwarf star and the last Encyclopod with it but, thanks to the intervention of Philip J. Fry and the Legion of Mad Fellows, it was defeated and killed by the newborn Encyclopod, finally bringing the war to an end.

Additional Info

 * The earlier evolutions that occurred include:
 * The frog evolving to eat the snake while it were standing on the frog to get to the fruit.
 * The snake inflating itself to not fit in the frog's mouth and to crush the frog.
 * The frog growing a sharp limb, which it used to deflate the inflated snake.
 * The snake gaining an armor-like hide which it could activate and deactivate at will, which was impenetrable to the frog's sharp limb. It also was able to turn itself inside out and devour the frog.
 * Strangely, the Arms Race of Evolution was originally two species evolving to get food, without any moral or ethnical implications. It is possible that as the snakes or the frogs evolved, they gained sentient minds, and were able to have opinions.