Pokemon are ever-changing creatures shaped by their environments and
experiences, prompting sudden physical changes and the loss or gain of
abilities as they grow. All Pokemon retain this ability, even if there
is no documentation of their species having drastic physiological form
changes. As such, they are able to gain racial hit die through
experience as if they were class levels. For every 4 hit die they gain,
they either choose a new ability from the pool of evolution adaptations
either found below, or more commonly from the pool of evolutions unique
to their individual species. These unique lists can be located on the
page detailing these individual species. Additionally, they are able to
gain gain evolution points (EP) like an unchained eidolon. The total of
points in this pool depends on the total number of hit die the pokemon
has gained, and in place of an evolution adaptation, can be spent on
spells which can be cast like spell-like abilities.
These updates should reflect the adaptations the Pokemon has gone through, and represent the physical adaptations that the pokemon has undergone during evolution. Once an evolution adaptation has been chosen or an evolution point has been spent, this choice cannot be changed. Should a listed trait have the same wording as a trait attributed to class, like a sorcerer bloodline, cleric domain, wizard school, shaman spirit, so that it's clear that the two are one and the same, then that trait's bonuses stack with that granted by the class. Custom abilities can be created at the GM's digression.