As an acorn needs to be planted in the dark underground to grow into a mighty oak tree, so must the soul unfold out of the darkness of the malas to full maturity and realization of its innate oneness with God. The soul is not created at the moment of conception of a physical body. Rather, it is created in the Sivaloka. Then it experiences many lives, maturing through the reincarnation process. Thus, from birth to birth, souls learn and mature.
Evolution is the result of experience and the lessons derived from it.
There are young souls just beginning to evolve, and old souls nearing the end of their earthly sojourn. In Saiva Siddhanta, evolution is understood as the removal of fetters which comes as a natural unfoldment, realization and expression of one's true, self-effulgent nature. Self Realization leads to moksha, liberation from the three malas and the reincarnation cycles. Then evolution continues in the celestial worlds until the soul finally merges fully and indistinguishably into Supreme God Siva, the Primal Soul, Parameshvara. The evolution of the soul is not a linear progression, but an intricate, circular, many-faceted mystery.