Just random changes that somehow (accidently ?) lead to improved (new) species .... or death.
According to evolutionary theory, the "species" CANNOT change itself, it just randomly mutates into something better.
There is no "Mother Nature" allowed, no design, allowed, nothing but random mutations. No species can change based on any amount of "want to" or "need to" change.
Actually, "need to" sounds a lot like "selection pressure" to me.