the biggest problem with this is that you still have to have a way for life to evolve in the first place while not being intrinsically resistant to its environment. to answer this requirements, how about something like this: the planet and its star were originally well-suited for normal terrestrial-style life. in these conditions, a form of life that would be recognizable to us as such would evolve, without being naturally able to survive extreme conditions. but suddenly (!), some outside force (such as a rogue planet/star/singularity approaching the system, the star aging and expanding, etc) begins changing the conditions on the surface of the planet at an increasing rate over a relatively short time (millions of years). the increased pressures placed on the native species of the planet cause most to die out, as the normal limits of evolution make it nigh-impossible for normal life to change fast enough. but one or more species manage to evolve around even this: not only selected for adaptation to environment, but for adaptation itself. these species gain the ability to do in a few dozen generations what the rest of the universe does in a few hundred.
some possible mechanisms to actually achieve this:
1. like you already said, having a built-in selection of various traits and adaptations that can be emphasized or de-emphasized at will. this could manifest in two ways: either the individuals are able to selectively modify the traits present in their young well they are t=still being developed, or they are able to modify their own genetic code, allowing them to take different forms as their cells die and are replaced by new, better-adapted cells. a problem with this is that it still limits the general pool to traits which are specifically focused on survival on the single planet the species developed on.
2. to take the above even further, this organism could theoretically be able to alter its entire cellular structure at will, allowing it to change in response to any environment. in some ways, though, this is less likely and maybe even less useful than the first idea stated; to start, the creature would have to know what adaptions it needed to develop, which would also require it to be very good at analyzing and sensing its environment. additionally, this raises questions about how much of this control is conscious and how much is automatic; if it is entirely a conscious ability, how does the organisms deal with mental trauma? and if it is controlled by instinct or the subconscious, how does this limit the possible forms that the organism could take? and this ignores the questoin of what mechanism is used to transmit precise, widely varied signals to every single cell of the organism. may be too much trouble to justify and explain if you want a hard sci-fi story
3. finally and least possibly, say the organism exists partially on another dimension, allowing it to retreat to avoid damage. if this dimension is time rather than space, this also lets it anticipate when to withdraw and when to return to best maximize survival, along with any other time control powers you feel like throwing in there.
or just make them sentient, that has worked out pretty well for us so far! ;D