Guppies at ambient temperature?

Hi,
fishes "in the wild" usually don't live in an temperature-stable
environment, oposite an aquarium where you can set a temperature and
then it's keept forever.

Thinking about this, i was wondering what would be the survival chances
of a guppy in an acuarium with no heater? I know these are warm water
fishes, but would it be very dangerous for it? is it a very bad idea?

Thanks.