Basically it just comes down to this, if you able to survive under the current conditions you will live, if you're not, you'll die.
You don't have to be perfectly suited to your environment, you only have to survive even if it's only through luck.
Eventually though, a life form that's better suited to its environment has a greater chance of survival, just like its offspring, and the traits that make them different from the ones who have a smaller chance of survival will become more common within the population: then we say nature has "selected" that trait and thus the life forms that carry it.