Natural selection is not a conscious process. Rather that artificial selection, when someone encourages specific traits they find favorable, and encourages their being passed on, nature selects against unfavorable characteristics (those out of synch with the environment). Too slow? Nature weeds out that trait pretty quickly. Likewise too stupid, or too clumsy, or too noisy. Unfavorable traits are selected against by nature as those individuals who possess such traits more rapidly fall to predation. Jim Z brings up another aspect of evolution: sexual selection. Survival is only half of the equation. You have to find a mate in order to get your genes into the next generation. Some species spend a lot of energy on this aspect of the genetic lottery (sexual fitness being a gauge of biological fitness).
Since evolution does not pick the fittest for survival, the is no guarantee that individuals who do carry advantageous traits will not die of some random event (like getting hit by a bus).