If an organism dies, it does not reproduce. And since traits are passed along in the genetics of each organism to its offspring, survival traits tend to survive themselves. This is called natural selection. It is more complicated than that, however, because environmental conditions change. Several years of drought will change a population of a species into one that can better get through a drought.
Evolution is a conservative process.
Genes for variations don't always disappear. They get passed down as recessive traits that can reappear in successive generations. Also, mutations occur regularly in each generation, and that can change the picture. Then there are genetic drift and the effect of recombination, which merges the genetic make-up of the two parents into the offspring. Overall, however, populations tend not to change unless outside environmental forces induce them to.