Answer:
It occurs over many generations.
It applies to an entire population.
It happens at the genetic level.
Step-by-step explanation:
Evolution is how the species change over many generation by passing along the genes that allow the species to be more adapted to the environment in which it is inserted, making it easier to procreate and survive. That's why we call it "the survival of the fittest". It isn't about strength, but about how adapted to the surroundings in order to keep living and procreating a species is. This change happens over the course of many generations since the more adapted genes are passed along to the offspring of the members of that species that have been surviving.