Two concepts are that there is already variation in the population and those variations allow some individuals better chances to survive and reproduce. The variation in a population comes from mutations to the DNA, independent assortment of chromosomes during meiosis, crossing over during meiosis, and fertilization. This change in DNA gives organisms different genetic codes which show up as different color, height, body shape, etc. If these changes produce traits that make the organism better able to survive and find a mate—better camouflaged color, faster, able to eat a new food, then that organism will find a mate and produce offspring that may have that new trait. Over time, the offspring with that new trait will compete with other organisms and also survive better and reproduce so the trait will be more prevalent in the population.
