Explain the salient features of darwin's theory of natural selection

The key to Darwin's theory of natural selection is genetic variation. Genetic variation can happen for a variety of reasons, but sexual reproduction maximizes genetic diversity. As the gene pool goes through changes, different features develop on individual organisms. Those features could be a thicker fur coat, a more acute sense of smell, etc. Those are adaptations, and adaptations are key for Darwin's theory. Beneficial adaptations result in an organism being better adapted to its environment. It is more fit for its environment. Organisms that have the greatest level of fitness are then more likely to live. Darwin called this the "survival of the fittest." A fit organism that is living longer has a greater chance of reproducing. Its genes, and consequently its adaptions, are likely to be passed on to its offspring. After enough time has passed, an entire population (or even species) might have that particular adaptation. Nature has essentially naturally selected which traits get passed on by allowing the fittest organisms to survive and reproduce.
https://www.britannica.com/science/natural-selection