1. All organisms have the ability to produce more offspring than possibly can survive. For instance, a female fish or frog lays million of eggs.
2. The environment contains things that kill organisms. The environment is often hostile: hot or cold, dry or flooded, predators are common, and the resources needed to survive and reproduce (food, water, living space, light) are often in short supply. Darwin describes this as 'struggle for existence'.
3. Individuals vary or differ in their traits. They may differ in size, coloration, running speed, resistance to disease, and many other traits.
4. Some inherited traits give individuals an advantage in coping with environmental challenges allowing them to survive longer and produce more offspring.
5. Individuals with advantageous traits have more offspring. Each new generation contains a greater proportion of the offspring with these advantageous traits than did the previous generation. Since these traits are inherited, offspring of individuals with the advantageous traits become more numerous. Individuals without the trait become less numerous. Gradually, over generations, the population contains more and more individuals with the advantageous trait. This process is evolution by natural selection.