Describe the process of evolution by natural selection

Think of two animals of the same species. One of them has a gene that contains mutation X. Mutation X helps this animal to be better adapted to survive as it makes them stronger, and more likely to find food. As the other animal does not have this mutation, it is less strong and is not able to get enough food, and dies. This is a notion called 'survival of the fittest'. The survivor gets to reproduce and is able to pass on its genes, containing the mutation. This means the offspring will now have this quality and can continue the cycle of passing it on when they reproduce.