In the last paragraph of Chapter 6 of The Origin of Species, Darwin wrote: 'Evolution is descent with modification'.

Evolution is what happens over time when individuals with certain heritable traits have reproductive advantages over other individuals of the same species who lack those traits. When this happens, the average of the population with respect to those traits changes over time: it is said that the population evolves.

In other words, evolution simply boils down to the idea that when there is reproduction that involves heritable genetic variation, with enough time this has far-reaching consequences as it is an accumulative process.

Reproduction does not happen at random. In any given population, there will always be better adapted individuals that will leave more offspring than others. Individuals will be selected naturally. Those who survive do not do so by chance, they survive because they are better adapted to the environment. They have certain traits that give them reproductive advantage.

Heritable variation does occur randomly. In any given population, individuals are not all identical, there are always small differences between them. Whether a descendant inherits a certain trait depends on chance.

Additionally, the trait may undergo small modifications, called mutations, which also depend on chance.