Evolution is about how living things have over millions of years. Darwin proposed that organisms with the best to their survive and have offspring which inherit those adaptations.
Useful characteristics become more common. Less well adapted organisms die out.
All over-produce, so individual organisms have to compete, particularly for food. Disease and predation cause large numbers of organisms to die. This is called the struggle for , and leads to the survival of the . In other words, those individuals with the most suitable are the most likely to survive.
So, selects the characteristics that are going to aid organisms. This is called selection. These changes are the mechanism by which evolution occurs.