Evolution is a change in the inherited characteristics of a population over a number of generations, through the process of natural selection, which may result in the formation of new species.
A species is a group of organisms that are capable of interbreeding together to form fertile offspring. If, through evolution, organisms become so different that they can no longer interbreed, then speciation has occurred. Speciation is more likely to occur if organisms become isolated from one another.
For example, a population of birds become isolated on an island with different food sources to the mainland. Through natural selection, the birds on the island will evolve to have different beaks in order to eat the food. Eventually, they will look very different to the population on the mainland and so will be a new species.
The idea behind the theory of evolution through the process of natural selection is that all species of living things have evolved from simple life forms over a period of time. The Earth is about 4.5 billion years old and there is scientific evidence to suggest that life on Earth began more than 3 billion years ago.
This slideshow shows key events in evolution, from the first bacteria to humans.
An evolution timeline