What is natural selection?

A process by which organisms which are most suited to the environment that they live in are more likely to survive and therefore to reproduce, passing the characteristics which made them most suitable on to the next generation.

Individual organisms in a species are not all the same. There is variation within the population, for example a species of butterflies may have different coloured wings, brown and blue. If one of the patterns means that the predator of the butterflies finds it harder to see them then more butterflies with this pattern will reach an age where they can reproduce. This means that this pattern will be more likely to be passed on to the next generation.