Evolution is a change in the inherited characteristics of a population over time, through a process of natural selection, which may result in the formation of a new species. The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
Natural selection works on the variation in a population. Mutations produce new variants, and some variants give phenotypes that are best suited to their environment. Individuals with these phenotypes are more likely to survive, breed and pass on their genes to their offspring. Over many generations the helpful characteristic becomes more common in the population. For example, in a cold climate, rabbits with thicker fur are more likely to survive and breed than rabbits with thin fur.
If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.
