Charles Darwin explained how species change over time. He noticed that individuals of the same species are not identical. They show variation, mainly because they have different genes and so different alleles.
Organisms produce more offspring than the environment can support. The offspring compete for food, space and mates, and many die. Individuals whose characteristics suit the environment best are more likely to survive and reproduce. This is called natural selection, or survival of the fittest.
The survivors pass on the alleles for their useful characteristics to their offspring. Over many generations the proportion of individuals with these characteristics increases, so the species gradually changes. If the changes build up enough, a new species can form. For example, in a cold winter rabbits with thicker fur survive better, so more of the next generation have thick fur.
