Charles Darwin proposed that species change over long periods of time by natural selection. His theory starts with variation. Individuals of the same species are not identical, because they have different alleles of their genes. Some new alleles are produced by mutations, which are random changes in DNA.
Organisms produce more offspring than the environment can support, so there is a struggle for survival. Individuals whose characteristics suit the environment better are more likely to find food, avoid predators and resist disease. They survive and breed. This is often called survival of the fittest.
The survivors pass on the alleles for their useful characteristics to their offspring. Over many generations the proportion of the population with those alleles increases. When the population has changed so much that it can no longer breed with the original population to produce fertile offspring, a new species has formed. This is how evolution happens.
