Natural selection is the evolutionary process which selects the variation(s) of organisms best suited for a particular environment. Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life, as well as for the molecular and structural similarities observed among the diverse species of living organisms.
The degree of kinship between organisms or species can be estimated from the similarity of their DNA sequences; this similarity often closely matches organisms' or species' classification based on anatomical similarities.
DNA Sequence Similarities in Some Primates The graphic above shows that:
1. All of these primates had a common ancestor, the ancestral primate.
2. The human and chimpanzee have the closest evolutionary relationship as their DNA is the most similar.

Sources of Variation
1. The exchanging and recombining of genes during meiosis and fertilization result in a great variety of new possible gene combinations from that of the parents.
2. Mutations are random changes in the genes or DNA of sex cells may result in new gene combinations creating variation in the offspring formed from these. Only mutations that occur in sex cells can be passed on to the offspring. Mutations which occur in other cells can be passed on to other body cells only. The experiences an organism has during its lifetime can affect its offspring only if the genes in its own sex cells are changed by the experience.