Gene duplication is a process where a DNA region coding for a gene duplicates, making additional copies of itself within the same genome. These duplicated copies of the gene - called paralogs can later mutate and diverge in one of the following ways.

The first is formation of the pseudogenes. Here, one of the gene paralogs may acquire deleterious mutations and turn into a nonfunctional copy called a pseudogene.

The second is sub-functionalization where both the paralogs acquire mutations in different protein coding domains or exons, thus partitioning the original gene function between them. However, the protein products of the two paralogous genes complement each other and exhibit the original gene function.

For example, in primitive fish and marine animals, a single chain globin protein served as the oxygen carrying molecule in the blood.

During the course of evolution, the globin gene duplicated and sub functionalized into two slightly different genes coding for α- and β-globin proteins, that associate to form the hemoglobin molecule with 4 subunits found in most present day vertebrates.

The third is Neo-functionalization. Here, one paralog acquires novel, advantageous mutations that can lead to the evolution of a new gene. In contrast, the other paralog retains the original function.

For example, the human β-globin gene duplicated and acquired mutations to produce a new gene called fetal β-globin that is expressed exclusively in the human fetus. However, soon after birth, the β-globin gene takes over production of the β-globin proteins.

The evolution of tricolor vision in humans is another interesting example of neofunctionalization. Much before the evolution of modern apes, the early primates had dichromatic vision due to the presence of the Blue and Green opsin genes.

Later on, the Green opsin gene duplicated and neo-functionalized into a novel red opsin gene.

Therefore, the species which evolved after the duplication event, such as the old world monkeys, apes, and humans have three opsin genes which impart tricolor vision.