Gene families arise from gene duplication and evolutionary divergence.

Small duplications play an important role in the origin of new genes in the course of evolution. In most cases, when a gene is duplicated, one of the copies is free to change without causing harm to the organism because the other copy continues to carry out the normal function of the gene. Occasionally, a mutation in the “extra” copy of the gene may result in a beneficial effect on survival or reproduction, and gradually a new gene is formed from the duplicate. These new genes usually have a function similar to that of the original gene.

This process of creating new genes from duplicates of old ones is known as duplication and divergence. The term divergence refers to the slow accumulation of differences between duplicate copies of a gene that occurs on an evolutionary time scale. Multiple rounds of duplication and divergence can give rise to a group of genes with related functions known as a gene family. The largest gene family in the human genome has about 400 genes and encodes for proteins that detect odors. These proteins are structurally very similar, but differ in the region that binds small odor molecules. It is the diversity of the odorant binding sites that allows us to identify so many different smells.

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Most gene families are not as large and diverse as that for odor detection. Fig. 14.13 shows the evolutionary origin of the family of globin genes, which in humans are spread across about 50 kb of chromosome 11. The globin gene family consists of five different genes that are expressed at various times during development (embryo, fetus, or adult). The two γ-(gamma-)globin polypeptides are nearly identical in amino acid sequence but expressed at different levels in the fetus, whereas the adult δ-(delta) and β-polypeptides differ somewhat more and are expressed at very different levels. The sequences are all similar enough to imply that the genes arose through duplication and divergence.

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FIG. 14.13 Origin of the β-globin gene family. The β-globin gene family evolved through several episodes of duplication and divergence. Data from: Y. A. Trusov and P. H. Dear, 1996, “A Molecular Clock Based on the Expansion of Gene Families,” Nucleic Acids Research 24(6):995–999.
Data from: Y. A. Trusov and P. H. Dear, 1996, “A Molecular Clock Based on the Expansion of Gene Families,” Nucleic Acids Research 24(6):995–999.

Globin sequences have changed through evolutionary time at a relatively constant rate, and so the number of differences between any two sequences is proportional to the time since they were created by duplication. The relative constancy of rates of evolutionary change in a DNA nucleotide sequence or a protein amino acid sequence is known as a molecular clock, and it allows molecular differences among genomes to be correlated with the fossil record and dated accordingly (Chapter 21). For example, the earliest duplication event in the tree in Fig. 14.13, which produced distinct genes for fetal and adult hemoglobin, took place at about the same time (200 million years ago) that the common ancestor of today’s placental mammals became a distinct species from the common ancestor of today’s marsupial mammals.