291Generating Genetic Variation Each of these forms of genetic variation—from the simple mutations that occur within a gene to the more extensive duplications, deletions, rear-rangements, and additions that occur within a genome—has played an important part in the evolution of modern organisms. And they still play that part today, as organisms continue to evolve. In this section, we dis-cuss these basic mechanisms of genetic change, and we consider their consequences for genome evolution. But first, we pause to consider the contribution of sex—the mechanism that many organisms use to pass genetic information on to future generations.In Sexually Reproducing Organisms, Only Changes to the Germ Line Are Passed On To ProgenyFor bacteria and unicellular organisms that reproduce mainly asexually, the inheritance of genetic information is fairly straightforward. Each indi-vidual duplicates its genome and donates one copy to each daughter cell when the individual divides in two. The family tree of such unicellular organisms is simply a branching diagram of cell divisions that directly links each individual to its progeny and to its ancestors. Figure 9–2 Genes and genomes can be altered by several different mechanisms. Small mutations, duplications, deletions, rearrangements, and even the infusion of fresh genetic material all contribute to genome evolution. Although the mobile genetic element here is shown interrupting a gene regulatory sequence, the movement of these parasitic elements can promote a variety of genetic variations, including gene duplication, exon shuffling, and other regulatory and structural alterations.QUESTION 9–1In this chapter, it is argued that genetic variability is beneficial for a species because it enhances that species’ ability to adapt to changing conditions. Why, then, do you think that cells go to such great lengths to ensure the fidelity of DNA replication?geneMUTATION WITHIN A GENEORIGINAL GENOME ALTERED GENOMEGENEDUPLICATIONEXONSHUFFLINGHORIZONTALTRANSFER+ ++ +gene Agene Bexonorganism Borganism B with newgene from organism Aorganism AECB4 e9.02/9.02mutationMUTATION INREGULATORY DNAgenegenemRNAregulatoryDNAmutationTRANSPOSITIONgeneregulatoryDNAmutationmobile genetic elementintronsCommon% mech anisms%by%which%diversity%is%gen erated291Generating Genetic Variation Each of these forms of genetic variation—from the simple mutations that occur within a gene to the more extensive duplications, deletions, rear-rangements, and additions that occur within a genome—has played an important part in the evolution of modern organisms. And they still play that part today, as organisms continue to evolve. In this section, we dis-cuss these basic mechanisms of genetic change, and we consider their consequences for genome evolution. But first, we pause to consider the contribution of sex—the mechanism that many organisms use to pass genetic information on to future generations.In Sexually Reproducing Organisms, Only Changes to the Germ Line Are Passed On To ProgenyFor bacteria and unicellular organisms that reproduce mainly asexually, the inheritance of genetic information is fairly straightforward. Each indi-vidual duplicates its genome and donates one copy to each daughter cell when the individual divides in two. The family tree of such unicellular organisms is simply a branching diagram of cell divisions that directly links each individual to its progeny and to its ancestors. Figure 9–2 Genes and genomes can be altered by several different mechanisms. Small mutations, duplications, deletions, rearrangements, and even the infusion of fresh genetic material all contribute to genome evolution. Although the mobile genetic element here is shown interrupting a gene regulatory sequence, the movement of these parasitic elements can promote a variety of genetic variations, including gene duplication, exon shuffling, and other regulatory and structural alterations.QUESTION 9–1In this chapter, it is argued that genetic variability is beneficial for a species because it enhances that species’ ability to adapt to changing conditions. Why, then, do you think that cells go to such great lengths to ensure the fidelity of DNA replication?geneMUTATION WITHIN A GENEORIGINAL GENOME ALTERED GENOMEGENEDUPLICATIONEXONSHUFFLINGHORIZONTALTRANSFER+ ++ +gene Agene Bexonorganism Borganism B with newgene from organism Aorganism AECB4 e9.02/9.02mutationMUTATION INREGULATORY DNAgenegenemRNAregulatoryDNAmutationTRANSPOSITIONgeneregulatoryDNAmutationmobile genetic elementintronsCommon% mech anisms%by%which%diversity%is%gen erated296 CHAPTER 9 How Genes and Genomes EvolveMany gene duplications are believed to be generated by homologous recombination. As discussed in Chapter 6, homologous recombination provides an important mechanism for mending a broken double helix; it allows an intact chromosome to be used as a template to repair a dam-aged sequence on its homolog. Homologous recombination normally takes place only after two long stretches of nearly identical DNA become paired, so that the information in the intact piece of DNA can be used to “restore” the sequence in the broken DNA. On rare occasions, how-ever, a recombination event can occur between a pair of shorter DNA sequences—identical or very similar—that fall on either side of a gene. If these short sequences are not aligned properly during recombination, a lopsided exchange of genetic information can occur. Such unequal cross-overs can generate one chromosome that has an extra copy of the gene and another with no copy (Figure 9–8). Once a gene has been dupli-cated in this way, subsequent unequal crossovers can readily add extra copies to the duplicated set by the same mechanism. As a result, entire sets of closely related genes, arranged in series, are commonly found in genomes.The Evolution of the Globin Gene Family Shows How Gene Duplication and Divergence Can Produce New Proteins The evolutionary history of the globin gene family provides a striking example of how gene duplication and divergence has generated new pro-teins. The unmistakable similarities in amino acid sequence and structure among the present-day globin proteins indicate that all the globin genes must derive from a single ancestral gene.The simplest globin protein has a polypeptide chain of about 150 amino acids, which is found in many marine