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Recombination, Lateral Gene Transfer, and Gene Duplication Can Result in New Features 15.6 Sexual Recombination Amplifies the Number of Genotypes Several evolutionary processes can result in the acquisition of major new characteristics in populations, as seen in previous sections of chapter 15. Each one of these processes results in a larger and more rapid evolutionary changes than do a single point mutation. Sexual reproduction results in new gene combinations and produces genetic variety that increases evolutionary potential. Sexual Recombination Amplifies the Number of Genotypes Asexual reproducing organisms are genetically identical unless there is a mutation but when organisms reproduce sexually, offspring differ from their parents because of crossing over, independent assortment, and fusion of different gametes. Sexual recombination produces an endless variety of genotypes which increases the evolutionary potential of populations This would be considered a long-term advantage of sex In the short term, sexual reproduction has disadvantages: • Recombination can break up adaptive combinations of genes • Reduced rate at which females pass genes to offspring • Dividing offspring into genders reduces the overall reproductive rate Sexual Recombination Amplifies the Number of Genotypes Exactly how these process of genetic recombination came about is puzzling, in the short term, sexual reproduction has three distinct disadvantages: • Recombination can break up adaptive combinations of genes • Reduced rate at which females pass genes to offspring • Dividing offspring into genders reduces the overall reproductive rate Sexual Recombination Amplifies the Number of Genotypes To see why separating into different genders reduces overall reproductive rate consider • A asexual mother who has two offspring • A sexual mother has two offspring •Assume both females produce two offspring, but that half of the sexual females are males •The asexual offspring will produce two females •In the next generation, there are two females for asexual and only one in the sexual offspring •Thus, effective reproductive rate in the asexual lineage is twice that of the sexual lineage Sexual Recombination Amplifies the Number of Genotypes So then why did sexual reproduction evolve? Possible advantages: • Facilitates repair of damaged DNA— damage on one chromosome can be repaired by copying intact sequences on the other chromosome • Elimination of deleterious mutations through recombination followed by selection • The variety of genetic combinations in each generation can be advantageous (e.g., as defense against pathogens and parasites) Sexual recombination does not directly influence the frequencies of alleles, it generates new combinations of alleles on which natural selection can act. Sexual Recombination Amplifies the Number of Genotypes • In asexually reproducing species, deleterious mutations can accumulate; only death of the lineage can eliminate them ◦ Muller called this the genetic ratchet—mutations accumulate or “ratchet up” at each replication; known as Muller’s ratchet. Lateral Gene Transfer Can Result In the Gain of New Functions We have see the tree of life branching as new adaptations and specialization occurs within individual organisms, however there are processes that can result in lateral gene transfer Lateral gene transfer—individual genes, organelles, or genome fragments move horizontally from one lineage to another • Species may pick up DNA fragments directly from the environment. • Genes may be transferred to a new host in a viral genome. • Hybridization results in the transfer of many genes. Lateral Gene Transfer Can Result In the Gain of New Functions Lateral gene transfer can be advantageous; it increases genetic variation. ◦ Most common in bacteria; genes that confer antibiotic resistance are often transferred among species ◦ Relatively uncommon in eukaryotes, but hybridization in plants leads to gene exchange The endosymbiosis events that gave rise to mitochondria and chloroplasts were lateral gene transfers. Lateral Gene Transfer Explained Lateral Gene Transfer: Rethinking Evolution Bozeman – Mechanisms that Increase Genetic Variation Many New Functions Arise Following Gene Duplication Gene duplication is another way genomes can gain new functions When a gene is duplicated, one copy of that gene is potentially freed from having to perform its original function The identical copies have one of four different fates ◦ Both copies retain original function, which can increase the amount of gene product. ◦ Gene expression may diverge in different tissues or at different times in development. ◦ One copy may accumulate deleterious mutations and become a functionless pseudogene. ◦ One copy retains original function, the other changes and evolves a new function. Many New Functions Arise Following Gene Duplication Sometimes entire genomes may be duplicated, providing massive opportunities for new functions to evolve. In vertebrate evolution, genomes of the jawed vertebrates have four diploid sets of many genes. Two genome-wide duplication events occurred in the ancestor of these species. This allowed specialization of individual vertebrate genes. These duplications allowed for considerable specialization of individual vertebrate genes, which are now highly specific Transfer, and Gene Duplication Can Result in New Features Successive rounds of duplication and sequence evolution may result in a gene family, a group of homologous genes with related functions. The globin gene family probably arose via gene duplications. A duplication event led to the α and β gene clusters Numbers indicate the estimated number of DNA sequence changes along that branch of a tree Evolutionary Theory Has Practical Applications 15.7 Knowledge of gene evolution is used to study protein function Evolutionary theory has many practical applications across biology, and new ones are being developed every day A few applications can be applied to fields such as agriculture, industry and medicine. Molecular evolutionary principles can be used to understand protein structure and function. Knowledge of gene evolution is used to study protein function Puffer fish produce a toxin (TTX) that blocks Na+ channels and prevents nerve and muscle function. People that eat the toxin of puffer fish can become paralyzed and die because the toxin will block their nerves and muscles from functioning properly. But Na+ channels in the puffer fish itself are not blocked by the toxin. Nucleotide substitutions in puffer fish genes result in changes in the channel proteins that prevent TTX from binding. TTX Video TTX Video #2 Knowledge of gene evolution is used to study protein function Mutations in human Na+ channel genes cause several neurological diseases. Study of these gene substitutions aids in understanding how Na+ channels function. Biologists compare rates of synonymous and nonsynonymous substitutions across Na+ channel genes in various animals that have evolved TTX resistance. In vitro evolution produces new molecules Living organisms produce many compounds useful to humans. The search for such compounds is called “bioprospecting.” These molecules result from millions of years of evolution. But biologists can imagine molecules that have not yet evolved. In vitro evolution—new molecules are produced in the laboratory to perform novel functions 2. Select the RNA molecules with the highest ligase activity 1. Start with a random pool of RNA sequences 5. Transcribe back into RNA, and then repeat the cycle for 10 rounds 3. Reverse transcribe the RNA into DNA 4. Use Polymerase Chain Reaction (PCR) amplification to introduce new mutations into the DNA population 6. After several rounds, an effective ribozyme has evolved from the pool of random RNA sequences Evolutionary theory provides multiple benefits to agriculture In agriculture, breeding programs have benefited from evolutionary principles, including incorporation of beneficial genes from wild species. An understanding of how pest species evolve resistance to pesticides has resulted in more effective pesticide application and rotation schemes. Resistance video Knowledge of molecular evolution is used to combat diseases Molecular evolution is also used to study disease organisms. All new viral diseases have been identified by evolutionary comparison of their genomes with those of known viruses. Studies of the origins, timing of emergence, and global diversity of human pathogens (including HIV & SARS) depend on evolutionary principles and methods, as do efforts to develop effective vaccines. Once biologists have collected genome data for enough infectious organisms, it will be possible to identify an infection by sequencing a portion of the pathogens genome and comparing this sequence with other sequences on an evolutionary tree.