Chapter Summary
23.1 DNA Sequences Record the History of Gene Evolution
A genome is an organism’s full set of genes, regulatory sequences, and structural elements as well as noncoding DNA.
The field of molecular evolution concerns relationships between the structures of genes and proteins and the functions of organisms.
Sequence alignments of proteins or nucleic acids from different organisms allow us to compare the sequences and identify homologous positions. Review Figure 23.1, Activity 23.1
The minimum number of changes between sequences can be calculated from a similarity matrix. Models of sequence evolution can be used to account for changes that cannot be observed directly. Review Figure 23.2, Activity 23.2
23.2 Genomes Reveal Both Neutral and Selective Processes of Evolution
Nonsynonymous substitutions of nucleotides result in changes to the amino acid sequences of proteins, but synonymous substitutions do not. Review Figure 23.4
The neutral theory of molecular evolution states that much of the molecular change in nucleotide sequences does not change genome function. The rate of fixation of neutral mutations is independent of population size and is equal to the mutation rate.
Positive selection for change in a protein-
coding gene may be detected by a higher rate of nonsynonymous than synonymous substitutions. The reverse is true of purifying selection. Common selective constraints can lead to convergent evolution of amino acid sequences in distantly related species. Review Figure 23.6
The total size of genomes varies much more widely across multicellular species than does the number of functional genes. Review Figures 23.7, 23.8
23.3 Lateral Gene Transfer and Gene Duplication Can Produce Major Changes
Lateral gene transfer can result in the rapid acquisition of new functions from distantly related species.
Gene duplications can result in increased production of a gene’s product, in nonfunctional pseudogenes, or in new gene functions. Several rounds of gene duplication can give rise to multiple genes with related functions, collectively known as a gene family. Review Figures 23.9, 23.10
Gene trees describe the evolutionary history of particular genes or gene families. See Activity 23.3
Some highly repeated genes undergo concerted evolution, in which the multiple copies within the genome maintain their similarity, even as the genes diverge among species. Review Figure 23.11, Animation 23.1
23.4 Molecular Evolution Has Many Practical Applications
Orthologs are genes that are related through speciation events, whereas paralogs are genes that are related through gene duplication events. Review Figure 23.12
Protein function can be studied by examining gene evolution. Detection of positive selection can be used to identify molecular changes that have resulted in functional changes.
In vitro evolution is used to produce synthetic molecules with particular desired functions. Review Figure 23.13
Many diseases are identified, studied, and combated through molecular evolutionary investigations. Review Investigating Life: Why Was the 1918–1919 Influenza Pandemic So Severe?
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