Summary

Concept 15.1: Evolution Is Both Factual and the Basis of Broader Theory

• Evolution is genetic change in populations over time. Evolution can be observed directly in living populations as well as in the fossil record of life.
• Evolutionary theory refers to our understanding and application of the processes of evolutionary change.
• Charles Darwin in best known for his ideas on the common ancestry of divergent species and on natural selection as a process of evolution. Review ANIMATED TUTORIAL 15.1 and ACTIVITY 15.1
• Since Darwin’s time, many biologists have contributed to the development of evolutionary theory, and rapid progress in our understanding continues today. Review Figure 15.2

Concept 15.2: Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution

• Mutation produces new genetic variants (alleles).
• Within populations, natural selection acts to increase the frequency of beneficial alleles and decrease the frequency of deleterious alleles.
• Adaptation refers both to a trait that evolves through natural selection and to the process that produces such traits.
• Migration or mating of individuals between populations results in gene flow.
• Genetic drift—the random loss of individuals and the alleles they possess—may produce large changes in allele frequencies from one generation to the next and greatly reduce genetic variation.
• Population bottlenecks occur when only a few individuals survive a random event, resulting in a drastic shift in allele frequencies within the population and the loss of variation. Similarly, a population established by a small number of individuals colonizing a new region may lose variation via a founder effect. Review Figure 15.8
• Nonrandom mating may result in changes in genotype frequencies in a population.
• Sexual selection results from differential mating success of individuals based on their phenotype. Review Figure 15.10

Concept 15.3: Evolution Can Be Measured by Changes in Allele Frequencies

• Allele frequencies measure the amount of genetic variation in a population. Genotype frequencies show how a population’s genetic variation is distributed among its members. Together, allele and genotype frequencies describe a population’s genetic structure. Review Figure 15.11 and ANIMATED TUTORIAL 15.2
• Hardy–Weinberg equilibrium predicts genotype frequencies from allele frequencies in the absence of evolution. Deviation from these frequencies indicates that evolutionary processes are at work. Review Figure 15.12 and ANIMATED TUTORIAL 15.3

Concept 15.4: Selection Can Be Stabilizing, Directional, or Disruptive

• Qualitative traits differ by discrete qualities (e.g., black versus white) and often are determined by alleles of a single gene.
• Quantitative traits differ along a continuum (e.g., small to large size), and usually are influenced by variation at multiple genes.
• Natural selection can act on characters with quantitative variation in three different ways. Review Figure 15.13
• Stabilizing selection acts to reduce variation without changing the mean value of a trait. When applied to selection that maintains a particular genetic variant in a population, stabilizing selection is called purifying selection. Review Figure 15.14
• Directional selection acts to shift the mean value of a trait toward one extreme. When applied to selection for change at a single genetic locus, directional selection is called positive selection. Review Figure 15.15
• Disruptive selection favors both extremes of trait values, resulting in a bimodal character distribution. Review Figure 15.16

Concept 15.5: Genomes Reveal Both Neutral and Selective Processes of Evolution

• Nonsynonymous substitutions of nucleotides result in amino acid replacements in proteins, but synonymous substitutions do not. Review Figure 15.17
• Rates of synonymous substitution are typically higher than rates of nonsynonymous substitution in protein-coding genes (a result of stabilizing selection). Review Figure 15.18
• Much of the change in nucleotide sequences over time is a result of neutral evolution. 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 substitution.
• Specific codons within a given gene sequence can be under different modes of selection. Review Figure 15.20
• The total size of genomes varies much more widely across multicellular organisms than does the number of functional genes. Review Figures 15.21 and 15.22
• Even though many noncoding regions of the genome may not have direct functions, these regions can affect the phenotype of an organism by influencing gene expression.
• Functionless pseudogenes can serve as the raw material for the evolution of new genes.

Concept 15.6: Recombination, Lateral Gene Transfer, and Gene Duplication Can Result in New Features

• Despite its short-term disadvantages, sexual reproduction generates countless genotype combinations that increase genetic variation in populations.
• In the absence of genetic recombination (as in some asexual organisms), deleterious mutations accumulate with each replication—a phenomenon known as Muller’s ratchet.
• Lateral gene transfer can result in the rapid acquisition of new functions from distantly related species.
• Gene duplications can result in increased production of the gene’s product, in divergence of the duplicated genes’ expression, in pseudogenes, or in new gene functions. Several rounds of gene duplication can give rise to multiple genes with related functions, known as a gene family. Review Figure 15.23 and ACTIVITY 15.2

Concept 15.7: Evolutionary Theory Has Practical Applications

• 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.
• Agricultural applications of evolution include the development of new crop plants and domesticated animals, as well as a reduction in the rate of evolution of pesticide resistance.
• In vitro evolution is used to produce synthetic molecules with particular desired functions. Review Figure 15.24
• Many diseases are identified, studied, and combated through molecular evolutionary investigations.