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Mader Inquiry into Life, Thirteenth Edition Chapter Outline CHAPTER 27: EVOLUTION OF LIFE 27.1 Origin of Life The common ancestor for all living things was the first cell or cells. The very first living thing had to have come from nonliving chemicals. Evolution of Small Organic Molecules Most chemical reactions take place in water, and the first protocell undoubtedly arose in the ocean. The Miller-Urey experiment supports the hypothesis that small organic molecules were formed at the ocean’s surface. Macromolecules Once formed, the first small organic molecules gave rise to still larger molecules and then macromolecules. The RNA-first hypothesis, protein-first hypothesis, and Cairns-Smith hypothesis help explain this stage of life. The Protocell After macromolecules formed, something akin to a modern plasma membrane was needed to separate them from the environment. Thus, before the first true cell arose, there would likely have been a protocell. The Heterotroph Hypothesis It has been suggested that the protocell likely was a heterotroph. The True Cell A true cell is a membrane-bounded structure that can carry on protein synthesis to produce the enzymes that allow DNA replication. The three main hypotheses about the origin of life have varying explanations of how this cell arose. 27.2 Evidence of Evolution Evolution is all the changes that have occurred in living things since the beginning of life due to differential reproductive success. Evolution is defined as “common descent.” Fossil Evidence Fossils are the remains and traces of past life or any other direct evidence of past life. Geological Timescale As a result of studying strata, scientists have divided Earth’s history into eras, and then periods and epochs. The absolute dating method relies on radioactive dating techniques to assign an actual date to a fossil. Biogeographical Evidence Biogeography is the study of the distribution of species throughout the world. Mass Extinctions Extinction is the death of every member of a species. During mass extinctions, a large percentage of species become extinct within a relatively short period of time. So far, there have been five major mass extinctions. Anatomical Evidence The fact that anatomical similarities exist among organisms provides further support for evolution via descent with modification. Biochemical Evidence When the degree of similarity in DNA nucleotide sequences or the degree of similarity in amino acid sequences of proteins is examined, the more similar the sequences are, generally the more closely related the organisms are. 1 27.3 The Process of Evolution Evolution occurs at the population level. Change in gene frequencies within a population over time is defined as microevolution. Population Genetics A population is all the members of a single species that occupy a particular area at the same time and that interbreed and exchange genes. The total number of alleles at all the gene loci in all the members make up a gene pool for the population. The Hardy-Weinberg Principle The Hardy-Weinberg principle states that allele frequencies in a gene pool will remain at equilibrium after one generation of random mating in a large, sexually reproducing population as long as five conditions are met: 1) no mutations, 2) no genetic drift, 3) no gene flow, 4) random mating, and 5) no selection. Five Agents of Evolutionary Change Mutations Mutations are genetic changes that provide the raw material for evolutionary change. Genetic Drift Genetic drift refers to changes in the allele frequencies of gene pool due to chance. The founder effect and the bottleneck effect are both examples of genetic drift. Gene Flow Gene flow is the movement of alleles between populations, as occurs when individuals migrate from one population to another. Nonrandom Mating Nonrandom mating occurs when individuals pair up, not by chance, but according to their genotypes or phenotypes. Natural Selection Natural selection is the process by which some individuals produce more offspring than others. Evolution by natural selection requires individual variation, inheritance, overproduction, and differential reproductive success. Stabilizing Selection With stabilizing selection, extreme phenotypes are selected against, and individuals near the average are favored. Directional Selection Directional selection occurs when an extreme phenotype is favored and the distribution curve shifts in that direction. Disruptive Selection In disruptive selection, two or more extreme phenotypes are favored over any intermediate phenotype. Maintenance of Variation Sickle cell disease illustrates how genetic variation is sometimes maintained in a population despite deleterious effects. Diploidy and the Heterozygote Only alleles that are expressed are subject to natural selection. In diploid organisms, this fact makes the heterozygote a potential protector of recessive alleles that might otherwise be weeded out of the gene pool by natural selection. 2 27.4 Speciation A species is defined as a group of subpopulations that are capable of interbreeding and are isolated reproductively from other species. Prezygotic isolating mechanisms are in place before fertilization while postzygotic isolating mechanisms are in place after fertilization. The Process of Speciation Speciation has occurred when one species gives rise to two species. One type of speciation, called allopatric speciation, usually occurs when populations become separated by a geographic barrier and gene flow is no longer possible. Adaptive Radiation Darwin’s finches provide an example of adaptive radiation or the proliferation of a species by adaptation to different ways of life. The Pace of Speciation Currently there are two hypotheses about the pace of speciation: phyletic gradualism and punctuated equilibrium. 27.5 Systematics Systematics is very analytical and relies on a combination of data from the fossil record and comparative anatomy and development, with an emphasis today on molecular data, to reconstruct a phylogeny, the evolutionary history of a group of organisms. Linnaean Classification The binomial system of nomenclature assigns a two-part name (genus and species) to each type of organism. Taxonomists use several categories of classification: species, genus, family, order, class, phylum, kingdom, and domain. Phylogenetics Phylogeneticists arrange species and higher classification categories into clades. Clades may be represented on a diagram called a cladogram. A clade contains a most recent common ancestor (presumed but unidentified) and all its descendant species. Linnaean Classification Versus Phylogenetics Two major problems arise in integrating Linnaean classification with phylogenetics. Many more clades may exist than do Linnaean taxonomic categories, and taxonomic groups are not necessarily equivalent in the Linnaean system. Three Domain Classification System Domain Eukarya contains the protists, animals, fungi, and plants. Systematists are in the process of sorting out what kingdoms belong within domain Bacteria and domain Archaea. Learning Outcomes 27.1 Origin of Life 1. Describe the steps that may have led to the formation of the first cells. 2. Explain how the Miller-Urey experiment supports our current understanding of the origin of small organic molecules. 3. Relate three theories about the origin of macromolecules to the origin of the protocell and eventually the true cell. 27.2 Evidence of Evolution 1. Define biological evolution. 2. Discuss four lines of evidence for evolution. 27.3 The Process of Evolution 1. List the five conditions necessary to maintain Hardy- Weinberg equilibrium. 3 2. Describe the agents of evolutionary change. 3. Compare and contrast the three types of natural selection. 27.4 Speciation 1. Give examples to illustrate the process of speciation. 2. Explain how adaptive radiation can lead to speciation. 3. Compare and contrast phyletic gradualism with punctuated equilibrium. 27.5 Systematics 1. Discuss how phylogenetics is used to classify organisms. 2. Cite the differences between a five-kingdom and a three domain system. 4