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Figure 13.0-1 DARWIN’S THEORY OF EVOLUTION © 2015 Pearson Education, Inc. I Can… 1. Explain how Darwin’s voyage on the Beagle influenced his thinking. 2. Explain why the concept of evolution is regarded as a theory with great significance. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • Charles Darwin is best known for his book On the Origin of Species by Means of Natural Selection, commonly referred to as The Origin of Species, which launched the era of evolutionary biology. • Darwin’s early career gave no hint of his future fame. • He enrolled in but left medical school. • Then he entered Cambridge University to become a clergyman. © 2015 Pearson Education, Inc. Figure 13.1a-0 HMS Beagle in port Darwin in 1840 North America Great Britain Asia Europe ATLANTIC OCEAN Africa Pinta Marchena Santiago Galápagos Islands Genovesa Equator Daphne Islands Pinzón Fernandina Isabela 0 0 40 km South America Santa Cruz Florenza 40 miles © 2015 Pearson Education, Inc. Santa Fe San Cristobal Española PACIFIC OCEAN Andes PACIFIC OCEAN PACIFIC OCEAN Equator Australia Cape of Good Hope Cape Horn Tierra del Fuego Tasmania New Zealand 13.1 A sea voyage helped Darwin frame his theory of evolution • Darwin’s observations indicated that geographic proximity is a better predictor relationships between organisms than similarity of environment. • Darwin was particularly intrigued by the geographic distribution of organisms on the Galápagos Islands, including • marine iguanas • giant tortoises © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • While on his voyage, Darwin was strongly influenced by the newly published Principles of Geology, by Scottish geologist Charles Lyell. • The book presented the case for an ancient Earth sculpted over millions of years by gradual geologic processes that continue today. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • By the time Darwin returned to Great Britain five years after the Beagle first set sail, he had begun to seriously doubt that Earth and all its living organisms had been specially created only a few thousand years earlier. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • As he reflected on his observations, analyzed his collections, and discussed his work with colleagues, Darwin hypothesized: • Present-day species are the descendants of ancient ancestors that they still resemble in some ways. • As the descendants of a remote ancestor spread into various habitats millions of years, they accumulated diverse modifications, or adaptations. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • By the early 1840s, Darwin had composed a long essay describing the major features of his theory of evolution by natural selection. • But he delayed publishing his essay, continued to compile evidence, and finally released his essay when learning of the work of another British naturalist, Alfred Wallace, who had a nearly identical hypothesis as Darwin. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • The next year, Darwin published The Origin of Species, a book that supported his hypothesis with immaculate logic and hundreds of pages of evidence from observations and experiments in biology, geology, and paleontology. • The hypothesis of evolution generated predictions that have been tested and verified by more than 150 years of research and millions of experiments. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • Consequently, scientists regard Darwin’s concept of evolution by means of natural selection as a theory, a widely accepted explanatory idea that • is broader in scope than a hypothesis, • generates new hypotheses, and • is supported by a large body of evidence. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • Darwin’s theory of evolution states that: • living species are descendants of ancestral species (different from present-day) and that natural selection is the mechanism for evolutionary change. © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Evidence of Evolution © 2015 Pearson Education, Inc. I Can… 1. Explain how homologies, the fossil record, and molecular biology support evolution. © 2015 Pearson Education, Inc. 13.2 The study of fossils provides strong evidence for evolution • Fossils • are the imprints or remains of organisms that lived in the past, • document differences between past and present organisms, and • reveal that many species have become extinct. • For example, the fossilized skull of one of our early relatives, Homo erectus, represents someone who lived 1.5 million years ago in Africa. © 2015 Pearson Education, Inc. 13.2 The study of fossils provides strong evidence for evolution • The sequence in which fossils appear within strata, layers of sedimentary rocks, is a historical record of life on Earth. • The fossil record is the chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata. Upper strata have younger fossils, deeper strata host older fossils. © 2015 Pearson Education, Inc. 13.2 The study of fossils provides strong evidence for evolution • The fossil record is incomplete because • many of Earth’s organisms did not live in areas that favor fossilization, • fossils that did form were in rocks later distorted or destroyed by geologic processes, and • not all fossils that have been preserved are accessible to paleontologists. © 2015 Pearson Education, Inc. 13.3 SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution • In The Origin of Species, Darwin predicted the existence of fossils of transitional forms linking very different groups of organisms. • Thousands of fossil discoveries have since shed light on the evolutionary origins of many groups of plants and animals, including • the transition of fish to amphibian, • the origin of birds from a lineage of dinosaurs, and • the evolution of mammals from a reptilian ancestor. © 2015 Pearson Education, Inc. 13.4 Homologies provide strong evidence for evolution • Evolution is a process of descent with modification. • Characteristics present in an ancestral organism are altered over time by natural selection as its descendants face different environmental conditions. • Related species can have characteristics that have an underlying similarity yet function differently. • Similarity resulting from common ancestry is known as homology. © 2015 Pearson Education, Inc. 13.4 Homologies provide strong evidence for evolution • Darwin cited the anatomical similarities among vertebrate forelimbs as evidence of common ancestry. • Biologists call anatomical similarities in different organisms homologous structures. © 2015 Pearson Education, Inc. 13.4 Homologies provide strong evidence for evolution • Darwin’s boldest hypothesis was that all life-forms are related. Molecular biology, comparing similarities in DNA, RNA, or Amino Acids between diverse organisms, provides strong evidence for this claim. • All forms of life use the same genetic language of DNA and RNA. • The genetic code—how RNA triplets are translated into amino acids—is essentially universal. © 2015 Pearson Education, Inc. 13.4 Homologies provide strong evidence for evolution • An understanding of homology helps explain why • early stages of development in different species reveal similarities not visible in adults • at some point in their development, all vertebrate embryos have • a tail • pharyngeal (throat) pouches. © 2015 Pearson Education, Inc. 13.4 Homologies provide strong evidence for evolution • Some of the most interesting homologies are “leftover” structures that are of marginal or perhaps no importance to the organism. • These vestigial structures are remnants of features that served important functions in the organism’s ancestors. Sci Show Vestigial Structures: https://www.youtube.com/watch?v=OAfw3akpRe8 Stated Clearly Evidence for Evolution: https://www.youtube.com/watch?v=lIEoO5KdPvg © 2015 Pearson Education, Inc. 13.5 Homologies indicate patterns of descent that can be shown on an evolutionary tree • Darwin was the first to view the history of life as a tree, with multiple branches from a common ancestral trunk to the descendant species at the tips of the twigs. • Today, biologists represent these patterns of descent with an evolutionary tree or phylogenic tree. © 2015 Pearson Education, Inc. 13.5 Homologies indicate patterns of descent that can be shown on an evolutionary tree • Homologous structures, including anatomical structure and/or molecular structure, can be used to determine the branching sequence of an evolutionary tree. © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Evolution by Natural Selection © 2015 Pearson Education, Inc. I Can… • • • Describe Darwin’s observations and inferences in developing the concept of natural selection. Explain how the work of Thomas Malthus and the process of artificial selection influenced Darwin’s development of the idea of natural selection. Explain why individuals cannot evolve and why evolution does not lead to perfectly adapted organisms. © 2015 Pearson Education, Inc. 13.6 Darwin proposed natural selection as the mechanism of evolution • Darwin’s greatest contribution to biology was his explanation of how life evolves. • Because he thought that species formed gradually over time, he knew that he would not be able to study evolution by direct observation. • However, examples of incremental change can be seen in artificial selection, in which humans have modified species through selective breeding. © 2015 Pearson Education, Inc. Artificial Selection © 2015 Pearson Education, Inc. 13.6 Darwin proposed natural selection as the mechanism of evolution • Darwin knew that individuals in natural populations have small but measurable differences. But forces led to the choosing of which individuals became the breeding stock for the next generation? • Darwin found inspiration from economist Thomas Malthus, who said that human suffering—disease, famine, and war—was the result of human populations increasing faster than their resources (food, etc.). © 2015 Pearson Education, Inc. 13.6 Darwin proposed natural selection as the mechanism of evolution • Darwin deduced that the production of more individuals than the limited resources can support leads to a struggle for existence, with only some offspring surviving in each generation. • The essence of natural selection is this unequal reproduction. • Individuals whose traits better enable them to obtain food or escape predators or tolerate physical conditions will survive and reproduce more successfully, passing these adaptive traits to their offspring. • Show Booby Chick Fight © 2015 Pearson Education, Inc. Natural Selection = Heritable Variation + Differential Reproductive Success © 2015 Pearson Education, Inc. 13.6 Darwin proposed natural selection as the mechanism of evolution • Darwin reasoned that if artificial selection can bring about so much change in a relatively short period of time, then natural selection could modify species considerably over hundreds or thousands of generations. © 2015 Pearson Education, Inc. 13.6 Darwin proposed natural selection as the mechanism of evolution • Three key points about evolution by natural selection. 1. Although natural selection occurs through interactions between individuals and the environment, individuals do not evolve. Rather, it is the population that evolves over time. 2. Natural selection can amplify or diminish only heritable traits. Traits gained during an organisms lifetime can not be inherited. 3. Evolution is not goal directed; it does not lead to perfectly adapted organisms. Traits that are “good enough” to allow an organism to survive and reproduced will get passed on. © 2015 Pearson Education, Inc. 13.7 Scientists can observe natural selection in action • An example of natural selection in action is the evolution of pesticide resistance in insects. © 2015 Pearson Education, Inc. 13.7 Scientists can observe natural selection in action • These examples of evolutionary adaptation highlight two important points about natural selection. 1. Natural selection is more of an editing process than a creative mechanism. 2. Natural selection is contingent on time and place, favoring those heritable traits in a varying population that fit the current, local environment. If the environment changes so too can the forces of natural selection. © 2015 Pearson Education, Inc. • Video 1 http://www.youtube.com/watch?v=zuqqnTOM1po • Video 2 • https://www.youtube.com/watch?v=rltZfejDVuw © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. THE EVOLUTION OF POPULATIONS © 2015 Pearson Education, Inc. I can… • Explain how mutation and sexual reproduction produce genetic variation. • Describe the five conditions required for the HardyWeinberg equilibrium. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Organisms typically show individual variation. • However, in The Origin of Species, Darwin could not explain • the cause of variation among individuals or • how variations were passed from parents to offspring. • Just a few years after the publication of The Origin of Species, Gregor Mendel wrote a groundbreaking paper on inheritance in pea plants. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Individual variation occurs in all species and it is essential for evolution. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Mutations are • changes in the nucleotide sequence of DNA and • the ultimate source of new alleles. • Thus, mutation is the ultimate source of the genetic variation that serves as raw material for evolution. • A change as small as a single nucleotide in a protein-coding gene can have a significant effect on phenotype. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • On rare occasions, however, a mutated allele may actually improve the adaptation of an individual to its environment and enhance its reproductive success. • Rock Pocket Mice • http://www.hhmi.org/biointeractive/making-fittestnatural-selection-and-adaptation © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Genetic variation of existing alleles arise every generation from three random components of sexual reproduction: 1. crossing over, 2. independent orientation of homologous chromosomes at metaphase I of meiosis, and 3. random fertilization. © 2015 Pearson Education, Inc. Animation: Genetic Variation from Sexual Recombination © 2015 Pearson Education, Inc. 13.9 Evolution occurs within populations • A population is a group of individuals of the same species, that live in the same area, and interbreed. • Different populations of the same species may be geographically isolated from each other to such an extent that an exchange of genetic material never occurs, or occurs only rarely. • A gene pool consists of all copies of every type of allele, at every locus, in all members of the population. © 2015 Pearson Education, Inc. 13.9 Evolution occurs within populations • Microevolution is • change in the relative frequencies of alleles in a population over a number of generations and evolution occurring on its smallest scale. © 2015 Pearson Education, Inc. 13.10 The Hardy-Weinberg equation can test whether a population is evolving • If a population is in Hardy-Weinberg equilibrium, allele and genotype frequencies will remain constant, generation after generation. • The Hardy-Weinberg principle tells us that something other than the reshuffling processes of sexual reproduction is required to change allele frequencies in a population. © 2015 Pearson Education, Inc. 13.10 The Hardy-Weinberg equation can test whether a population is evolving • For a population to be in Hardy-Weinberg equilibrium and not evolving, it must satisfy five main conditions. There must be 1. 2. 3. 4. 5. a very large population, no gene flow between populations, no mutations, random mating, and no natural selection. • Rarely are all five conditions met in real populations, and thus allele and genotype frequencies often do change. © 2015 Pearson Education, Inc. 13.10 The Hardy-Weinberg equation can test whether a population is evolving • Rarely are all five conditions met in real populations, and thus allele and genotype frequencies often do change. TED Ed: 5 Fingers of Evolution http://ed.ted.com/lessons/five-fingers-of-evolution © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. MECHANISMS OF MICROEVOLUTION © 2015 Pearson Education, Inc. I Can… • Define genetic drift and gene flow. Explain how the bottleneck effect and the founder effect influence microevolution. • Distinguish between stabilizing selection, directional selection, and disruptive selection. Describe an example of each. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • The three main causes of evolutionary change are 1. natural selection, 2. genetic drift, and 3. gene flow. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution 1. Natural selection • If individuals differ in their survival and reproductive success, natural selection will alter allele frequencies. • Genetic equilibrium would be disturbed as the frequency of the beneficial allele increased in the gene pool from one generation to the next. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution 2. Genetic drift • In a process called genetic drift, chance events can cause allele frequencies to fluctuate unpredictably from one generation to the next. • The smaller the population, the more impact genetic drift is likely to have. • Catastrophes (hurricanes, floods, or fires) may kill a lot of individuals, leaving a small population that is unlikely to have the same genetic makeup as the original population. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • The bottleneck effect leads to a loss of genetic diversity when a population is greatly reduced. • Human activities such as overhunting and habitat destruction may create severe bottlenecks for other species. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • Genetic drift is also likely when a few individuals colonize an island or other new habitat, producing what is called the founder effect. • The smaller the group, the less likely the genetic makeup of the colonists will represent the gene pool of the larger population they left. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution 3. Gene flow • Where a population may gain or lose alleles when fertile individuals move into or out of a population or when gametes (such as plant pollen) are transferred between populations. • Gene flow tends to reduce differences between populations. © 2015 Pearson Education, Inc. 13.13 Natural selection is the only mechanism that consistently leads to adaptive evolution • Genetic drift, gene flow, and mutations could each result in microevolution, but only by chance could these events improve a population’s fit to its environment. • In natural selection, only the genetic variation produced by mutation and sexual reproduction results from random events. • The process of natural selection, in which some individuals are more likely than others to survive and reproduce, is not random. • Because of this sorting, only natural selection consistently leads to adaptive evolution. © 2015 Pearson Education, Inc. 13.13 Natural selection is the only mechanism that consistently leads to adaptive evolution • Relative fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. © 2015 Pearson Education, Inc. 13.14 Natural selection can alter variation in a population in three ways • Natural selection can affect the distribution of phenotypes in a population. • Stabilizing selection favors intermediate phenotypes. • Directional selection shifts the overall makeup of the population by acting against individuals at one of the phenotypic extremes. • Disruptive selection typically occurs when environmental conditions vary in a way that favors individuals at both ends of a phenotypic range over individuals with intermediate phenotypes. © 2015 Pearson Education, Inc. Figure 13.14 Frequency of individuals Original population Original population Evolved population Stabilizing selection © 2015 Pearson Education, Inc. Phenotypes (fur color) Directional selection Disruptive selection © 2015 Pearson Education, Inc. I Can… • Define and compare intrasexual selection and intersexual selection. © 2015 Pearson Education, Inc. 13.15 Sexual selection may lead to phenotypic differences between males and females • Sexual selection is a form of natural selection in which individuals with certain characteristics are more likely than other individuals to obtain mates. • Booby Dance • In many animal species, males and females may have secondary sexual characteristics, noticeable differences not directly associated with reproduction or survival, called sexual dimorphism. • Peacocking © 2015 Pearson Education, Inc. 13.15 Sexual selection may lead to phenotypic differences between males and females • In some species, individuals compete directly with members of the same sex for mates. • This is called intrasexual selection (within the same sex, most often the males). © 2015 Pearson Education, Inc. 13.15 Sexual selection may lead to phenotypic differences between males and females • In a more common type of sexual selection, called intersexual selection (between sexes) or mate choice, individuals of one sex (usually females) are choosy in selecting their mates. • Human Sexual Selection © 2015 Pearson Education, Inc. Giraffe Clips http://www.arkive.org/giraffe/giraffacamelopardalis/video-09e.html http://www.arkive.org/giraffe/giraffacamelopardalis/video-09a.html © 2015 Pearson Education, Inc. Sexual Selection: Giraffe • Males use the flehmen (urine test) sequence to determine which females are in estrus • According to Pratt (1985), female giraffes prefer older, more dominant males. • When approached for urine-testing, Pratt found that females urinated more often for dominant males than subordinate males. • Thus, dominant males have more success in determining which females are receptive. • This advantageous for the female because it allows her to get the best genes for her offspring: the most dominant male is the one who ultimately fertilizes her eggs © 2015 Pearson Education, Inc. 13.15 Sexual selection may lead to phenotypic differences between males and females • What is the advantage to females of being choosy? • One hypothesis is that females prefer male traits that are correlated with “good genes.” • In several bird species, research has shown that traits preferred by females, such as bright beaks or long tails, are related to overall male health. • The “good genes” hypothesis was also tested in gray tree frogs. Female frogs prefer to mate with males that give long mating calls. © 2015 Pearson Education, Inc. 13.16 EVOLUTION CONNECTION: The evolution of drug-resistant microorganisms is a serious public health concern • In the same way that pesticides select for resistant insects, antibiotics select for resistant bacteria. • Again we see both the random and nonrandom aspects of natural selection: • random genetic mutations in bacteria • nonrandom selective effects as the environment favors the antibiotic-resistant phenotype. © 2015 Pearson Education, Inc. 13.16 EVOLUTION CONNECTION: The evolution of drug-resistant microorganisms is a serious public health concern • Each year in the United States nearly 100,000 people die from infections they contract in the hospital, often because the bacteria are resistant to multiple antibiotics. • A formidable “superbug” known as MRSA (methicillin-resistant Staphylococcus aureus) can cause “flesh-eating disease” and potentially fatal systemic (whole-body) infections. © 2015 Pearson Education, Inc. 13.17 Diploidy and balancing selection preserve genetic variation • In some cases, genetic variation is preserved rather than reduced by natural selection. • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population. © 2015 Pearson Education, Inc. 13.17 Diploidy and balancing selection preserve genetic variation • Heterozygote advantage is a type of balancing selection in which heterozygous individuals have greater reproductive success than either type of homozygote, with the result that two or more alleles for a gene are maintained in the population. © 2015 Pearson Education, Inc. 13.17 Diploidy and balancing selection preserve genetic variation • Frequency-dependent selection is a type of balancing selection that maintains two different phenotypic forms in a population. • In this case, selection acts against either phenotypic form if it becomes too common in the population. • Scale-eating fish in Lake Tanganyika, Africa, which attacks other fish from behind, darting in to remove a few scales from the side of its prey. © 2015 Pearson Education, Inc. 13.18 Natural selection cannot fashion perfect organisms • The evolution of organisms is constrained. 1. 2. 3. 4. Selection can act only on existing variations. New, advantageous alleles do not arise on demand. Evolution is limited by historical constraints. Evolution co-opts existing structures and adapts them to new situations. Adaptations are often compromises. The same structure often performs many functions. Chance, natural selection, and the environment interact. Environments often change unpredictably. © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. You should now be able to 1. Explain how Darwin’s voyage on the Beagle influenced his thinking. 2. Explain why the concept of evolution is regarded as a theory with great significance. 3. Explain how fossils form and why the fossil record is incomplete. 4. Explain how homologies, the fossil record, and molecular biology support evolution. 5. Explain how evolutionary trees are constructed and used to represent ancestral relationships. © 2015 Pearson Education, Inc. You should now be able to 6. Describe Darwin’s observations and inferences in developing the concept of natural selection. 7. Explain how the work of Thomas Malthus and the process of artificial selection influenced Darwin’s development of the idea of natural selection. 8. Explain why individuals cannot evolve and why evolution does not lead to perfectly adapted organisms. 9. Describe two examples of natural selection known to occur in nature. © 2015 Pearson Education, Inc. You should now be able to 10. Explain how mutation and sexual reproduction produce genetic variation. 11. Explain why prokaryotes can evolve more quickly than eukaryotes. 12. Describe the five conditions required for the HardyWeinberg equilibrium. 13. Explain why the Hardy-Weinberg equilibrium is significant to understanding the evolution of natural populations and to public health science. 14. Define genetic drift and gene flow. Explain how the bottleneck effect and the founder effect influence microevolution. © 2015 Pearson Education, Inc. You should now be able to 15. Distinguish between stabilizing selection, directional selection, and disruptive selection. Describe an example of each. 16. Define and compare intrasexual selection and intersexual selection. 17. Explain how antibiotic resistance has evolved. 18. Explain how genetic variation is maintained in populations. 19. Explain why natural selection cannot produce perfection. © 2015 Pearson Education, Inc. Figure 13.UN01 Heritable variations in individuals Observations Overproduction of offspring Inferences Individuals well-suited to the environment tend to leave more offspring and Over time, favorable traits accumulate in the population © 2015 Pearson Education, Inc. Figure 13.UN02 Allele frequencies p + q = 1 Genotype frequencies p2 + 2pq + q2 = 1 Dominant Heterozygotes homozygotes © 2015 Pearson Education, Inc. Recessive homozygotes Figure 13.UN03 Original population Stabilizing selection © 2015 Pearson Education, Inc. Evolved population Directional selection Pressure of natural selection Disruptive selection Figure 13.UN04 Microevolution is the may result from change in allele frequencies in a population (a) (c) (b) random fluctuations due to more likely in a movement of individuals or gametes (d) may be result of (e) © 2015 Pearson Education, Inc. (f) due to leads to (g) of individuals best adapted to environment adaptive evolution Figure 13.UN05 © 2015 Pearson Education, Inc.