Chapter 13 How Populations Evolve

Name: Chapter 13 How Populations Evolve
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Description: Chapter 13 How Populations Evolve

Chapter 13 How Populations Evolve
Laura Coronado Bio Chapter 13

Biology and Society: Persistent Pests
Mosquitoes and malaria In the 1960s, the World Health Organization (WHO) began a campaign to eradicate the mosquitoes that transmit malaria. It used DDT, to which some mosquitoes have evolved resistance. The evolution of pesticide-resistant insects is just one of the ways that evolution affects our lives. An understanding of evolution informs every field of biology, for example: Agriculture Medicine Biotechnology Conservation biology Laura Coronado Bio Chapter 13

CHARLES DARWIN AND THE ORIGIN OF SPECIES
Charles Darwin published On the Origin of Species by Means of Natural Selection, November 24, 1859. Darwin presented two main concepts: Life evolves Change occurs as a result of “descent with modification,” with natural selection as the mechanism Laura Coronado Bio Chapter 13

Laura Coronado Bio 10 Chapter 13
A Trinidad tree mantid that mimics dead leaves Figure 13.1 Camouflage as an example of evolutionary adaptation A flower mantid in Malaysia A leaf mantid in Costa Rica Laura Coronado Bio Chapter 13 Figure 13.1

Natural Selection Is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are individuals with other characteristics. Natural selection leads to: A population (a group of individuals of the same species living in the same place at the same time) changing over generations Evolutionary adaptation In one modern definition of evolution, the genetic composition of a population changes over time. Laura Coronado Bio Chapter 13

1837 Darwin begins analyzing his specimens and writing his notebooks on the origin of species. 1809 Lamarck publishes his theory of evolution. 1844 Darwin writes his essay on the origin of species. 1830 Lyell publishes Principles of Geology. 1865 Mendel publishes papers on genetics. 1800 1870 1809 Charles Darwin is born. 1859 Darwin publishes The Origin of Species. 1858 Wallace sends an account of his theory to Darwin. 1831–36 Darwin travels around the world on the HMS Beagle. Figure 13.2 The historical context of Darwin's life and ideas Green sea turtle in the Galápagos Islands Laura Coronado Bio Chapter 13 Figure 13.2

Darwin’s Cultural and Scientific Context
The Origin of Species challenged the notion that the Earth was: Relatively young Populated by unrelated species The Greek philosopher Aristotle held the belief that species are fixed and do not evolve. The Judeo-Christian culture fortified this idea with a literal interpretation of the Bible and suggested the Earth may only be 6,000 years old. Laura Coronado Bio Chapter 13

Lamarck and Evolutionary Adaptations
In the mid-1700s, the study of fossils began to take form as a branch of science. Naturalist Georges Buffon noted that: The Earth may be more than 6,000 years old There are similarities between fossils and living species Fossil forms might be ancient versions of similar living species Jean Baptiste Lamarck suggested that organisms evolved by the process of adaptation by the inheritance of acquired characteristics, now known to be incorrect. Laura Coronado Bio Chapter 13

Darwin was born on February 12, In 1831 he left Great Britain on the HMS Beagle on a five-year voyage around the world. Student Misconceptions and Concerns 1. Students often think that the application of pesticides somehow causes the changes necessary for members of a species to survive. It is important to point out that selection simply favors traits already present. The short phrase, first diversity, then selection, can help students remember this important point. 2. Continuing the point above, species do not evolve because of need. Biological diversity exists and the environment selects. Evolution is not deliberate, it is reactive. As teachers, we must be careful in how we express evolution to best reflect this process. For example: birds evolved wings seems as if birds did something deliberately, that there might have been a planning committee! wings evolved in birds is more accurate, in that something was done to birds in the process. This use of the passive voice in our descriptions of evolution better reflects the reactive nature of evolution. 3. Students often think of evolution as a process that improves. However, an adaptation in one context might be a handicap in another. Bats are not better animals than sharks. Neither could survive long in the other’s environment. Instead, the adaptations found in bats best reflect their terrestrial environment, distinct from the aquatic lifestyle and adaptations of sharks. 4. Students often believe that Charles Darwin first suggested that life evolves: the early contributions by Greek philosophers (such as Anaximander) and the work of Jean Baptiste Lamarck remain unappreciated. Consider emphasizing this earlier work. 5. Students might become confused by some scientific debates. Evolution can be considered on three levels, sometimes referred to as a) fact, b) course, and c) mechanism: a) Does evolution occur? b) Who gave rise to whom? and c) Is natural selection the only mechanism of evolution that produces adaptations? Students that listen to scientific debates about the course of evolution might think that evolution itself is under attack. Doubts about ancestry can be misconstrued as doubts about evolution itself. Consider sharing these distinctions with your class. Teaching Tips 1. Consider beginning your unit on evolution by asking each student to explain how a particular adaptation evolved. Reviewing these student explanations can provide great insight into the misconceptions and confusion that your students bring to the class. 2. Many excellent evolution resources are available: a. Two extensive sites rich with details and references are ( and ( b. The complete works of Charles Darwin can be accessed at ( c. An extensive usenet newsgroup devoted to the discussion and debate of biological and physical origins can be found at ( 3. Consider asking your students to consider “Can individuals evolve?” Sometimes such simple questions require complex answers. Might Lamarck have answered this question differently than Darwin? Laura Coronado Bio Chapter 13

Beagle Journey On his journey on the Beagle, Darwin: Collected thousands of specimens Observed various adaptations in organisms Darwin was intrigued by: The geographic distribution of organisms on the Galápagos Islands Similarities between organisms in the Galápagos and those in South America Laura Coronado Bio Chapter 13

Figure 13.4 A marine iguana, an example of the unique species inhabiting the Galápagos Laura Coronado Bio Chapter 13 Figure 13.4

Darwin was strongly influenced by the writings of geologist Charles Lyell. Lyell suggested that the Earth: Is very old Was sculpted by gradual geological processes that continue today Darwin applied Lyell’s principle of gradualism to the evolution of life on Earth. Darwin made two main points in The Origin of Species: Organisms inhabiting Earth today descended from ancestral species Natural selection was the mechanism for descent with modification Laura Coronado Bio Chapter 13

EVIDENCE OF EVOLUTION Biological evolution leaves observable signs. We will examine five of the many lines of evidence in support of evolution: The fossil record Biogeography Comparative anatomy Comparative embryology Molecular biology Laura Coronado Bio Chapter 13

The Fossil Record Fossils are: Imprints or remains of organisms that lived in the past Often found in sedimentary rocks The fossil record: Is the ordered sequence of fossils as they appear in rock layers Reveals the appearance of organisms in a historical sequence Fits the molecular and cellular evidence that prokaryotes are the ancestors of all life Paleontologists: Are scientists that study fossils Have discovered many transitional forms that link past and present Laura Coronado Bio Chapter 13

Figure 13.5 Strata of sedimentary rock in the Grand Canyon Laura Coronado Bio Chapter 13 Figure 13.5

Figure 13.6 A transitional fossil linking past and present (Step 3) Laura Coronado Bio Chapter 13 Figure

Biogeography Biogeography is the study of the geographic distribution of species that first suggested to Darwin that today’s organisms evolved from ancestral forms. Many examples from biogeography would be difficult to understand, except from an evolutionary perspective. One example is the distribution of marsupial mammals in Australia. Laura Coronado Bio Chapter 13

Australia Koala Common ringtail possum Figure 13.7 Biogeography Common wombat Red kangaroo Laura Coronado Bio Chapter 13 Figure 13.7

Comparative Anatomy Comparative anatomy Is the comparison of body structure between different species Confirms that evolution is a remodeling process Homology is: The similarity in structures due to common ancestry Illustrated by the remodeling of the pattern of bones forming the forelimbs of mammals Vestigial structures: Are remnants of features that served important functions in an organism’s ancestors Now have only marginal, if any, importance Laura Coronado Bio Chapter 13

Figure 13.8 Homologous structures: anatomical signs of descent with modification Human Cat Whale Bat Laura Coronado Bio Chapter 13 Figure 13.8

Comparative Embryology
Early stages of development in different animal species reveal additional homologous relationships. For example, pharyngeal pouches appear on the side of the embryo’s throat, which: Develop into gill structures in fish Form parts of the ear and throat in humans Comparative embryology of vertebrates supports evolutionary theory. Laura Coronado Bio Chapter 13

Pharyngeal pouches Post-anal tail Figure 13.9 Evolutionary signs from comparative embryology Chicken embryo Human embryo Laura Coronado Bio Chapter 13 Figure 13.9

Molecular Biology The hereditary background of an organism is documented in: Its DNA The proteins encoded by the DNA Evolutionary relationships among species can be determined by comparing: Genes Proteins of different organisms Laura Coronado Bio Chapter 13

Primate Percent of selected DNA sequences that match a chimpanzee’s DNA 92% 96% 100% Chimpanzee Human Gorilla Orangutan Figure Genetic relationships among some primates Gibbon Old World monkey Laura Coronado Bio Chapter 13 Figure 13.10

NATURAL SELECTION Darwin noted the close relationship between adaptation to the environment and the origin of new species. The evolution of finches on the Galápagos Islands is an excellent example. Darwin based his theory of natural selection on two key observations: All species tend to produce excessive numbers of offspring & leads to a struggle for existence. Variation exists among individuals in a population & much of this variation is heritable Laura Coronado Bio Chapter 13

(a) The large ground finch Figure Galápagos finches with beaks adapted for specific diets (b) The small tree finch Laura Coronado Bio Chapter 13 (c) The woodpecker finch Figure 13.11

NATURAL SELECTION Figure Overproduction of offspring Figure Color variations within a single species of Asian lady beetles Laura Coronado Bio Chapter 13

NATURAL SELECTION Inference: Differential reproductive success (natural selection) Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring. Examples of natural selection include: Pesticide-resistant insects Antibiotic-resistant bacteria Drug-resistant strains of HIV Laura Coronado Bio Chapter 13

Insecticide application Chromosome with gene conferring resistance to pesticide Figure Evolution of pesticide resistance in insect populations (Step 3) Survivors Reproduction Laura Coronado Bio Chapter 13 Figure

The Process of Science: Does Predation Drive the Evolution of Lizard Horn Length? Observation: Flat-tailed horned lizards defend against attack by: Thrusting their heads backward Stabbing a shrike with the spiked horns on the rear of their skull Question: Are longer horns a survival advantage? Hypothesis: Longer horns are a survival advantage. Prediction: Live horned lizards have longer horn lengths than dead ones. Experiment: Measure the horn lengths of dead and living lizards. Results: The average horn length of live lizards is about 10% longer than that of dead lizards. Laura Coronado Bio Chapter 13

Live Killed (a) A flat-tailed horned lizard 20 Length (mm) Live 10 Killed Figure The effect of predation on lizard horn length Rear horns Side horns (tip to tip) (b) The remains of a lizard impaled by a shrike (c) Results of measurement of lizard horns Laura Coronado Bio Chapter 13 Figure 13.15

EVOLUTIONARY TREES Darwin saw the history of life as analogous to a tree: The first forms of life on Earth form the common trunk At each fork is the last common ancestor to all the branches extending from that fork Laura Coronado Bio Chapter 13

Lungfishes Amphibians Tetrapods Mammals Tetrapod limbs Amniotes Lizards and snakes Amnion Crocodiles Figure An evolutionary tree of tetrapods Ostriches Birds Feathers Hawks and other birds Laura Coronado Bio Chapter 13 Figure 13.16

The Modern Synthesis: Darwinism Meets Genetics
The modern synthesis is the fusion of genetics with evolutionary biology. Laura Coronado Bio Chapter 13

Populations as the Units of Evolution
A population is: A group of individuals of the same species, living in the same place, at the same time The smallest biological unit that can evolve The total collection of alleles in a population at any one time is the gene pool. When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale, which is sometimes called microevolution Laura Coronado Bio Chapter 13

Figure Populations (a) Two dense populations of trees separated by a lake (b) A nighttime satellite view of North America Laura Coronado Bio Chapter 13 Figure 13.17

Genetic Variation in Populations
Individual variation abounds in populations. Not all variation in a population is heritable. Only the genetic component of variation is relevant to natural selection. Variable traits in a population may be: Polygenic, resulting from the combined effects of several genes or Determined by a single gene Polygenic traits tend to produce phenotypes that vary more or less continuously. Single gene traits tend to produce only a few distinct phenotypes. Laura Coronado Bio Chapter 13

Figure Variation in a garter snake population Laura Coronado Bio Chapter 13 Figure 13.18

Sources of Genetic Variation
Genetic variation results from: Mutations, changes in the DNA of an organism Sexual recombination, the shuffling of alleles during meiosis For any one gene, mutation alone has little effect on a large population in a single generation. Organisms with very short generation spans, such as bacteria, can evolve rapidly with mutations as the only source of genetic variation. Laura Coronado Bio Chapter 13

Analyzing Gene Pools The gene pool is a reservoir from which the next generation draws its genes. Alleles in a gene pool occur in certain frequencies. Alleles can be symbolized by: p for the relative frequency of the dominant allele in the population q for the frequency of the recessive allele in the population Genotype frequencies: Can be calculated from allele frequencies Are symbolized by the expressions p2, 2pq, and q2 Laura Coronado Bio Chapter 13

Frequency of one allele Frequency of alternate allele Figure 13.UN1 Allele frequencies for a wildflower population with two varieties of flower color Laura Coronado Bio Chapter 13 Figure 13.UN1

Frequency of one allele Frequency of alternate allele Frequency of homozygotes for one allele Frequency of heterozygotes Frequency of homozygotes for alternate allele Figure 13.UN4 Summary: Hardy-Weinberg formula Laura Coronado Bio Chapter 13 Figure 13.UN4

Allele frequencies p  0.8 (R) q  0.2 (r) Eggs R r p  0.8 q  0.2 RR Rr p2  0.64 pq  0.16 R p  0.8 Sperm rR rr q2  0.04 r qp  0.16 Figure A mathematical swim in the gene pool q  0.2 p2  0.64 q2  0.04 Genotype frequencies 2pq  0.32 (RR) (Rr) (rr) Laura Coronado Bio Chapter 13 Figure 13.20

The Hardy-Weinberg formula
Used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles. Used to calculate the percentage of a human population that carries the allele for a particular inherited disease. PKU: Is a recessive allele that prevents the breakdown of the amino acid phenylalanine Occurs in about one out of every 10,000 babies born in the United States Laura Coronado Bio Chapter 13

INGREDIENTS: SORBITOL, MAGNESIUM STEARATE, ARTIFICIAL FLAVOR, ASPARTAME† (SWEETENER), Figure A warning to individuals with PKU ARTIFICIAL COLOR (YELLOW 5 LAKE, BLUE 1 LAKE), ZINC GLUCONATE. †PHENYLKETONURICS: CONTAINS PHENYLALANINE Laura Coronado Bio Chapter 13 Figure 13.21

Microevolution as Change in a Gene Pool
How can we tell if a population is evolving? A non-evolving population is in genetic equilibrium, called the Hardy-Weinberg equilibrium, in which the population gene pool remains constant over time. From a genetic perspective evolution can be defined as a generation-to-generation change in a population’s frequencies of alleles, sometimes called microevolution. Laura Coronado Bio Chapter 13

MECHANISMS OF EVOLUTION
The main causes of evolutionary change are: Genetic drift Genetic drift: A change in the gene pool of a small population Due to chance Gene flow: Is genetic exchange with another population Tends to reduce genetic differences between populations Natural selection: of all causes of microevolution, only natural selection promotes adaptation Laura Coronado Bio Chapter 13

RR rr RR RR RR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring Rr Rr RR RR rr RR RR rr RR RR Rr Rr RR RR RR rr RR Rr RR Figure Genetic drift (Step 3) RR Rr Rr Rr RR RR Generation 1 Generation 2 Generation 3 p (frequency of R)  0.7 q (frequency of r)  0.3 p  0.5 q  0.5 p  1.0 q  0.0 Laura Coronado Bio Chapter 13 Figure

The Bottleneck Effect The bottleneck effect: Is an example of genetic drift Results from a drastic reduction in population size Bottlenecking in a population usually reduces genetic variation because at least some alleles are likely to be lost from the gene pool. Cheetahs appear to have experienced at least two genetic bottlenecks in the past 10,000 years. Laura Coronado Bio Chapter 13

Figure Bottleneck effect (Step 3) Original population Bottlenecking event Surviving population Laura Coronado Bio Chapter 13 Figure

The Founder Effect The founder effect is likely when a few individuals colonize an isolated habitat and represent genetic drift in a new colony. The founder effect explains the relatively high frequency of certain inherited disorders among some small human populations. Laura Coronado Bio Chapter 13

Africa South America Tristan da Cunha Figure Residents of Tristan daCunha in the early 1900s Laura Coronado Bio Chapter 13 Figure 13.25

Figure Human gene flow Laura Coronado Bio Chapter 13 Figure 13.26

Darwinian Fitness Fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. Figure Darwinian fitness of some flowering plants depends in part on competition in attracting pollinators Laura Coronado Bio Chapter 13

Three General Outcomes of Natural Selection
Directional selection: Shifts the phenotypic “curve” of a population Selects in favor of some extreme phenotype Disruptive selection can lead to a balance between two or more contrasting phenotypic forms in a population. Stabilizing selection: Favors intermediate phenotypes Is the most common Laura Coronado Bio Chapter 13

Phenotypes (fur color) Original population
of individuals Frequency Original population Evolved population Phenotypes (fur color) Original population Figure Three general effects of natural selection on a phenotypic character (a) Directional selection (b) Disruptive selection (c) Stabilizing selection Laura Coronado Bio Chapter 13 Figure 13.28

Sexual Selection Sexual dimorphism is: A distinction in appearance between males and females Not directly associated with reproduction or survival Sexual selection is a form of natural selection in which inherited characteristics determine mating preferences. Laura Coronado Bio Chapter 13

Figure Sexual dimorphism (a) Sexual dimorphism in a finch species (b) Competing for mates Laura Coronado Bio Chapter 13 Figure 13.29

Evolution Connection: The Genetics of the Sickle-Cell Allele
Sickle-cell disease: Is a genetic disorder Affects about one out of every 400 African-Americans Abnormally shaped red blood cells cause painful and life-threatening complications. Heterozygous individuals for the sickle-cell allele: Do not develop sickle-cell anemia Are more resistant to malaria In the African tropics, where malaria is most common, the frequency of the sickle-cell allele is highest. Laura Coronado Bio Chapter 13

Colorized SEM Frequencies of the sickle-cell allele 0–2.5% Figure Mapping malaria and the sickle-cell allele 2.5–5.0% 5.0–7.5% 7.5–10.0% Areas with high incidence of malaria 10.0–12.5% 12.5% Laura Coronado Bio Chapter 13 Figure 13.30

Chapter 13 How Populations Evolve

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