How Populations Evolve Chapter 13 How Populations Evolve Lecture by Joan Sharp

Homework Reading 13.1, 13.2, 13.3 for Monday

Introduction: Clown, Fool, or Simply Well Adapted? Living organisms are adapted to their environment Copyright © 2009 Pearson Education, Inc.

Introduction: Clown, Fool, or Simply Well Adapted? What is an adaptation? Behavioral adaptations - hibernation Structural adaptations – beak shape, giraffe neck Biochemical adaptations – antibiotic resistance Physiological adaptations - salt glands in aquatic birds Adaptations can be behaviors (for example, the swaying of the leaf insect), structures (for example, mosquito mouthparts; cocklebur spines), biochemical and physiological modifications (for example, the booby’s salt glands) that enhance an organism’s ability to survive and reproduce in its environment. Copyright © 2009 Pearson Education, Inc.

Creation http://www.youtube.com/watch?v=fkgoS3vSE0k

DARWIN’S THEORY OF EVOLUTION Copyright © 2009 Pearson Education, Inc.

13.1 A sea voyage helped Darwin frame his theory of evolution The primary mechanism of evolutionary change which results in adaptations of organisms to their environment is natural selection. the differential survival and reproduction of individuals within a population So how do you think the evolution of flight in birds evolved? Copyright © 2009 Pearson Education, Inc.

Greek Natural Philosophers Around 520 BC - Anaximander The Greek philosopher, Anaximander of Miletus, wrote a text called "On Nature" in which he introduced an idea of evolution, stating that life started as slime in the oceans and eventually moved to drier places. He also brought up the idea that species evolved over time. Around 500 BC - Xenophanes Xenophanes studied fossils and put forth various theories on the evolution of life. Around 350 BC - Aristotle The Greek philosopher, Aristotle, viewed species as perfect and permanent. In the century prior to Darwin (mid 1700s), the study of fossils suggested that species had changed over time Copyright © 2009 Pearson Education, Inc.

1800s, Jean Baptiste Lamarck suggested that life on Earth evolves His proposed mechanisms: Use and disuse Inheritance of acquired characteristics

Voyage of the Beagle (1831–1836) Great Britain Europe North America ATLANTIC OCEAN Africa PACIFIC OCEAN Brazil Equator The Galápagos Islands PACIFIC OCEAN South America Pinta Genovesa Australia Marchena Equator Santiago Cape of Good Hope Daphne Islands Andes Argentina Fernandina Pinzón Tasmania Isabela Santa Cruz Cape Horn New Zealand Santa Fe San Cristobal 40 km Florenza Española 40 miles

13.1 A sea voyage helped Darwin frame his theory of evolution Darwin was influenced by Lyell’s Principles of Geology which suggested that natural forces have gradually changed and continue to change the Earth’s surface He came to realize that the Earth was very old and that, over time, present day species have arisen from ancestral species by natural processes Alfred Russel Wallace (1823 –1913) He is best known for independently proposing a theory of evolution due to natural selection that prompted Charles Darwin to publish his own theory. Copyright © 2009 Pearson Education, Inc.

On the Origin of Species by Means of Natural Selection In 1859, Darwin published On the Origin of Species by Means of Natural Selection, presenting a strong, logical explanation of descent with modification, evolution by the mechanism of natural selection Darwin observed that Organisms produce more offspring than the environment can support Organisms vary in many traits Copyright © 2009 Pearson Education, Inc.

13.2 Darwin proposed natural selection as the mechanism of evolution Darwin reasoned that traits that increase their chance of surviving and reproducing in their environment tend to leave more offspring than others As a result, favorable traits accumulate in a population over generations Student Misconceptions and Concerns 1. Students often misunderstand the basic process of evolution and instead express a Lamarckian point of view. Organisms do not evolve structures because of want or need. Evolution is a passive process in which the environment favors certain traits that exist within a population. Adaptations evolve in populations. Organisms do not actively or willingly evolve. 2. Students often think of evolution as a process that improves. As the text notes, an adaptation in one context might be a handicap in another context. Reptiles are not “better” animals than fish. Neither could survive long in the other’s environment. Instead, the adaptations found in reptiles allow them to survive in a terrestrial environment, as those of fish allow them to survive in an aquatic one. Teaching Tips 1. Students may be asked to consider this question: Can individuals evolve? Sometimes such simple questions require complex answers. Might Lamarck have answered this question differently from Darwin? Module 13.2 addresses this question. Copyright © 2009 Pearson Education, Inc.

13.2 Darwin proposed natural selection as the mechanism of evolution Darwin found convincing evidence for his ideas in the results of artificial selection, the selective breeding of domesticated plants and animals Copyright © 2009 Pearson Education, Inc.

Terminal bud Lateral buds Cabbage Brussels sprouts Flower clusters Leaves Kale Cauliflower Stem Wild mustard Flowers and stems Broccoli Kohlrabi

13.2 Darwin proposed natural selection as the mechanism of evolution Note these important points Individuals do not evolve: populations evolve Natural selection can amplify or diminish only heritable traits; acquired characteristics cannot be passed on to offspring Evolution is not goal directed and does not lead to perfection; favorable traits vary as environments change Copyright © 2009 Pearson Education, Inc.

13.2 Darwin proposed natural selection as the mechanism of evolution Will natural selection act on variation in hair style in a human population? Copyright © 2009 Pearson Education, Inc.

13.2 Darwin proposed natural selection as the mechanism of evolution Will natural selection act on tongue rolling in a human population? (Note: Tongue rolling is an inherited trait, caused by a dominant allele) Copyright © 2009 Pearson Education, Inc.

Darwin’s Finches

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Week 4 Day 1 Do Now (While I check your textbook for readings) How do you think the example of pesticide resistance in insects relates to the problem of treating bacterial infections? Homework: Read 13.4 and 13.5 and (scientific article)

Required vocabulary: DNA – The genetic material of an organism. A double-stranded helical grouping of molecules. Gene – A discrete unit of hereditary information within the DNA. Chromosome –Thin strand of coiled DNA (in eukaryotes). Circular strand of DNA (in prokaryotes) Allele – an alternative version of a gene

Link to Discovery Darwin mpg

Darwin’s Finches

13.3 Scientists can observe natural selection in action Rosemary and Peter Grant have worked on Darwin’s finches in the Galápagos for over 20 years In wet years, small seeds are more abundant and small beaks are favored In dry years, large strong beaks are favored because large seeds remain Copyright © 2009 Pearson Education, Inc.

13.3 Scientists can observe natural selection in action Development of pesticide resistance in insects Initial use of pesticides favors those few insects that have genes for pesticide resistance With continued use of pesticides, resistant insects flourish and vulnerable insects die Proportion of resistant insects increases over time Copyright © 2009 Pearson Education, Inc.

Chromosome with allele conferring resistance to pesticide Pesticide application Survivors Additional applications will be less effective, and the frequency of resistant insects in the population will grow Figure 13.3B Evolution of pesticide resistance in an insect.

13.4 The study of fossils provides strong evidence for evolution The fossil record shows that organisms have evolved in a historical sequence What is a fossil? What is the fossil record? Copyright © 2009 Pearson Education, Inc.

Fossils The oldest known fossils are prokaryote cells The oldest eukaryotic fossils are a billion years younger Multicellular fossils are even more recent

Skull of Homo erectus a unicellular eukaryote Ammonite casts Dinosaur tracks

Insect in amber Figure 13.4E A gallery of fossils. E “Ice Man”

13.4 The study of fossils provides strong evidence for evolution Many fossils link early extinct species with species living today A series of fossils documents the evolution of whales from a group of land mammals Copyright © 2009 Pearson Education, Inc.

13.5 A mass of other evidence reinforces the evolutionary view of life Biogeography, the geographic distribution of species, suggested to Darwin that organisms evolve from common ancestors Darwin noted that animals on islands resemble species on nearby mainland more closely than they resemble animals on similar islands close to other continents Eg distribution of lungfish in Australia, southern South America, and southern Africa. All once part of Gondwanaland Copyright © 2009 Pearson Education, Inc.

Comparative anatomy the comparison of body structures in different species Homology is the similarity in characteristics that result from common ancestry Vertebrate forelimbs Copyright © 2009 Pearson Education, Inc.

13.5 A mass of other evidence reinforces the evolutionary view of life Which of the following pairs are homologous structures? Human limb and whale flipper Insect wing and bat wing Human thumb and chimpanzee thumb Copyright © 2009 Pearson Education, Inc.

13.5 A mass of other evidence reinforces the evolutionary view of life Comparative embryology is the comparison of early stages of development among different organisms Many vertebrates have common embryonic structures, revealing homologies When you were an embryo, you had a tail and pharyngeal pouches (just like an embryonic fish) Copyright © 2009 Pearson Education, Inc.

Pharyngeal pouches Post-anal tail Chick embryo Human embryo

13.5 A mass of other evidence reinforces the evolutionary view of life Some homologous structures are vestigial organs For example, the pelvic and hind-leg bones of some modern whales Copyright © 2009 Pearson Education, Inc.

13.5 A mass of other evidence reinforces the evolutionary view of life Molecular biology: Comparisons of DNA and amino acid sequences between different organisms reveal evolutionary relationships All living things share a common DNA code for the proteins found in living cells We share genes with bacteria, yeast, and fruit flies Copyright © 2009 Pearson Education, Inc.

Chimpanzees and Humans: We really are related!!

13.6 Homologies indicate patterns of descent that can be shown on an evolutionary tree Darwin was the first to represent the history of life as a tree Copyright © 2009 Pearson Education, Inc.

Homologous structures and genes can be used to determine the branching sequence of an evolutionary tree Copyright © 2009 Pearson Education, Inc.

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THE EVOLUTION OF POPULATIONS Copyright © 2009 Pearson Education, Inc.

13.7 Populations are the units of evolution A population is a group of individuals of the same species living in the same place at the same time Evolution is the change in heritable traits in a population over generations Populations may be isolated from one another (with little interbreeding), or individuals within populations may interbreed Copyright © 2009 Pearson Education, Inc.

13.7 Populations are the units of evolution A gene pool is the total collection of genes in a population at any one time Microevolution is a change in the relative frequencies of alleles in a gene pool over time Copyright © 2009 Pearson Education, Inc.

13.7 Populations are the units of evolution Population genetics studies how populations change genetically over time The modern synthesis connects Darwin’s theory with population genetics Copyright © 2009 Pearson Education, Inc.

13.8 Mutation and sexual reproduction produce genetic variation, making evolution possible Mutation, or changes in the nucleotide sequence of DNA, is the ultimate source of new alleles Occasionally, mutant alleles improve the adaptation of an individual to its environment and increase its survival and reproductive success (for example, DDT resistance in insects) Copyright © 2009 Pearson Education, Inc.

13.8 Mutation and sexual reproduction produce genetic variation, making evolution possible Chromosomal duplication is an important source of genetic variation If a gene is duplicated, the new copy can undergo mutation without affecting the function of the original copy For example, an early ancestor of mammals had a single gene for an olfactory receptor The gene has been duplicated many times, and humans now have 1,000 different olfactory receptor genes Copyright © 2009 Pearson Education, Inc.

Animation: Genetic Variation from Sexual Recombination 13.8 Mutation and sexual reproduction produce genetic variation, making evolution possible Sexual reproduction shuffles alleles to produce new combinations Homologous chromosomes sort independently as they separate during anaphase I of meiosis During prophase I of meiosis, pairs of homologous chromosomes cross over and exchange genes Further variation arises when sperm randomly unite with eggs in fertilization Animation: Genetic Variation from Sexual Recombination Copyright © 2009 Pearson Education, Inc.

Parents ´ Meiosis Gametes Random fertilization Offspring, with new A1 A1 A2 A3 Parents ´ Meiosis A1 A2 A3 Gametes Figure 13.8 New allele combinations from sexual reproduction. Random fertilization Offspring, with new combinations of alleles A1 A2 A1 A3 and

13.8 Mutation and sexual reproduction produce genetic variation, making evolution possible How many possible combinations of chromosomes are possible in a human sperm or egg due to independent assortment during meiosis? 23 combinations 46 combinations 232 = 529 combinations 223 = ~ 8 million combinations Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving Sexual reproduction alone does not lead to evolutionary change in a population Although alleles are shuffled, the frequency of alleles and genotypes in the population does not change Similarly, if you shuffle a pack of cards, you’ll deal out different hands, but the cards and suits in the deck do not change Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving The Hardy Weinberg principle states that allele and genotype frequencies within a sexually reproducing, diploid population will remain in equilibrium unless outside forces act to change those frequencies Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving Imagine that there are two alleles in a blue-footed booby population: W and w W is a dominant allele for a nonwebbed booby foot w is a recessive allele for a webbed booby foot Copyright © 2009 Pearson Education, Inc.

Webbing No webbing

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving Consider the gene pool of a population of 500 boobies 320 (64%) are homozygous dominant (WW) 160 (32%) are heterozygous (Ww) 20 (4%) are homozygous recessive (ww) Copyright © 2009 Pearson Education, Inc.

Phenotypes Genotypes WW Ww ww Number of animals (total = 500) 320 160 Genotype frequencies 320 ––– 500 = 0.64 160 ––– 500 = 0.32 20 ––– 500 = 0.04 Number of alleles in gene pool (total = 1,000) 640 W 160 W + 160 w 40 w Figure 13.9B Gene pool of original population of boobies. Allele frequencies 800 1,000 = 0.8 W 200 1,000 = 0.2 w

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving Frequency of dominant allele (W) = 80% = p 80% of alleles in the booby population are W Frequency of recessive allele (w) = 20% = q 20% of alleles in the booby population are w Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving Frequency of all three genotypes must be 100% or 1.0 p2 + 2pq + q2 = 100% = 1.0 homozygous dominant + heterozygous + homozygous recessive = 100% Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving What about the next generation of boobies? Probability that a booby sperm or egg carries W = 0.8 or 80% Probability that a sperm or egg carries w = 0.2 or 20% Copyright © 2009 Pearson Education, Inc.

Gametes reflect allele frequencies of parental gene pool Sperm W sperm WW p2 = 0.64 Ww pq = 0.16 W egg p = 0.8 Eggs wW qp = 0.16 ww q2 = 0.04 w egg q = 0.2 Figure 13.9C Gene pool of next generation of boobies. Next generation: Genotype frequencies 0.64 WW 0.32 Ww 0.04 ww Allele frequencies 0.8 W 0.2 w

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving What is the probability of a booby chick with a homozygous dominant genotype (WW)? What is the probability of a booby chick with a homozygous recessive genotype (ww)? What is the probability of a booby chick with a heterozygous genotype (Ww)? Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving If a population is in Hardy-Weinberg equilibrium, allele and genotype frequencies will not change unless something acts to change the gene pool Copyright © 2009 Pearson Education, Inc.

13.9 The Hardy-Weinberg equation can be used to test whether a population is evolving For a population to remain in Hardy-Weinberg equilibrium for a specific trait, it must satisfy five conditions: Very large population No gene flow between populations No mutations Random mating No natural selection Copyright © 2009 Pearson Education, Inc.

13.10 CONNECTION: The Hardy-Weinberg equation is useful in public health science Public health scientists use the Hardy-Weinberg equation to estimate frequencies of disease-causing alleles in the human population One out of 3,300 Caucasian newborns in the United States have cystic fibrosis This disease, which causes digestive and respiratory problems, is caused by a recessive allele Copyright © 2009 Pearson Education, Inc.

13.10 CONNECTION: The Hardy-Weinberg equation is useful in public health science The frequency of individuals with this disease is approximately q2 = 1/3300 = 0.0003 The frequency of the recessive allele is q = .0174 or 1.7% The frequency of heterozygous carriers of cystic fibrosis is 2pq = 2 x 0.983 x 0.017 = 0.034 Around 3.4% of Caucasian Americans are carriers for cystic fibrosis Copyright © 2009 Pearson Education, Inc.

MECHANISMS OF MICROEVOLUTION Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population If the five conditions for the Hardy-Weinberg equilibrium are not met in a population, the population’s gene pool may change Mutations are rare and random and have little effect on the gene pool If mating is nonrandom, allele frequencies won’t change much (although genotype frequencies may) Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population The three main causes of evolutionary change are Natural selection Genetic drift Gene flow Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Natural selection If individuals differ in their survival and reproductive success, natural selection will alter allele frequencies Consider the boobies: Would webbed or nonwebbed boobies be more successful at swimming and capturing fish? Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Genetic drift Genetic drift is a change in the gene pool of a population due to chance In a small population, chance events may lead to the loss of genetic diversity Copyright © 2009 Pearson Education, Inc.

Animation: Causes of Evolutionary Change 13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Genetic drift The bottleneck effect leads to a loss of genetic diversity when a population is greatly reduced For example, the northern elephant seal was hunted to near extinction in the 1700s and 1800s A remnant population of fewer than 100 seals was discovered and protected; the current population of 175,000 descended from those few seals and has virtually no genetic diversity Animation: Causes of Evolutionary Change Copyright © 2009 Pearson Education, Inc.

Figure 13.11A The bottleneck effect. Original population

Original population Bottlenecking event Figure 13.11A The bottleneck effect. Original population Bottlenecking event

Original population Bottlenecking event Surviving population Figure 13.11A The bottleneck effect. Original population Bottlenecking event Surviving population

Figure 13. 11B Greater prairie chicken (Tympanuchus cupido) Figure 13.11B Greater prairie chicken (Tympanuchus cupido). Illinois populations have reduced genetic variation as a result of a bottleneck.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Genetic drift Genetic drift produces the founder effect when a few individuals colonize a new habitat The smaller the group, the more different the gene pool of the new population will be from the gene pool of the original population Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Gene flow Gene flow is the movement of individuals or gametes/spores between populations and can alter allele frequencies in a population Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Four moose were taken from the Canadian mainland to Newfoundland in 1904. These two males and two females rapidly formed a large population of moose that now flourishes in Newfoundland. Which mechanism is most likely to have contributed to the genetic differences between the mainland and Newfoundland moose? Gene flow Founder effect Novel mutations Copyright © 2009 Pearson Education, Inc.

13.11 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population The fossil remains of pygmy (or dwarf) mammoths (1.5 m to 2 m tall) have been found on Santa Rosa and San Miguel Islands off the coast of California. This population of pygmy mammoths is descended from a population of mammoths of normal size (4 m tall). Dwarfing is common in island populations and is not the result of chance events. What mechanism do you think best accounts for the decrease in mammoth size on these islands? Gene flow Genetic drift Natural selection Copyright © 2009 Pearson Education, Inc.

13.12 Natural selection is the only mechanism that consistently leads to adaptive evolution An individual’s fitness is the contribution it makes to the gene pool of the next and subsequent generations The fittest individuals are those that pass on the most genes to the next generation Copyright © 2009 Pearson Education, Inc.

Figure 13.12 The flexible spine of a cheetah stretches out in the middle of its 7 m (23 ft) stride.

13.13 Natural selection can alter variation in a population in three ways Stabilizing selection favors intermediate phenotypes, acting against extreme phenotypes Stabilizing selection is very common, especially when environments are stable Copyright © 2009 Pearson Education, Inc.

Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Stabilizing selection Directional selection Disruptive selection

13.13 Natural selection can alter variation in a population in three ways Example of stabilizing selection In Swiss starlings, clutch size varies from 1 to 8; the average clutch size is 4 Researchers marked chicks from different clutch sizes and recaptured fledglings after 3 months Birds from clutches with 3, 4, or 5 birds had higher recapture rates than birds from clutches of 1 or 2 or 6, 7, or 8 Copyright © 2009 Pearson Education, Inc.

13.13 Natural selection can alter variation in a population in three ways Directional selection acts against individuals at one of the phenotypic extremes Directional selection is common during periods of environmental change, or when a population migrates to a new and different habitat Copyright © 2009 Pearson Education, Inc.

13.13 Natural selection can alter variation in a population in three ways Disruptive selection favors individuals at both extremes of the phenotypic range This form of selection may occur in patchy habitats Copyright © 2009 Pearson Education, Inc.

13.14 Sexual selection may lead to phenotypic differences between males and females In many animal species, males and females show distinctly different appearance, called sexual dimorphism Intrasexual competition involves competition for mates, usually by males Copyright © 2009 Pearson Education, Inc.

Figure 13.14B A contest for access to mates.

13.14 Sexual selection may lead to phenotypic differences between males and females In intersexual competition (or mate choice), individuals of one sex (usually females) are choosy in picking their mates, often selecting flashy or colorful mates Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria As a result, natural selection is favoring bacteria that are resistant to antibiotics Natural selection for antibiotic resistance is particularly strong in hospitals Many hospital-acquired infections are resistant to a variety of antibiotics Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern The fruit fly Drosophila melanogaster has an allele that confers resistance to DDT and similar insecticides Laboratory strains of D. melanogaster have been established from flies collected in the wild in the 1930s (before the widespread use of insecticides) and the 1960s (after 20 years of DDT use) Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern Lab strains established in the 1930s have no alleles for DDT resistance; in lab strains established in the 1960s, the frequency of the DDT-resistance allele is 37% Copyright © 2009 Pearson Education, Inc.

Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 18.17 Key developmental events in the life cycle of Drosophila; (a) Adult. Left Ventral (a) Adult

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern Some fruit flies evolved resistance to DDT in order to survive—true or false? Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern Fruit flies became more resistant to DDT over time—true or false? Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern When DDT was widely used, fruit flies with DDT resistance had greater evolutionary fitness than fruit flies lacking DDT resistance—true or false? Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern Alleles for DDT resistance may have been present but rare prior to DDT use—true or false? Copyright © 2009 Pearson Education, Inc.

13.15 EVOLUTION CONNECTION: The evolution of antibiotic resistance in bacteria is a serious public health concern Alleles for DDT resistance arose by mutation during the period of DDT use because of selection for pesticide resistance—true or false? Copyright © 2009 Pearson Education, Inc.

13.16 Diploidy and balancing selection preserve genetic variation Why doesn’t natural selection act to eliminate genetic variation in populations, retaining only the most favorable alleles? Diploidy preserves variation by “hiding” recessive alleles A recessive allele is only subject to natural selection when it influences the phenotype in homozygous recessive individuals For example, cystic fibrosis Copyright © 2009 Pearson Education, Inc.

13.16 Diploidy and balancing selection preserve genetic variation Balancing selection maintains stable frequencies of two or more phenotypes in a population In heterozygote advantage, heterozygotes have greater reproductive success than homozygous For example, sickle-cell anemia Copyright © 2009 Pearson Education, Inc.

13.16 Diploidy and balancing selection preserve genetic variation In frequency-dependent selection, two different phenotypes are maintained in a population For example, Indonesian silverside fishes Some variations may be neutral, providing no apparent advantage or disadvantages For example, human variation in fingerprints Copyright © 2009 Pearson Education, Inc.

“left-mouthed” individuals “Right-mouthed” 1.0 “Left-mouthed” “left-mouthed” individuals Frequency of 0.5 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year

13.17 Natural selection cannot fashion perfect organisms Selection can only act on existing variation Natural selection cannot conjure up new beneficial alleles Evolution is limited by historical constraints Birds arose as the forelimb of a small dinosaur evolved into a wing Copyright © 2009 Pearson Education, Inc.

Wing claw (like dinosaur) Teeth (like dinosaur) Long tail with Figure 19.7A Archaeopteryx, an extinct bird. Archaeopteryx flew by modification of its forelimbs into wings, not by sprouting wings from its back, like Pegasus. Long tail with many vertebrae (like dinosaur) Feathers

13.17 Natural selection cannot fashion perfect organisms Adaptations are often compromises Chance, natural selection and the environment interact Copyright © 2009 Pearson Education, Inc.

You should now be able to Describe Darwin’s concept of natural selection Describe two examples of natural selection known to occur in nature Explain how the fossil record, biogeography, comparative anatomy, comparative embryology, and molecular biology support evolution Explain how mutation and sexual recombination produce genetic variation Describe the five conditions required for a population to be in Hardy-Weinberg equilibrium Copyright © 2009 Pearson Education, Inc.

You should now be able to Explain the significance of the Hardy-Weinberg equilibrium to natural populations and to public health science Define genetic drift and gene flow Explain why natural selection is the only mechanism that leads to adaptive evolution Distinguish between stabilizing selection, directional selection, and disruptive selection, and describe an example of each Copyright © 2009 Pearson Education, Inc.

You should now be able to Distinguish between intrasexual selection and intersexual selection Describe how antibiotic resistance has evolved Explain how genetic variation is maintained in populations Copyright © 2009 Pearson Education, Inc.