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Chapter 13 How Populations Evolve PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE • TAYLOR • SIMON • DICKEY • HOGAN © 2015 Pearson Education, Inc. Lecture by Edward J. Zalisko DARWIN’S THEORY OF EVOLUTION © 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. 13.1 A sea voyage helped Darwin frame his theory of evolution • The cultural and scientific context of his time instilled Darwin with a conventional view of Earth and its life. • Most scientists accepted the views of the Greek philosopher Aristotle, who generally held that species are fixed, permanent forms that do not evolve. • Judeo-Christian culture taught a literal interpretation of the biblical book of Genesis, that each form of life is individually created in its present-day form. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • Earlier religious scholars estimated the age of Earth at 6,000 years. • Thus, the idea that all living species came into being relatively recently and are unchanging in form dominated the intellectual climate of the Western world at the time. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • At the age of 22, Darwin took a position on HMS Beagle, a survey ship preparing for a long expedition to chart poorly known stretches of the South American coast. • As the ship’s naturalist (field biologist), Darwin • spent most of his time on shore collecting thousands of specimens of fossils and living plants and animals and • kept detailed journals of his observations. © 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 • Many of Darwin’s observations indicated that geographic proximity is a better predictor of relationships among organisms than similarity of environment. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • Darwin was particularly intrigued by the geographic distribution of organisms on the Galápagos Islands, including • marine iguanas and • 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, he concluded that the evidence was better explained by the hypothesis that presentday species are the descendants of ancient ancestors that they still resemble in some ways. © 2015 Pearson Education, Inc. 13.1 A sea voyage helped Darwin frame his theory of evolution • He hypothesized that as the descendants of a remote ancestor spread into various habitats over millions and millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life in their environment. © 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 in support of his hypothesis, and finally released his essay to the scientific community when learning of the work of another British naturalist, Alfred Wallace, who had a nearly identical hypothesis. © 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 drawn 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. © 2015 Pearson Education, Inc. Last paragraph from Origin of Species • Thus, from the war of nature, from famine and death, the most exalted object of which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. © 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 • Next we examine lines of evidence for Darwin’s theory of evolution, the idea that living species are descendants of ancestral species that were different from present-day ones and that natural selection is the mechanism for evolutionary change. © 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. Figure 13.2a © 2015 Pearson Education, Inc. Figure 13.2b © 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. • Paleontologists (scientists who study fossils) sometimes gain access to very old fossils when erosion carves through upper (younger) strata, revealing deeper (older) strata that had been buried. © 2015 Pearson Education, Inc. Figure 13.2c © 2015 Pearson Education, Inc. 13.2 The study of fossils provides strong evidence for evolution • 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. © 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 • Beginning in the late 1970s, paleontologists unearthed an extraordinary series of transitional fossils in Pakistan and Egypt. • The new fossil discoveries were consistent with the earlier hypothesis, and paleontologists became more firmly convinced that whales did indeed arise from a wolf-like carnivore. © 2015 Pearson Education, Inc. Figure 13.3b Pakicetus Rodhocetus Dorudon Living cetaceans © 2015 Pearson Education, Inc. Key Pelvis Femur Tibia Foot 13.3 SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution • But molecular biologists • found a close relationship between whales and hippopotamuses and • hypothesized that whales and hippos were both descendants of a cloven-hoofed ancestor. © 2015 Pearson Education, Inc. 13.3 SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution • Paleontologists were taken aback by the contradictory results, but openness to new evidence is a hallmark of science. • Two fossils discovered in 2001 provided the answer. • Both Pakicetus and Rodhocetus had the distinctive ankle bone of a cloven-hoofed mammal. • Thus, as is often the case in science, scientists are becoming more certain about the evolutionary origin of whales as mounting evidence from different lines of inquiry converge. © 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. • Evolution is a remodeling process. • 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. • As Figure 13.4A shows, the same skeletal elements make up the forelimbs of humans, cats, whales, and bats, but the functions of these forelimbs differ. • Biologists call such anatomical similarities in different organisms homologous structures. © 2015 Pearson Education, Inc. Figure 13.4a Humerus Radius Ulna Carpals Metacarpals Phalanges Human © 2015 Pearson Education, Inc. Cat Whale Bat 13.4 Homologies provide strong evidence for evolution • Molecular comparisons between diverse organisms have allowed biologists to develop hypotheses about the evolutionary divergence of major branches on the tree of life. • Darwin’s boldest hypothesis was that all life-forms are related. Molecular biology 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 animal species reveal similarities not visible in adult organisms and • at some point in their development, all vertebrate embryos have • a tail posterior to the anus, and • structures called pharyngeal (throat) pouches. © 2015 Pearson Education, Inc. Figure 13.4b-0 Pharyngeal pouches Post-anal tail Chick embryo © 2015 Pearson Education, Inc. Human embryo 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. © 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, although today they often turn the trees sideways. © 2015 Pearson Education, Inc. 13.5 Homologies indicate patterns of descent that can be shown on an evolutionary tree • Homologous structures can be used to determine the branching sequence of an evolutionary tree. • These homologies can include • anatomical structure and/or • molecular structure. © 2015 Pearson Education, Inc. Figure 13.5 Each branch point represents the common ancestor of the lineages beginning there and to the right of it Lungfishes Amniotes Mammals 2 Tetrapod limbs Amnion Lizards and snakes 3 4 5 Ostriches 6 Feathers © 2015 Pearson Education, Inc. Hawks and other birds Birds A hatch mark represents a homologous character shared by all the groups to the right of the mark Crocodiles Tetrapods Amphibians 1 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 long periods of time, he knew that he would not be able to study the evolution of new species by direct observation. • But insights into how incremental change occurs could be seen in examples of artificial selection, in which humans have modified species through selective breeding. © 2015 Pearson Education, Inc. Figure 13.6-0 Frillback Fantail Rock pigeon Old Dutch Capuchine © 2015 Pearson Education, Inc. Trumpeter 13.6 Darwin proposed natural selection as the mechanism of evolution • Darwin knew that individuals in natural populations have small but measurable differences. • But what forces in nature played the role of the breeder, choosing which individuals became the breeding stock for the next generation? • Darwin found inspiration in an essay written by economist Thomas Malthus, who contended that much of human suffering—disease, famine, and war—was the consequence of human populations increasing faster than food supplies and other resources. © 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. © 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 • It is important to emphasize three key points about evolution by natural selection. 1. Although natural selection occurs through interactions between individual organisms and the environment, individuals do not evolve. Rather, it is the population, the group of organisms, that evolves over time. 2. Natural selection can amplify or diminish only heritable traits. 3. Evolution is not goal directed; it does not lead to perfectly adapted organisms. © 2015 Pearson Education, Inc. 13.7 Scientists can observe natural selection in action • Biologists have documented evolutionary change in thousands of scientific studies. • Peter and Rosemary Grant have worked on Darwin’s finches in the Galápagos for over 30 years. They found that • in dry years, large strong beaks are favored because all seeds are in short supply and birds must eat more larger seeds, but • in wet years, small seeds are more abundant and small beaks are favored. © 2015 Pearson Education, Inc. 13.7 Scientists can observe natural selection in action • Another example of natural selection in action is the evolution of pesticide resistance in insects. • A relatively small amount of poison initially kills most of the insects, but subsequent applications are less and less effective. • The few survivors are individuals that are genetically resistant, carrying an allele that somehow enables them to survive the chemical attack. • When these resistant insects reproduce, the percentage of the population resistant to the pesticide increases. © 2015 Pearson Education, Inc. Figure 13.7-0 Pesticide application Chromosome with allele conferring resistance to pesticide Survivors Additional applications of the same pesticide will be less effective, and the frequency of resistant insects in the population will grow © 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. © 2015 Pearson Education, Inc. THE EVOLUTION OF POPULATIONS © 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. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Just a few years after the publication of The Origin of Species, Gregor Mendel wrote a groundbreaking paper on inheritance in pea plants. • Although the significance of Mendel’s work was not recognized during his or Darwin’s lifetime, its rediscovery in 1900 set the stage for understanding the genetic differences on which evolution is based. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Each person has a unique genome, reflected in individual phenotypic variations, such as appearance and other traits. • Indeed, individual variation occurs in all species. © 2015 Pearson Education, Inc. Figure 13.8 © 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, as in sickle-cell disease. © 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. • This kind of effect is more likely when the environment is changing such that mutations that were once disadvantageous are favorable under new conditions. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Chromosomal duplication is an important source of genetic variation. • If a repeated segment of DNA can persist over the generations, mutations may accumulate in the duplicate copies without affecting the function of the original gene, eventually leading to new genes with novel functions. • For example, the remote ancestors of mammals carried a single gene for detecting odors that has since been duplicated repeatedly. As a result, mice have about 1,300 different olfactory receptor genes. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • In organisms that reproduce sexually, most of the genetic variation in a population results from the unique combination of alleles that each individual inherits. © 2015 Pearson Education, Inc. 13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Fresh assortments 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. 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. • We can measure evolution as a change in the prevalence of certain heritable traits in a population over a span of generations. © 2015 Pearson Education, Inc. 13.9 Evolution occurs within populations • 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. • Such isolation is common in populations confined to different lakes. © 2015 Pearson Education, Inc. Figure 13.9 © 2015 Pearson Education, Inc. 13.9 Evolution occurs within populations • A gene pool consists of all copies of every type of allele, at every locus, in all members of the population. • 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. MECHANISMS OF MICROEVOLUTION © 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. • Consider the imaginary iguana population. Individuals with webbed feet (genotype ww) might survive better and produce more offspring because they are more efficient at swimming and catching food than individuals that lack webbed feet. • Genetic equilibrium would be disturbed as the frequency of the w 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. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • Catastrophes such as hurricanes, floods, or fires may kill large numbers of individuals, leaving a small surviving population that is unlikely to have the same genetic makeup as the original population. • The bottleneck effect leads to a loss of genetic diversity when a population is greatly reduced. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • Analogous to shaking just a few marbles through a bottleneck, certain alleles may be present at higher frequency in the surviving population than in the original population, others may be present at lower frequency, and some (orange marbles) may not be present at all. • After a population is drastically reduced, genetic drift may continue for many generations until the population is again large enough for fluctuations due to chance to have less of an impact. © 2015 Pearson Education, Inc. Figure 13.12a-3 Original population © 2015 Pearson Education, Inc. Bottlenecking event Surviving population 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • One reason it is important to understand the bottleneck effect is that human activities such as overhunting and habitat destruction may create severe bottlenecks for other species. • Examples of species affected by bottlenecks include the endangered Florida panther, the African cheetah, and the greater prairie chicken. © 2015 Pearson Education, Inc. Figure 13.12b © 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. • The founder effect explains the relatively high frequency of certain inherited disorders among some human populations established by small numbers of colonists. © 2015 Pearson Education, Inc. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution 3. Gene flow • Allele frequencies in a population can also change as a result of 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. 13.12 Natural selection, genetic drift, and gene flow can cause microevolution • To counteract the lack of genetic diversity in the remaining Illinois greater prairie chickens, researchers added 271 birds from neighboring states to the Illinois populations, which successfully introduced new alleles. • This strategy worked. New alleles entered the population, and the egg-hatching rate improved to more than 90%. © 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 • The adaptations of organisms include many striking examples. • Consider some of the features that make the bluefooted booby suited to its home on the Galapagos Islands. • The bird’s large, webbed feet make great flippers, propelling the bird through the water at high speeds. • Its body and bill are streamlined, minimizing friction as it dives from heights up to 24 m (over 75 feet) into the shallow water below. • To pull out of this high-speed dive once it hits the water, the booby uses its large tail as a brake. © 2015 Pearson Education, Inc. Figure 13.13 © 2015 Pearson Education, Inc. 13.13 Natural selection is the only mechanism that consistently leads to adaptive evolution • Let’s take a closer look at natural selection. • The commonly used phrases “struggle for existence” and “survival of the fittest” are misleading if we take them to mean direct competition between individuals. Reproductive success is generally more subtle and passive. • 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 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. • In many animal species, males and females may have secondary sexual characteristics, noticeable differences not directly associated with reproduction or survival, called sexual dimorphism. © 2015 Pearson Education, Inc. Figure 13.15a © 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 type of sexual selection is called intrasexual selection (within the same sex, most often the males). • Contests may involve physical combat, but are more often ritualized displays. © 2015 Pearson Education, Inc. Figure 13.15b © 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. © 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. Figure 13.15c © 2015 Pearson Education, Inc. 13.16 EVOLUTION CONNECTION: The evolution of drug-resistant microorganisms is a serious public health concern • Antibiotics are drugs that kill infectious microorganisms. • During the 1950s, some medical experts even thought the age of human infectious diseases would soon be over. • Why didn’t that optimistic forecast come true? • Answer: It did not take into account the force of evolution. © 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: the random genetic mutations in bacteria and the nonrandom selective effects as the environment favors the antibioticresistant phenotype. © 2015 Pearson Education, Inc. 13.16 EVOLUTION CONNECTION: The evolution of drug-resistant microorganisms is a serious public health concern • In what ways do we contribute to the problem of antibiotic resistance? • Doctors may overprescribe antibiotics. • Patients may misuse prescribed antibiotics by prematurely stopping the medication because they feel better. • Livestock producers add antibiotics to animal feed as a growth promoter and to prevent illness. © 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. Figure 13.16 © 2015 Pearson Education, Inc. 13.17 Diploidy and balancing selection preserve genetic variation • What prevents natural selection from eliminating all variation as it selects against unfavorable genotypes? • Why aren’t less adaptive alleles eliminated as the “best” alleles are passed to the next generation? • It turns out that the tendency for natural selection to reduce variation in a population is countered by mechanisms that maintain variation. © 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. • An example is a 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. Figure 13.17 “Left-mouthed” “Right-mouthed” © 2015 Pearson Education, Inc. 13.18 Natural selection cannot fashion perfect organisms • The evolution of organisms is constrained. 1. Selection can act only on existing variations. New, advantageous alleles do not arise on demand. 2. Evolution is limited by historical constraints. Evolution co-opts existing structures and adapts them to new situations. 3. Adaptations are often compromises. The same structure often performs many functions. 4. Chance, natural selection, and the environment interact. Environments often change © 2015 Pearson Education, Inc.