How Populations Evolve Chapter 13 How Populations Evolve

Biology and Society: Mosquitoes, Microbes, and Malaria In the 1960s, the World Health Organization (WHO) launched a campaign to eradicate the mosquitoes that transmit malaria. It used DDT, to which some mosquitoes have evolved resistance. © 2013 Pearson Education, Inc. 2

Figure 13.0 Figure 13.0 These children in Uganda are being tested for malaria.

Biology and Society: Mosquitoes, Microbes, and Malaria 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, medicine, agriculture, biotechnology, and conservation biology. © 2013 Pearson Education, Inc. 4

CHARLES DARWIN AND THE ORIGIN OF SPECIES Biology came of age on November 24, 1859. Charles Darwin published On the Origin of Species by Means of Natural Selection, an assemblage of facts about the natural world. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 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. Many resources related to Charles Darwin are available on the Internet. The following are only a few examples: a. www.ucmp.berkeley.edu/history/evolution.html and ncse.com are extensive sites rich with details and references. b. www.literature.org/authors/darwin-charles/ includes the texts of The Voyage of the Beagle, The Origin of Species first and sixth editions, and The Descent of Man. c. http://williamcalvin.com/bookshelf/down_hse.htm includes details of Charles Darwin’s home. 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? 5

CHARLES DARWIN AND THE ORIGIN OF SPECIES Darwin made three observations from these facts. Life shows rich diversity. There are similarities in life that allow the classification of organisms into groups nested within broader groups. Organisms display many striking ways in which they are suited for their environments. © 2013 Pearson Education, Inc. 6

(b) Patterns of similarities Figure 13.1 (b) Patterns of similarities (a) The diversity of life (c) An insect suited to its environment Figure 13.1 Key observations about life

CHARLES DARWIN AND THE ORIGIN OF SPECIES In The Origin of Species, Darwin proposed a hypothesis, a scientific explanation for his observations, used hundreds of pages in his book to describe the evidence supporting his hypothesis, made testable predictions, and reported the results of numerous experiments he had performed. © 2013 Pearson Education, Inc. 8

CHARLES DARWIN AND THE ORIGIN OF SPECIES Darwin hypothesized that present-day species are the descendents of ancient ancestors that they still resemble in some ways and change occurs as a result of “descent with modification,” with natural selection as the mechanism. © 2013 Pearson Education, Inc. 9

Figure 13.2 Figure 13.2 A sketch made by Darwin as he pondered the origin of species

CHARLES DARWIN AND THE ORIGIN OF SPECIES 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. As a result of natural selection, a population, a group of individuals of the same species living in the same place at the same time, changes over generations. © 2013 Pearson Education, Inc. 11

CHARLES DARWIN AND THE ORIGIN OF SPECIES Natural selection leads to evolutionary adaptation, a population’s increase in the frequency of traits suited to the environment. Natural selection thus leads to evolution, seen either as a change in the genetic composition of a population over time or on a grander scale, the entire biological history, from the earliest microbes to the enormous diversity of organisms that live on Earth today. © 2013 Pearson Education, Inc. 12

CHARLES DARWIN AND THE ORIGIN OF SPECIES 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 and evolutionary adaptation. In one modern definition of evolution, the genetic composition of a population changes over time. © 2013 Pearson Education, Inc. 13

Darwin’s Cultural and Scientific Context The Origin of Species was fundamentally different from the prevailing scientific and cultural views of Darwin’s time. Let’s place Darwin’s ideas in their historical context. © 2013 Pearson Education, Inc. 14

The Idea of Fixed 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 biblical book of Genesis and the suggestion that Earth may only be 6,000 years old. Naturalists were grappling with the interpretation of fossils, imprints or remains of organisms that lived in the past. © 2013 Pearson Education, Inc. 15

(b) Ichthyosaur skull and paddle-like forelimb Figure 13.3 (a) “Snakestone” (b) Ichthyosaur skull and paddle-like forelimb Figure 13.3 Perplexing fossils

Lamarck and Evolutionary Adaptations Naturalists compared fossil forms with living species and noted patterns of similarities and differences. In the early 1800s, French naturalist Jean Baptiste Lamarck suggested that life evolves, and explained this evolution as the refinement of traits that equip organisms to perform successfully in their environment. © 2013 Pearson Education, Inc. 17

Lamarck and Evolutionary Adaptations Lamarck suggested a mechanism that we now know is wrong. Lamarck proposed that by using or not using its body parts, an individual may develop certain traits that it passes on to its offspring, thus, acquired traits are inherited. Lamarck helped set the stage for Darwin by proposing that species evolve as a result of interactions between organisms and their environment. © 2013 Pearson Education, Inc. 18

The Voyage of the Beagle Darwin was born on February 12, 1809, the same day that Abraham Lincoln was born. In December 1831, Darwin left Great Britain on the HMS Beagle on a five-year voyage around the world. Video: Galápagos Islands Overview © 2013 Pearson Education, Inc. 19

HMS Beagle Darwin in 1840 Great Britain Asia Europe North America Figure 13.4 HMS Beagle Darwin in 1840 Great Britain Asia Europe North America ATLANTIC OCEAN Africa Galápagos Islands PACIFIC OCEAN Pinta South America Equator Genovesa Marchena Equator Santiago Daphne Islands Australia Pinzón Fernandina Cape of Good Hope Isabela Santa Cruz PACIFIC OCEAN Santa Fe San Cristobal 40 km Florenza Española Cape Horn Tasmania 40 miles Tierra del Fuego New Zealand Figure 13.4 The voyage of the Beagle

Figure 13.4a Darwin in 1840 Figure 13.4 The voyage of the Beagle (part 1)

Figure 13.4b HMS Beagle Figure 13.4 The voyage of the Beagle (part 2)

Galápagos Islands PACIFIC OCEAN Pinta Genovesa Marchena Equator Figure 13.4c Galápagos Islands PACIFIC OCEAN Pinta Genovesa Marchena Equator Santiago Daphne Islands Pinzón Fernandina Isabela Santa Cruz Santa Fe San Cristobal 40 km Florenza Española 40 miles Figure 13.4 The voyage of the Beagle (part 3)

The Voyage of the Beagle On his journey on the Beagle, Darwin collected thousands of specimens and observed various adaptations in organisms. © 2013 Pearson Education, Inc. 24

The Voyage of the Beagle Darwin was intrigued by the geographic distribution of organisms on the Galápagos Islands and similarities between organisms in the Galápagos and those in South America. Video: Galápagos Marine Iguana Video: Galápagos Sea Lion Video: Galápagos Tortoise © 2013 Pearson Education, Inc. 25

Figure 13.5 Figure 13.5 A land iguana (left), and a marine iguana (right), an example of the unique species inhabiting the Galápagos

Figure 13.5a Figure 13.5 A land iguana, an example of the unique species inhabiting the Galápagos (part 1)

Figure 13.5b Figure 13.5 A marine iguana, an example of the unique species inhabiting the Galápagos (part 2)

The Voyage of the Beagle Darwin was strongly influenced by the writings of geologist Charles Lyell. Lyell suggested that Earth is very old and was sculpted by gradual geological processes that continue today. © 2013 Pearson Education, Inc. 29

The Voyage of the Beagle Darwin reasoned that the extended time scale would allow for gradual changes to occur in species and in geologic features. © 2013 Pearson Education, Inc. 30

Descent with Modification Darwin made two main points in The Origin of Species. Organisms inhabiting Earth today descended from ancestral species. Natural selection is the mechanism for descent with modification. © 2013 Pearson Education, Inc. 31

EVIDENCE OF EVOLUTION 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, and molecular biology. © 2013 Pearson Education, Inc. 32

The Fossil Record Fossils are imprints or remains of organisms that lived in the past often found in sedimentary rocks. © 2013 Pearson Education, Inc. 33

The Fossil Record The fossil record is the ordered sequence of fossils as they appear in rock layers, reveals the appearance of organisms in a historical sequence, and fits with the molecular and cellular evidence that prokaryotes are the ancestors of all life. Video: Grand Canyon © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students who struggle with the concept of evolution may also bring personal objections to the classroom. Distinguishing between the processes of science and the use of supernatural forces in explanations helps distinguish between scientific explanations and those that are not scientific. The evidence of evolution permits a demonstration of the scientific method, in which confidence in our conclusions increases based on multiple, independent lines of evidence. Science requires evidence and cannot rely upon faith or supernatural explanations. 2. Students might expect that every living organism will leave fossils and that we should be able to find them. Furthermore, they might falsely conclude that the absence of evidence is evidence of absence. Understanding the rare circumstances under which fossils form (the scientific study of fossil formation is referred to as taphonomy), the geological processes that distort and destroy layers (for example, earthquakes and glaciers), and the long odds against the discovery of fossils, help students understand why the fossil record is limited. 3. The experiences of beginning college students with the subject of fossils are likely to be uneven and weak. For example, the many ways that fossil ages are determined also demonstrates the strength of independent lines of evidence and consistency. Here is a chance to describe the many ways that a fossil’s age can be determined (for example, radiometric dating, association with other fossils of known age in the same layer, correlation to other strata of known ages). Teaching Tips 1. If you have any sort of a fossil, it makes a nice visual aid (of course, the larger the better). Many shops in natural history museums carry large and common fossils that can be purchased for less than $50.00. Crinoid stems or other marine fossils from the Midwest are the types of fossils that make the point that life and ecosystems change over time. Selecting fossils that are common in your region may stimulate students to search for their own. 2. The sequence, but not absolute ages, is revealed by the stratifications in sedimentary rocks. This is like peeling the layers of wallpaper from the walls of a very old house that has been inhabited by many owners. By the sequence, we can tell which layers are older, but not the absolute ages of each layer. 3. Another popular example of biogeography is the distribution of lungfish in Australia, southern South America, and southern Africa. At one time, these three continents were united as Gondwana. 4. One striking example of evolutionary remodeling is the shell of turtles, primarily made from vertebrae and modified ribs. If you find an old turtle shell and soak it in water, the shell may come apart with some effort, leaving isolated ribs that make interesting visual aids. 5. The evolution of the three inner ear bones in mammals, the malleus and incus joining the stapes (already present as the columella in reptiles), is another wonderful transitional sequence in the fossil record. 6. Building on the earlier point about the evolution of the middle ear bones from the lower jaw of reptiles, consider the conflicting problems of hearing while chewing. Ask your students to consider the problems of chewing carrots and listening to a lecture. The mammalian hearing apparatus clearly conflicts with the crunching noises of chewing, one of the costs of remodeling. Adaptations typically represent a compromise of functions and not perfection. 7. You can have some fun with this analogy to molecular evidence for evolution. As teachers, we have to be keenly aware of cheating. Certainly, if two students turn in written assignments that differ by just a few words, we would conclude that they had a “common heritage” or that one was derived from the other. 34

Figure 13.6 Figure 13.6 Strata of sedimentary rock at the Grand Canyon

The Fossil Record Paleontologists (scientists who study fossils) have discovered many transitional forms that link past and present. Transitional fossils include evidence that birds descended from one branch of dinosaurs and whales descended from four-legged land mammals. © 2013 Pearson Education, Inc. had a “common heritage” or that one was derived from the other. 36

Figure 13.7-3 Figure 13.7 A transitional fossil linking past and present (step 3)

Biogeography Biogeography, the study of the geographic distribution of species, first suggested to Darwin that today’s organisms evolved from ancestral forms. Darwin noted that Galápagos animals resembled species of the South American mainland more than they resembled animals on similar but distant islands. © 2013 Pearson Education, Inc. 38

Biogeography Many examples from biogeography would be difficult to understand, except from an evolutionary perspective. One example is the distribution of marsupial mammals in Australia. © 2013 Pearson Education, Inc. 39

Australia Common ringtail possum Koala Common wombat Red kangaroo Figure 13.8 Australia Common ringtail possum Koala Common wombat Red kangaroo Figure 13.8 Biogeography

Comparative Anatomy Vestigial structures are remnants of features that served important functions in an organism’s ancestors and now have only marginal, if any, importance. © 2013 Pearson Education, Inc. 41

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 and form parts of the ear and throat in humans. Comparative embryology of vertebrates supports evolutionary theory. © 2013 Pearson Education, Inc. 42

Pharyngeal pouches Post-anal tail Chicken embryo Human embryo Figure 13.10 Pharyngeal pouches Post-anal tail Chicken embryo Human embryo Figure 13.10 Evolutionary signs from comparative embryology

Molecular Biology The hereditary background of an organism is documented in its DNA and the proteins encoded by the DNA. Evolutionary relationships among species can be determined by comparing genes and proteins of different organisms. © 2013 Pearson Education, Inc. 44

Percent of selected DNA sequences that match a chimpanzee’s DNA Figure 13.11 Percent of selected DNA sequences that match a chimpanzee’s DNA Primate 92% 96% 100% Chimpanzee Human Gorilla Orangutan Gibbon Old World monkey Figure 13.11 Genetic relationships among some primates

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. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students must understand that the environment does the selecting (editing) in natural selection. Species do not evolve because of want or need. Biological diversity exists and the environment selects. Evolution is not deliberate, it is reactive. 2. A related point is what we learn from extinction. As Darwin noted, species become extinct because they do not get what they want or need! 3. The authors note that evolution is not goal directed and does not lead to perfectly adapted organisms. These excellent points represent two misunderstandings: (a) Instructors need to be clear that evolutionary change is a consequence of an immediate advantage, not a distant goal. Three-chambered hearts in amphibians evolved from two-chambered fish hearts because the three-chambered hearts conveyed an advantage, not because evolution was directed toward producing a four-chambered heart! (b) Evolutionary change only reflects “improvement” in the context of the immediate environment. What is better “today” may not be better “tomorrow.” Thus, species do not steadily get “better,” they respond to the environment or go extinct. 4. Students require practice with evolutionary trees to understand their meaning. For example, students may struggle to see the relationships as a family tree, or understand how time is represented. Teaching Tips 1. An analogy might be made between the specialized functions of finch beaks and the many types of screwdrivers that exist today. Each type of screwdriver (Phillips, flathead, long-handled, interchangeable tips) represents a specialization for a particular job. 2. Based on the human condition, students are likely to think that reproduction is largely a choice, and less a consequence of good health or other adaptations. Yet, in our natural world, there are thousands of examples of the overproduction of offspring and resulting death or failure to reproduce. Thousands of acorns hanging from one tree, millions of spores escaping from a puffball, or a salmon spawning thousands of eggs, are all easy examples of overproduction. Provide a few solid illustrations from your geographic region to bring home this point. 3. Challenge students to explain whether the evolutionary tree in Figure 13.17 would be different if the groups “Ostriches” and “Hawks” were reversed, such that the “Hawk” group would be positioned closer to “Crocodiles.” It would not. 4. Consider pointing out that evolutionary trees are testable hypotheses about evolutionary relationships. 46

(a) The large ground finch Figure 13.12 (a) The large ground finch (c) The woodpecker finch (b) The warbler finch Figure 13.12 Galápagos finches with beaks adapted for specific diets

Darwin’s Theory of Natural Selection Darwin based his theory of natural selection on two key observations. All species tend to produce excessive numbers of offspring. Organisms vary, and much of this variation is heritable. © 2013 Pearson Education, Inc. . 48

Darwin’s Theory of Natural Selection Observation 1: Overproduction and competition All species have the potential to produce many more offspring than the environment can support. This leads to inevitable competition among individuals. © 2013 Pearson Education, Inc. 49

Darwin’s Theory of Natural Selection Observation 2: Individual variation Variation exists among individuals in a population. Much of this variation is heritable. © 2013 Pearson Education, Inc. 50

Figure 13.14 Figure 13.14 Color variation within a population of Asian lady beetles

Darwin’s Theory of Natural Selection Inference: Unequal reproductive success (natural selection) Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring. © 2013 Pearson Education, Inc. 52

Natural Selection in Action Examples of natural selection include pesticide-resistant insects, antibiotic-resistant bacteria, and drug-resistant strains of HIV. Blast Animation: Evidence for Evolution: Antibiotic Resistance in Bacteria Blast Animation: Natural Selection © 2013 Pearson Education, Inc. 53

Insecticide application Figure 13.15-3 Insecticide application Chromosome with gene conferring resistance to pesticide Survivors Reproduction Figure 13.15 Evolution of pesticide resistance in insect populations (step 3)

(a) A flat-tailed horned lizard Live Killed Figure 13.16 (a) A flat-tailed horned lizard Live Killed 20 Length (mm) Live 10 Killed Rear horns Side horns (tip to tip) (b) The remains of a lizard impaled by a shrike (c) Results of measurement of lizard horns Figure 13.16 The effect of predation on the evolution of lizard horn length

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. The tips of millions of twigs represent the species living today. © 2013 Pearson Education, Inc. 56

Lungfishes Amphibians Tetrapods Mammals Amniotes Tetrapod limbs Figure 13.17 Common ancestor of lineages to the right Lungfishes Amphibians 1 Tetrapods Mammals 2 Amniotes Tetrapod limbs Lizards and snakes 3 Amnion 4 Crocodiles 5 Ostriches Birds Homologous trait shared by all groups to the right 6 Feathers Hawks and other birds Figure 13.17 An evolutionary tree of tetrapods (four-limbed animals)

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 and the smallest biological unit that can evolve. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students may suggest that individuals evolve. As this chapter section clarifies, populations are the smallest units that can evolve. Individuals cannot have diversity from which to select. However, individuals can change during their lifetime in response to activities. Muscles can grow stronger because of use. However, these individual changes are not passed on to the next generation (after all, boys have been circumcised for thousands of years but are still born with a foreskin). 2. Another misperception is that evolution results from need. Challenge your students to explain how “need” and “want” have nothing to do with evolution (because neither “need” nor “want” can generate genetic variation!). 3. Students may think of mutations in a positive sense, as if they come as needed; yet mutations in key genes are the cause of cancer and other diseases. Only rarely does a mutation lead to a change that increases the chances for survival. However, these rare events can become significant if given enough time. (As the authors note: A random mutation is like a shot in the dark; it is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance.) Teaching Tips 1. It might be interesting to discuss with students whether the Internet would have helped Mendel and Darwin. Is the Internet facilitating scientific communication? Has this technology created new problems in the process? 2. Try to find good, local examples of populations. If you live near a seashore, the many invertebrate populations (starfish, sea urchin, and kelp) would be great. Further inland we find somewhat isolated populations of fish and continuous and clumped populations of squirrels, separated by vast fields of corn, wheat, or soybeans! Bring the subject home with local examples. 3. No doubt about it, the Hardy-Weinberg equation is problematic for some students. Students should create a quick reference key to the definitions of the elements. Consider some practice problems varying the value of p and q. 4. Heterozygotes can form in two ways, the recessive from mom, the dominant from dad—or the reverse. This should serve to remind students that the 2pq portion of the equation represents the heterozygotes. 5. Another example that can be used for Hardy-Weinberg practice is cystic fibrosis. Cystic fibrosis strikes about one out of every 3,300 Caucasian children. It results from the homozygous recessive condition. Thus, q2 would equal 1/3,300 = 0.000303 and q = the square root of 0.000303 = 0.0174. The frequency of carriers = 2pq = 2  0.0174  0.9826 = 0.0342 = 3.42%, about one in every 29 Caucasian people. (Source of information is Fertil. Steril. 2006 Jan 85(1): 135–8.) 58

Populations as the Units of Evolution 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. © 2013 Pearson Education, Inc. 59

Genetic Variation in Populations Individual variation abounds in all species. Not all variation in a population is heritable. Only the genetic component of variation is relevant to natural selection. Animation: Genetic Variation from Sexual Recombination © 2013 Pearson Education, Inc. 60

Genetic Variation in Populations 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. © 2013 Pearson Education, Inc. 61

Sources of Genetic Variation Genetic variation results from processes that both involve randomness: mutations, changes in the nucleotide sequence of DNA, and sexual recombination, the shuffling of alleles during meiosis. © 2013 Pearson Education, Inc. 62

Sources of Genetic Variation For any given gene locus, 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 mutation as the only source of genetic variation. © 2013 Pearson Education, Inc. 63

Analyzing Gene Pools A gene pool consists of all the alleles in a population at any one time and is a reservoir from which the next generation draws its alleles. Alleles in a gene pool occur in certain frequencies. © 2013 Pearson Education, Inc. 64

Analyzing Gene Pools 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, and p + q = 1. Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele. © 2013 Pearson Education, Inc. 65

Analyzing Gene Pools Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving). The Hardy-Weinberg formula p2 + 2pq + q2 = 1 can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles. © 2013 Pearson Education, Inc. 66

Figure 13.20 Figure 13.20 A population of wildflowers with two varieties of color

Allele frequencies p  0.8 (R) q  0.2 (r) Eggs R r p  0.8 q  0.2 RR Figure 13.21 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 pq  0.16 r q  0.2 p2  0.64 q2  0.04 Genotype frequencies 2pq  0.32 (RR) (Rr) (rr) Figure 13.21 A mathematical swim in the gene pool

Population Genetics and Health Science The Hardy-Weinberg formula can be used to calculate the percentage of a human population that carries the allele for a particular inherited disease. © 2013 Pearson Education, Inc. 69

Population Genetics and Health Science PKU is a recessive allele that prevents the breakdown of the amino acid phenylalanine and occurs in about one out of every 10,000 babies born in the United States. People with PKU must strictly regulate their dietary intake of the amino acid phenylalanine. © 2013 Pearson Education, Inc. 70

INGREDIENTS: SORBITOL, MAGNESIUM STEARATE, ARTIFICIAL FLAVOR, Figure 13.22 INGREDIENTS: SORBITOL, MAGNESIUM STEARATE, ARTIFICIAL FLAVOR, ASPARTAME† (SWEETENER), ARTIFICIAL COLOR (YELLOW 5 LAKE, BLUE 1 LAKE), ZINC GLUCONATE. †PHENYLKETONURICS: CONTAINS PHENYLALANINE Figure 13.22 A warning to individuals with PKU

Microevolution as Change in a Gene Pool How can we tell if a population is evolving? A non-evolving population is in genetic equilibrium, also known as Hardy-Weinberg equilibrium, meaning the population’s gene pool is 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. © 2013 Pearson Education, Inc. 72

MECHANISMS OF EVOLUTION The main causes of evolutionary change are genetic drift, gene flow, and natural selection. Natural selection is the most important, because it is the only process that promotes adaptation. © 2013 Pearson Education, Inc. ”! 73

Genetic Drift Genetic drift is a change in the gene pool of a small population due to chance. Bioflix Animation: Mechanisms of Evolution Animation: Causes of Evolutionary Change © 2013 Pearson Education, Inc. 74

Only 5 of 10 plants leave offspring Only 2 of 10 plants leave Figure 13.23-3 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 rr RR RR Rr RR RR Rr Rr Rr RR RR Generation 1 Generation 2 Generation 3 p  0.7 q  0.3 p  0.5 q  0.5 p  1.0 q  0.0 Figure 13.23 Genetic drift (step 3)

Passing through a “bottleneck,” a severe reduction in population size, The Bottleneck Effect The bottleneck effect is an example of genetic drift and results from a drastic reduction in population size. Passing through a “bottleneck,” a severe reduction in population size, decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, and results in a loss of individual variation and hence adaptability. © 2013 Pearson Education, Inc. ”! 76

Original population Figure 13.24-1 Figure 13.24 The bottleneck effect (step 1)

Original population Bottleneck event Figure 13.24-2 Original population Bottleneck event Figure 13.24 The bottleneck effect (step 2)

Original population Bottleneck event Surviving population Figure 13.24-3 Original population Bottleneck event Surviving population Figure 13.24 The bottleneck effect (step 3)

Cheetahs appear to have experienced at least two genetic bottlenecks: The Bottleneck Effect Cheetahs appear to have experienced at least two genetic bottlenecks: during the last ice age, about 10,000 years ago, and during the 1800s, when farmers hunted the animals to near extinction. With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges. © 2013 Pearson Education, Inc. 80

Figure 13.25 Figure 13.25 Implications of the bottleneck effect in conservation biology

This represents genetic drift in a new colony. The Founder Effect The founder effect is likely when a few individuals colonize an isolated habitat. This represents genetic drift in a new colony. The founder effect explains the relatively high frequency of certain inherited disorders in some small human populations. © 2013 Pearson Education, Inc. 82

Africa South America Tristan da Cunha Figure 13.26 Figure 13.26 Residents of Tristan da Cunha in the early 1900s

Figure 13.26a Figure 13.26 Residents of Tristan da Cunha in the early 1900s (part 1)

Africa South America Tristan da Cunha Figure 13.26b Figure 13.26 Residents of Tristan da Cunha in the early 1900s (part 2)

Gene Flow Gene flow is another source of evolutionary change, is separate from genetic drift, is genetic exchange with another population, may result in the gain or loss of alleles, and tends to reduce genetic differences between populations. © 2013 Pearson Education, Inc. 86

Natural Selection: A Closer Look Of all causes of microevolution, only natural selection promotes adaptation. Evolutionary adaptation results from chance, in the random generation of genetic variability, and sorting, in the unequal reproductive success among the varying individuals. © 2013 Pearson Education, Inc. 87

(a) Sexual dimorphism in a finch species (b) Competing for mates Figure 13.30 (a) Sexual dimorphism in a finch species (b) Competing for mates Figure 13.30 Sexual dimorphism

Evolution Connection: An Evolutionary Response to Malaria We can see the results of past natural selection in present-day humans. Malaria first emerged as a serious threat to people in Africa just 10,000 years ago, long after humans had established populations around the globe, therefore only producing evolutionary responses in malarial regions. © 2013 Pearson Education, Inc. “winning streak”! 89

Evolution Connection: An Evolutionary Response to Malaria Sickle hemoglobin is a mutation that denies the malarial parasite essential access to human hemoglobin and distorts the shape of red blood cells. Individuals with one copy of this sickle allele (heterozygotes) are relatively resistant to malaria. Individuals with two copies (homozygotes) are usually fatally ill. © 2013 Pearson Education, Inc. one amazing “winning streak”! 90

Evolution Connection: An Evolutionary Response to Malaria In the African tropics, malaria is most common and the frequency of the sickle-cell allele is highest. © 2013 Pearson Education, Inc. 91

Asia Africa Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Figure 13.31 Colorized SEM Asia Africa Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% Areas with high incidence of malaria 10.0–12.5% 12.5% Figure 13.31 Mapping malaria and the sickle-cell allele

Figure 13.31b Colorized SEM Figure 13.31 Mapping malaria and the sickle-cell allele (part 2)

Frequency of homozygotes for one allele Frequency of heterozygotes Figure 13.UN02 Frequency of homozygotes for one allele Frequency of heterozygotes Frequency of homozygotes for alternate allele Figure 13.UN02 In-text figure, Hardy-Weinberg formula, p. 259

Overproduction of offspring Figure 13.UN09 Observations Conclusion Overproduction of offspring Natural selection: unequal reproductive success Individual variation Figure 13.UN09 Summary of Key Concepts: Darwin’s Theory of Natural Selection