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Evolution and Diversity Evolution of Life VI part 27 All living things are descended from the first cell(s) and are adapted to their environment. 549 Microbiology 28 Some viruses, bacteria, protists, and fungi cause human diseases, but most bacteria, protists, and fungi are free-living and perform environmental services. 575 Plants 29 Plants are photosynthetic organisms adapted to live on land. Reproduction in seed plants does not require a watery medium. 607 Animals: Part I 30 Animals are heterotrophic organisms that must take in organic food. Animals evolved in the water, and only certain forms live on land. 625 Animals: Part II 31 The more complex animals are divided into two major groups according to the way they develop. Arthropods in one group and vertebrates in the other have jointed skeletons suitable for locomotion on land. 649 548 mad86751_ch27_548-574.indd 548 12/21/06 7:31:51 PM We tend to think of evolution as happening over long timescales. However, we have recently 27 chapter learned that human activities can accelerate the process of evolution quite rapidly—even over a few years! For example, farmers are continuously Evolution of Life challenged by the fact that insects evolve pesticide resistance (see the Health Focus at the end of this chapter). That is, when new pesticides are sprayed, a certain (usually small) proportion of insect populations are resistant to the pesticide. These individuals survive and reproduce, while those that are susceptible die. Thus, over time, the proportion of resistant individuals increases to the point that the pesticide is no longer effective and crop damage increases. Then, new pesticides have to be used or developed. C h a p t e r C o n c e p t s 27.1 Origin of Life ■ What type of evolution preceded biological evolution? Explain. 550–551 ■ What conditions were needed for a true cell to come into being? 551–552 27.2 Evidence of Evolution ■ What types of evidence show that common descent with modification has occurred? 552–558 27.3 The Process of Evolution ■ What type of change occurs when a population evolves? Do individual organisms evolve? 559–562 ■ What five agents lead to evolutionary changes? 562–566 ■ Which agent allows populations to become better adapted to their environment? 564 27.4 Speciation Similar problems have arisen with ■ How does speciation occur? 566–569 antibiotics; much in the same way, ■ What is meant by the phrase “the pace of speciation”? 567 bacteria evolve resistance to antibiotics, making some ineffective for treating certain infections. The good news is that our understanding of evolutionary 27.5 Classification ■ What are the basic categories of classification? 569 ■ What does evolution have to do with classification of organisms? 569 ■ What are the five kingdoms and the three domains? What kingdoms are in Domain Eukarya? 569–570 ■ How do evolutionary trees of traditionalists differ from cladograms done by phylogeneticists? 571 biology has helped change human behavior to deal with the evolution of resistance in pests and bacteria. For example, many farmers now use “integrated pest management,” which involves rotating pesticide types as well as using natural enemies (predators) to fight crop pests. Similarly, doctors no longer prescribe antibiotics unless they are Colorado potato beetles, Leptinotarsa decimlineata relatively certain a patient has a bacterial infection, and they tend to prescribe the minimum strength antibiotic to do the job. These are examples of why evolution is important in people’s everyday lives. In this chapter, you will learn about evidence that indicates evolution has occurred and about the way the evolutionary process works. mad86751_ch27_548-574.indd 549 12/21/06 7:31:56 PM 550 27.1 Part Six Evolution and Diversity Origin of Life In order to understand evolution, it is first important to understand how life began. The common ancestor for all living things was the first cell or cells. The planet Earth was in existence a long time before the first cell arose—over a billion years, in fact. Earth is 4.6 billion years old, and the earliest fossils of prokaryotes are 3.5 billion years old. A billion years is about 13 million human life spans, assuming humans live 75 years. The origin of the first cell is an event of low probability, because a complex series of events would have had to occur—but this length of time is long enough for an event of low probability to have occurred. Today we do not believe that life arises spontaneously from nonlife, and we say that “life comes only from life.” However, the very first living thing had to have come from nonliving chemicals. Under the conditions of early Earth, it is possible that a chemical reaction produced the Biological Evolution photosynthesis cellular respiration cell DNA RNA origin of genetic code first cell(s). A particular mix of inorganic chemicals could have reacted to produce small organic molecules such as glucose, amino acids, and nucleotides. Then these would have polymerized into macromolecules. Once a plasma membrane formed, a structure called a protocell could have come into existence (Fig. 27.1). Evolution of Small Organic Molecules Most chemical reactions take place in water, and the first protocell undoubtedly arose in the ocean. But where? One possibility is that the protocell formed on the surface of the seas and in seaside pools where much energy was available. Ultraviolet radiation was intense in these areas because there was no ozone shield, the layer of O3 that today blocks much of the ultraviolet radiation coming from the sun. Oxygen molecules later reacted with one another to produce the ozone shield. In 1953, Stanley Miller and Harold Urey performed an experiment (known as the Miller-Urey experiment) that supports the hypothesis that small organic molecules were formed at the ocean’s surface. In the early Earth, volcanoes erupted constantly, and the first atmospheric gases would have consequently contained methane (CH 4 ), ammonia (NH3), and hydrogen (H2). These gases could then have been washed into the ocean by the first rains. Fierce lightning and unabated ultraviolet radiation would have allowed them to react and produce the first organic molecules. protocell electrode aggregation macromolecules plasma membrane Chemical Evolution polymerization small organic molecules energy capture stopcock for adding gases stopcock for withdrawing liquid abiotic synthesis electric spark CH4 NH3 H2 H2O condenser gases hot water out cool water in liquid droplets inorganic chemicals boiler cooling early Earth heat small organic molecules Figure 27.1 Origin of the first cell(s). Figure 27.2 Miller and Urey’s apparatus and A chemical evolution may have produced the first cell. First, inorganic chemicals reacted to produce small organic molecules, which polymerized to form macromolecules. With the origination of the plasma membrane, the first primitive cell (a protocell) evolved, and once this cell could replicate, life began. Gases that were thought to be present in the early Earth’s atmosphere were admitted to the apparatus, circulated past an energy source (electric spark), and cooled to produce a liquid that could be withdrawn. Upon chemical analysis, the liquid was found to contain various small organic molecules. mad86751_ch27_548-574.indd 550 experiment. 12/21/06 7:32:01 PM 551 Chapter Twenty–Seven Evolution of Life Macromolecules plume of hot water rich in iron-nickel sulfides hydrothermal vent Figure 27.3 Chemical evolution at hydrothermal vents. Minerals that form at deep-sea hydrothermal vents like this one can catalyze the formation of ammonia and even organic molecules. To test the hypothesis of chemical evolution, Miller placed the inorganic chemicals mentioned in a closed system, heated the mixture, and circulated it past an electric spark (simulating lightning). After a week, the solution contained a variety of amino acids and organic acids (Fig. 27.2). This and other similar experiments support the hypothesis that inorganic chemicals in the absence of oxygen (O2) and in the presence of a strong energy source can result in organic molecules. Is there any other place in the oceans where life could have evolved? Scientists have discovered mid-oceanic ridges within the depths of the sea. Hydrothermal (hot water) vents (openings) occur in the region of mid-oceanic ridges. A vent can be huge, measuring 10–15 meters wide with sides about 15–20 meters high. Hot water spewing out of these vents contains a mix of iron-nickel sulfides. Amazingly, scientists have discovered communities of organisms, including tube worms and giant clams, living in the regions of hydrothermal vents. It is possible that in the past, the right combination of conditions occurred at these vents to initiate life (Fig. 27.3). The formation of small organic molecules is thought to be the first step toward the origination of a cell, from which other life-forms evolved. mad86751_ch27_548-574.indd 551 Once formed, the first small organic molecules gave rise to still larger molecules and then macromolecules. There are three primary hypotheses concerning this stage in the origin of life. One is the RNA-first hypothesis, which suggests that only the macromolecule RNA (ribonucleic acid) was needed at this time to progress toward formation of the first cell or cells. Scientists formulated this hypothesis after discovering that RNA can sometimes be both a genetic substrate and an enzyme. Such RNA molecules are called ribozymes. The first genes and enzymes could thus both have been composed of RNA, since we now know that ribozymes exist. Scientists who support this hypothesis say it was an “RNA world” some 4 billion years ago. Another hypothesis is termed the protein-first hypothesis. Sidney Fox has shown that amino acids polymerize abiotically (without life) when exposed to dry heat. He suggests that amino acids collected in shallow puddles along the rocky shore, and the heat of the sun caused them to form proteinoids, small polypeptides that have some catalytic properties. When proteinoids are returned to water, they form microspheres, structures composed only of protein that have many of the properties of a cell. Some of these proteins could have had enzymatic properties. This hypothesis assumes that DNA genes came after protein enzymes arose. The third hypothesis is put forth by Graham CairnsSmith. He believes that clay was especially helpful in causing the polymerization of both proteins and nucleic acids at the same time. Clay attracts small organic molecules and contains iron and zinc, which may have served as inorganic catalysts for polypeptide formation. In addition, clay tends to collect energy from radioactive decay and then discharge it when the temperature or humidity changes, possibly providing a source of energy for polymerization. Cairns-Smith suggests that RNA nucleotides and amino acids became associated in such a way that polypeptides were ordered by, and helped synthesize, RNA. Chemical reactions likely produced the macromolecules we associate with living things. The Protocell After macromolecules formed, something akin to a modern plasma membrane was needed to separate them from the environment. Thus, before the first true cell arose, there would likely have been a protocell, a structure that had a lipid-protein membrane and carried on energy metabolism. Fox has shown that if lipids are made available to microspheres, the two tend to become associated, producing a lipid-protein membrane (Fig. 27.4a). 12/21/06 7:32:11 PM 552 Part Six Evolution and Diversity Figure 27.4 Protocell components. a. Microspheres, which are composed only of protein, have a number of cellular characteristics and could have evolved into the protocell. b. Liposomes form automatically when phospholipid molecules are put into water. Plasma membranes may have evolved similarly. a. Aleksandr Oparin, a Soviet biochemist, showed that under appropriate conditions of temperature, ionic composition, and pH, concentrated mixtures of macromolecules can give rise to complex units called coacervate droplets. Coacervate droplets have a tendency to absorb and incorporate various substances from the surrounding solution. Eventually, a semipermeable boundary may form around the droplet. In a liquid environment, phospholipid molecules automatically form droplets called liposomes (Fig. 27.4b). Perhaps the first plasma-like membrane formed in this manner. However it happened, development of the plasma membrane was key because it separated the genetic material from the outside environment. The Heterotroph Hypothesis It has been suggested that the protocell likely was a heterotroph, an organism that takes in preformed food. During the early evolution of life, the ocean contained abundant nutrition in the form of small organic molecules. This suggests that heterotrophs preceded autotrophs, organisms that make their own food. At first, the protocell may have used preformed ATP, but as this supply dwindled, cells that could extract energy from carbohydrates to transform ADP to ATP were favored. Glycolysis is a common metabolic pathway in living things, and this testifies to its early evolution in the history of life. Since there was no free oxygen, we can assume that the protocell carried on a form of fermentation. It seems logical that the protocell at first had limited ability to break down organic molecules and that it took millions of years for glycolysis to evolve completely. b. called reverse transcriptase that uses RNA as a template to form DNA, which then undergoes protein formation. Perhaps, with time, reverse transcription gave rise to DNA genes. Once DNA genes existed, they may have specified proteins. According to the protein-first hypothesis, proteins, or at least polypeptides, were the first of the three molecules (i.e., DNA, RNA, and protein) to arise. Only after the protocell developed sophisticated enzymes did it have the ability to synthesize DNA and RNA from small molecules provided by the ocean. Researchers point out that because nucleic acids are very complicated molecules, the likelihood that RNA arose on its own is minimal. Cairns-Smith proposes that polypeptides and RNA evolved simultaneously. Therefore, the first true cell would have contained RNA genes that could have replicated because of the presence of proteins. This eliminates the baffling chicken-and-egg paradox: which came first, proteins or RNA? It does mean, however, that two unlikely events would have had to happen at the same time. Once the protocells acquired genes that could replicate, they became cells capable of reproducing, and biological evolution began. The hypothesis that the origin of life followed a transition from small organic molecules to macromolecules to protocells to true cells is currently widely favored by scientists. However, recently some scientists have argued that early cells may have come from asteroids from Mars that hit the Earth. Recent expeditions to Mars have shown that water possibly existed there in the past. Nonetheless, it is thought that cells, the basic building block of life, arose at some point from nonliving matter. The True Cell A true cell is a membrane-bounded structure that can carry on protein synthesis to produce the enzymes that allow DNA to replicate. The central concept of genetics states that DNA directs protein synthesis and that information flows from DNA to RNA to protein. It is possible that this sequence developed in stages. According to the RNA-first hypothesis, RNA would have been the first genetic material to evolve, and the first true cell would have had RNA genes. These genes would have directed and enzymatically carried out protein synthesis, as in ribozymes. Also, today we know that some viruses have RNA as their genetic material. These viruses have a protein enzyme mad86751_ch27_548-574.indd 552 Once the protocell was capable of reproduction, it became a true cell, and biological evolution began. 27.2 Evidence of Evolution Evolution is all the changes that have occurred in living things since the beginning of life due to differential reproductive success. That is, some individuals reproduce more than others because they are better “fit” to their environment. Table 27.1 indicates that Earth is about 4.6 billion 12/21/06 7:32:13 PM 553 Chapter Twenty–Seven Evolution of Life years old and that prokaryotes, probably the first living organisms, evolved about 3.5 billion years ago. The eukaryotic cell arose about 2.1 billion years ago, but multicellularity didn’t begin until perhaps 700 million years ago. This means that only unicellular organisms were present for 80% of the time that life has existed on Earth. Most evolutionary events we will be discussing in future chapters occurred in less than 20% of the history of life! Evolution is defined as “common descent.” Because of descent with modification, all living things share the same fundamental characteristics: they are made of cells, take chemicals and energy from the environment, respond to external stimuli, and reproduce. Living things are diverse because individual organisms exist in the many environments throughout the Earth, and the features that enable them to survive in those environments are quite diverse. Many fields of biology provide evidence that evolution through descent with modification occurred in the past and is still occurring. Let us look at the various types of evidence for evolution. wing feathers feet a. Figure 27.5 Transitional fossils. teeth tail with vertebrae a. Archaeopteryx was a transitional link between reptiles and birds. Fossils indicate it had feathers and wing claws. Most likely, it was a poor flier. Perhaps it ran over the ground on strong legs and climbed into trees with the assistance of these claws. b. Archaeopteryx also had a feather-covered, reptilian tail that shows up well in this artist’s representation. (Orange labels=reptilian characteristics; green label=bird characteristic.) mad86751_ch27_548-574.indd 553 Fossils are the remains and traces of past life or any other direct evidence of past life. Most fossils consist only of hard parts of organisms, such as shells, bones, or teeth, because these are usually preserved after death. The soft parts of a dead organism are often consumed by scavengers or decomposed by bacteria. Occasionally, however, an organism is buried quickly and in such a way that decomposition is never completed or is completed so slowly that the soft parts leave an imprint of their structure. Traces include trails, footprints, burrows, worm casts, or even preserved droppings. The great majority of fossils are found embedded in sedimentary rock. Sedimentation, a process that has been going on since Earth formed, can take place on land or in bodies of water. The weathering and erosion of rocks produces particles that vary in size and are called sediment. As such particles accumulate, sediment becomes a stratum (pl., strata), a recognizable layer of rock. Any given stratum is older than the one above it and younger than the one immediately below it, so that the relative age of fossils can be determined based on their depth. Paleontologists are biologists who study the fossil record and from it draw conclusions about the history of life. Particularly interesting are the fossils that serve as transitional links between groups. For example, the famous fossils of Archaeopteryx are intermediate between reptiles and birds (Fig. 27.5). The dinosaur-like skeleton of this fossil has reptilian features, including jaws with teeth and a long, jointed tail, but Archaeopteryx also had feathers and wings, all suggesting that reptiles evolved from birds. Other transitional links among fossil vertebrates suggest that fishes evolved before amphibians, which evolved before reptiles, which evolved before both birds and mammals in the history of life. wing head tail Fossil Evidence claws b. 12/21/06 7:32:19 PM 554 Part Six Evolution and Diversity TABLE 27.1 Era The Geological Timescale: Major Divisions of Geological Time and Some of the Major Evolutionary Events of Each Time Period Period Epoch Millions of Years Ago Plant Life Animal Life Holocene 0.01 Human influence on plant life Age of Homo sapiens Significant Mammalian Extinction Quaternary Cenozoic* Tertiary Pleistocene Herbaceous plants spread and diversify. Presence of ice age mammals. Modern humans appear. First hominids appear. Pliocene (5–1.8) Herbaceous angiosperms flourish. Miocene (23–25) Grasslands spread as forests contract. Oligocene (36–23) Many modern families of flowering plants evolve. Browsing mammals and monkeylike primates appear. Eocene (57–36) Subtropical forests with heavy rainfall thrive. All modern orders of mammals are represented. Paleocene (65–57) Flowering plants continue to diversify. Primitive primates, herbivores, carnivores, and insectivores appear. Apelike mammals and grazing mammals flourish; insects flourish. Mass Extinction: Dinosaurs and Most Reptiles Mesozoic Cretaceous (144–65) Flowering plants spread; conifers persist. Placental mammals appear; modern insect groups appear. Jurassic (231–144) Flowering plants appear. Dinosaurs flourish; birds appear. Mass Extinction Triassic (248–118) Forests of conifers and cycads dominate. First mammals appear; first dinosaurs appear; corals and molluscs dominate seas. Mass Extinction Permian (280–248) Gymnosperms diversify. Reptiles diversify; amphibians decline. Carboniferous (360–280) Age of great coal-forming forests: ferns, club mosses, and horsetails flourish. Amphibians diversify; first reptiles appear; first great radiation of insects. Mass Extinction Paleozoic Devonian (408–360) First seed plants appear. Seedless vascular plants diversify. Jawed fishes diversify and dominate the seas; first insects and first amphibians appear. Silurian (438–408) Seedless vascular plants appear. First jawed fishes appear. Mass Extinction Precambrian Time Ordovician 488.3 Nonvascular land plants appear. Marine algae flourish. Invertebrates spread and diversify; jawless fishes (first vertebrates) appear. Cambrian 542 First plants appear on land. Marine algae flourish. All invertebrate phyla present; first chordates appear. 600 Oldest soft-bodied invertebrate fossils 1,400–700 Protists evolve and diversify. 2,200 Oldest eukaryotic fossils 2,700 O2 accumulates in atmosphere. 3,500 Oldest known fossils (prokaryotes) 4,600 Earth forms. * Many authorities divide the Cenozoic era into the Paleogene period (contains the Paleocene, Eocene, and Oligocene epochs) and the Neogene period (contains the Miocene, Pliocene, Pleistocene, and Holocene epochs). mad86751_ch27_548-574.indd 554 12/21/06 7:32:26 PM Figure 27.6 Dinosaurs of the late Cretaceous period. Parasaurolophus walkeri, although not as large as other dinosaurs, was one of the largest plant-eaters of the late Cretaceous period. The crest atop its head was about 2 meters long and was used to make booming calls. Also living at this time were the rhino-like dinosaurs represented here by Triceratops (left), another herbivore. Geological Timescale As a result of studying strata, scientists have divided Earth’s history into eras, and then periods and epochs (Table 27.1). The fossil record has helped determine the dates given in the table. There are two ways to date fossils. The relative dating method determines the relative order of fossils and strata depending on the layer of rock in which they were found, but it does not determine the actual date they were formed. The absolute dating method relies on radioactive dating techniques to assign an actual date to a fossil. All radioactive isotopes have a particular half-life, the length of time it takes for half of the radioactive isotope to change into another stable element. Carbon 14 (14C) is the only radioactive isotope in organic matter. Assuming a fossil contains organic matter, half of the 14C will have changed to nitrogen 14 (14N) in 5,730 years. To estimate how much 14C was in the organism to begin with, it is assumed that organic matter always begins with the same amount of 14C. Scientists compare the 14C radioactivity of the fossil to that of a modern sample of organic matter. For example, if a fossil has one-fourth the amount of radioactive 14C as a modern sample, then the fossil is approximately 11,460 years old (2 half-lives). After 50,000 years, however, the amount of 14 C radioactivity is so low that it cannot be used to measure the age of a fossil accurately. In that event, certain other ratios of isotopes with longer half-lives can be used to date rocks even billions of years old, and then the age of a fossil contained in the rock can be inferred in a similar way. Using both relative and absolute dating methods, we can learn from fossils about the various organisms and environments that existed across the planet during any time period. Mass Extinctions Extinction is the death of every member of a species. During mass extinctions, a large percentage of species become extinct within a relatively short period of time. So far, there mad86751_ch27_548-574.indd 555 have been five major mass extinctions. These occurred at the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods (see Table 27.1), and a sixth is likely occurring now, probably as a result of human activities (discussed in Chapter 36). Following mass extinctions, the remaining groups of organisms are likely to spread out and fill the habitats vacated by those that have become extinct. It was proposed in 1977 that the Cretaceous extinction (or “Cretaceous crisis”) was due to an asteroid that exploded, producing meteorites that fell to Earth. A large meteorite striking Earth could have produced a cloud of dust that mushroomed into the atmosphere, blocking out the sun and causing plants to freeze and die. A huge crater that could have been caused by a meteorite involved in the Cretaceous extinction was found in the Caribbean–Gulf of Mexico region on the Yucatán Peninsula. During the Cretaceous period, great herds of dinosaurs roamed the plains, as did Parasaurolophus walkeri and Triceratops (Fig. 27.6), but all dinosaur species went extinct near the end of the Cretaceous period. In 1984, paleontologists found that marine animals have a mass extinction about every 26 million years, and surprisingly, astronomers can offer an explanation. Our sun moves up and down as it orbits in the Milky Way, a starry galaxy. Astronomers predict that when this vertical movement causes our solar system to approach certain other members of the Milky Way, an unstable situation develops that could lead to a meteorite striking Earth. This evidence suggests that mass extinctions can be associated with extraterrestrial events, but these events are not necessarily the only cause of mass extinctions. Fossils allowed scientists to construct the geological timescale that traces the history of life. Several mass extinctions have occurred in the past, possibly due to extraterrestrial events. 12/21/06 7:32:38 PM 556 Part Six Evolution and Diversity Laurasia Laurasia a e ga n Pa Go nd Go n wa n a Permian period ~250 million years ago Figure 27.7 Continental drift. During the Permian period, all the continents were joined into a supercontinent called Pangaea. During the Triassic period, the joined continents of Pangaea began moving apart, forming two large continents called Laurasia and Gondwana. Then all the continents began to separate. This process is continuing today. North America and Europe are presently drifting apart at a rate of about 2 cm per year. Eurasia North America Eurasia Africa South America Africa India India South America Australia Australia Antarctica Cretaceous period 65 million years ago Biogeographical Evidence Another type of evidence that supports evolution through descent with modification is found in the field of biogeography, the study of the distribution of species throughout the world. The world’s six biogeographical regions each have their own distinctive mix of living things. For example, the mammals and flowering plants of North America are different from those in Africa, even though parts of the two continents have similar environmental conditions. If you want to see zebras and lions, you have to go to Africa, not to the midwestern United States. Similarly, cactuses flourish in the deserts of North America, but euphorbias, not cactuses, occupy similar arid habitats of Africa. What is the best explanation for this phenomenon? Different mammals and flowering plants evolved separately in each biogeographical region, and barriers such as mountain ranges and oceans prevented them from migrating to other regions. Many of these barriers arose through a process called continental drift. That is, the continents have never been fixed; rather, their positions and the positions of the oceans have changed over time (Fig. 27.7). During the Permian period, all the present landmasses belonged to one continent and then later drifted apart. As evidence of this, fossils of one species of seed fern (Glossopteris) have been found on all the southern continents separated by oceans. This species’ presence on Antarctica is evidence that this continent was not always frozen. In contrast, many Australian species are restricted to that continent, including the majority of marsupials (pouched mammals such as the kangaroo). What is the mad86751_ch27_548-574.indd 556 Jurassic period 144 million years ago Triassic period ~220 million years ago North America dw an a Antarctica Present day explanation for these distributions? Some organisms must have evolved and spread out before the continents broke up; then they became extinct. The distribution of many organisms on Earth is explainable by knowing when they evolved, either before or after the continents moved apart. Anatomical Evidence The fact that anatomical similarities exist among organisms provides further support for evolution via descent with modification. Vertebrate forelimbs are used for flight (birds and bats), orientation during swimming (whales and seals), running (horses), climbing (arboreal lizards), or swinging from tree branches (monkeys). Yet all vertebrate forelimbs contain the same sets of bones organized in similar ways, despite their dissimilar functions. The most plausible explanation for this unity is that the basic forelimb plan belonged to a common ancestor, and then the plan was modified in the succeeding groups as each continued along its own evolutionary pathway. Structures that are anatomically similar because they are inherited from a common ancestor are called homologous structures (Fig. 27.8). In contrast, analogous structures serve the same function, but are not constructed similarly, nor do they share a common ancestry. The wings of birds 12/21/06 7:32:58 PM Chapter Twenty–Seven Evolution of Life and insects and the eyes of octopi and humans are analogous structures and are similar due to a common environment, not common ancestry. The presence of homology, not analogy, is evidence that organisms are related. Vestigial structures are anatomical features that are fully developed in one group of organisms but that are reduced and may have no function in similar groups. Most birds, for example, have well-developed wings for flight. However, some bird species (e.g., ostrich) have greatly reduced wings and do not fly. Similarly, snakes have no use for hindlimbs, and yet some have remnants of hindlimbs in a pelvic girdle and legs. The presence of vestigial structures can be explained by common descent. Vestigial structures occur because organisms inherit their anatomy from their ancestors; they are traces of an organism’s evolutionary history. The homology shared by vertebrates extends to their embryological development (Fig. 27.9). At some time during development, all vertebrates have a postanal tail and exhibit paired pharyngeal pouches. In fishes and amphibian larvae, these pouches develop into functioning gills. In humans, the first pair of pouches becomes the cavity of the middle ear and the auditory tube. The second pair becomes the tonsils, while the third and fourth pairs become the thymus and parathyroid glands. Why should terrestrial vertebrates develop and then modify structures like pharyngeal pouches that have lost their original function? The most likely explanation is that fishes are ancestral to other vertebrate groups. bird humerous ulna radius metacarpals phalanges 557 In 1859, Charles Darwin (See Science Focus, p. 560) speculated that whales evolved from a land mammal. His hypothesis has now been substantiated. In recent years the fossil record has yielded an incredible parade of fossils that link modern whales and dolphins to land ancestors (Fig. 27.10). The presence of a vestigial pelvic girdle and legs in modern whales is also significant evidence. Organisms that share homologous structures are closely related and have a common ancestry. Studies of comparative anatomy and embryological development reveal homologous structures. fish salamander tortoise bat chick whale cat horse human pharyngeal pouches human postanal tail Figure 27.8 Significance of homologous structures. Figure 27.9 Significance of developmental similarities. Although the specific design details of vertebrate forelimbs are different, the same bones are present (note color-coding). Homologous structures provide evidence of a common ancestor. At these comparable developmental stages, vertebrate embryos have many features in common, which suggests they evolved from a common ancestor. (These embryos are not drawn to scale). mad86751_ch27_548-574.indd 557 12/21/06 7:33:34 PM 558 Part Six Evolution and Diversity Figure 27.10 Ancestor to whales. Ambulocetus, an ancestor to modern whales, dated 50 MYA. The presence of limbs is evidence that land-based mammals gave rise to whales. Biochemical Evidence Almost all living organisms use the same basic biochemical molecules, including DNA, ATP (adenosine triphosphate), and many identical or nearly identical enzymes. Further, organisms use the same DNA triplet code for the same 20 amino acids in their proteins. Since the sequences of DNA bases in the genomes of many organisms are now known, it has become clear that humans share a large number of genes with much simpler organisms. It appears that life’s vast diversity has come about by only a slight difference in many of the same genes. The result has been widely divergent types of bodies. yeast Number of Amino Acid Differences Compared to Human Cytochrome c 0 moth fish turtle When the degree of similarity in DNA nucleotide sequences or the degree of similarity in amino acid sequences of proteins is examined, the more similar the DNA sequences are, generally the more closely related the organisms are. For example, humans and chimpanzees are about 99% similar! Cytochrome c is a molecule that is used in the electron transport chain of all the organisms appearing in Figure 27.11. Data regarding differences in the amino acid sequence of cytochrome c show that the sequence in a human differs from that in a monkey by only one amino acid, from that in a duck by 11 amino acids, and from that in a yeast by 51 amino acids. These data are consistent with other data regarding the anatomical similarities of these organisms and, therefore, how closely they are related. Evolution is no longer considered a hypothesis. It is one of the great unifying theories of biology. In science, the word theory is reserved for those conceptual schemes that are supported by a large number of observations and scientific experiments. The theory of evolution has the same status in biology that the germ theory of disease has in medicine. Many lines of evidence support the theory of evolution by descent with modification. Recently, biochemical evidence has also been found to support evolution. A hypothesis is strengthened when it is supported by many different lines of evidence. duck pig monkey human 10 20 30 40 Cytochrome c is a small protein that plays an important role in the electron transport chain within mitochondria of all cells. 50 Figure 27.11 Significance of biochemical differences. The branch points in this diagram indicate the number of amino acids that differ between human cytochrome c and the organisms depicted. These biochemical data are consistent with those provided by a study of the fossil record and comparative anatomy. mad86751_ch27_548-574.indd 558 12/21/06 7:33:41 PM Chapter Twenty–Seven Evolution of Life 27.3 The Process of Evolution Some people have the misconception that individuals evolve; however, evolution occurs at the population level. As evolution takes place, genetic changes occur within a population, and over generations, these lead to phenotypic changes that are commonly seen in that population. In this section we will consider a change in gene frequencies within a population over time, defined as microevolution. Microevolution can be studied using population genetics, which investigates changes in gene frequencies. Population Genetics A population is all the members of a single species that occupy a particular area at the same time and that interbreed and exchange genes. A population could be all the green frogs in a frog pond, all the field mice on a farm, or all the English daisies on a hill. The members of a population reproduce with one another to produce the next generation. Each member of a population is assumed to be free to reproduce with any other member, and when reproduc- p 2 +2 pq+q2 p 2=frequency of homozygous dominant individuals (AA) p=frequency of dominant allele (A) q 2=frequency of homozygous recessive individuals (aa) q=frequency of recessive allele (a) 2 pq=frequency of heterozygous individuals (Aa) Realize that p+q=1 (There are only 2 alleles.) p 2 +2 pq+q 2 =1 (These are the only genotypes so the total frequency of the genotypes in a population must add up to 1, or 100%.) Example: An investigator has determined by inspection that 16% of a human population has a recessive trait. Using this information, we can complete all the genotype and allele frequencies for this population. q 2=16%=0.16 are homozygous recessive individuals Given: q= 0.16=0.4=frequency of recessive allele p=1.0-0.4=0.6=frequency of dominant allele p 2=(0.6)(0.6)=0.36=36% are homozygous dominant individuals 2 pq=2(0.6)(0.4)=0.48=48% are heterozygous individuals or 2 pq=1.00-0.52=0.48 84% have the dominant phenotype Therefore, Figure 27.12 Calculating gene pool frequencies using the Hardy-Weinberg equation. mad86751_ch27_548-574.indd 559 559 tion occurs, the genes of one generation are passed on in the manner described by Mendel’s laws. Therefore, in this so-called Mendelian population (as discussed in Ch. 23) of sexually reproducing individuals, the total number of alleles at all the gene loci in all the members make up a gene pool for the population. It is customary to describe this gene pool in terms of allele frequencies for the various genes. Using this methodology, two investigators, G. H. Hardy, an English mathematician, and W. Weinberg, a German physician, discovered a principle that now bears their names. Hardy and Weinberg decided to use the binomial equation p2+2pq+q2 to calculate the genotype and allele frequencies of a population. Figure 27.12 shows how this is done. Once you know the allele frequencies, you can calculate the ratio of genotypes in the next generation using a Punnett square. The data from Figure 27.12 reveal that the next generation will have exactly the same ratio of genotypes as before: eggs sperm 0.6 L 0.4 l 0.6 L 0.36 LL 0.24 Ll 0.4 l 0.24 Ll 0.16 ll Genotype frequencies: 0.36 LL+0.48 Ll+0.16 ll=1 or L2+2 Ll+l2 It is important to realize that the sperm and eggs represented in this Punnett square are actually the frequencies of alleles L and l in an entire population, not gametes produced by individuals. The Hardy-Weinberg Principle The Hardy-Weinberg principle states that allele frequencies in a gene pool will remain at equilibrium, and thus constant, after one generation of random mating in a large, sexually reproducing population as long as five conditions are met: 1. No mutations. Genetic mutations are an alteration in an allele, due to a change in DNA composition. Under Hardy-Weinberg assumptions, allele changes do not occur, or changes in one direction are balanced by changes in the opposite direction. 2. No genetic drift. Genetic drift is random changes in allele frequencies by chance. If a population is very large, changes in allele frequencies due to chance alone are insignificant. 3. No gene flow. Gene flow is the sharing of alleles between two populations through interbreeding. If there is no gene flow, migration of individuals, and therefore their genes, into or out of the population does not occur. 4. Random mating. Random mating occurs when individuals pair by chance, not according to their genotypes or phenotypes. 5. No selection. Often, the environment selects certain phenotypes to reproduce and have more offspring than other phenotypes. If selection does not occur, no phenotype is favored over another to reproduce. 12/21/06 7:33:43 PM 560 Part Six Evolution and Diversity a. b. Figure 27.13 Microevolution. Both dark-colored and light-colored individuals occur in populations of the peppered moth, Biston betularia. a. When tree trunks are light, dark-colored moths are seen and eaten by predatory birds, and the light-colored moths increase in number. b. When tree trunks are dark due to pollution, light-colored moths are seen and eaten by predatory birds, and the dark-colored moths increase in number. In real life, these conditions are rarely, if ever, met, and allele frequencies in the gene pool of a population do change from one generation to the next. Because a change in allele frequencies is our definition of microevolution, then evolution has occurred. A significance of the HardyWeinberg principle is that microevolution can be detected by noting deviations from a Hardy-Weinberg equilibrium of allele frequencies in the gene pool of a population. Such deviations suggest that one or more of the five conditions is occurring in a population. Figure 27.13 gives an example of microevolution due to selection in a population of peppered moths. Peppered moths can be dark colored or light colored, and the percentage of each in the population can vary. Predatory birds are the selective agent that causes the makeup of the population to vary. When dark-colored moths rest on light trunks in a nonpolluted area, they are seen and eaten by these birds. With pollution, the trunks of trees darken, so light-colored moths stand out and are eaten more than darkcolored moths. We know that evolution has occurred in Figure 27.13 because the population changes from 10% dark-colored phenotype to 80% dark-colored phenotype over time. In this example, evolution has occurred because a selective force (predatory birds) favored one genotype over another. The Hardy-Weinberg principle predicts that allele frequencies in a population will remain constant generation after generation, and this provides a baseline by which to judge whether evolution has occurred. A change of allele frequencies in the gene pool of a population signifies that evolution has occurred. mad86751_ch27_548-574.indd 560 Five Agents of Evolutionary Change The list of conditions for genetic equilibrium stated previously implies that the opposite conditions can cause evolutionary change. These conditions are mutations, genetic drift, gene flow, nonrandom mating, and natural selection. Mutations Mutations are genetic changes that provide the raw material for evolutionary change; mutations create new alleles. For example, a mutation can result in a nucleotide change in a gene. A mutation can be “silent” if the nucleotide change does not result in an amino acid change or if it is recessive and masked by a dominant allele in a diploid organism. If a mutation does, however, affect protein function, it can be harmful to an organism. Mutations are random and are most often thought to result in no change or a negative effect on an individual’s reproductive success. In a changing environment, however, even a seemingly harmful mutation that results in a phenotypic change can be a source of an adaptive variation. For example, the water flea Daphnia ordinarily thrives at temperatures around 20°C, but there is a mutation that requires Daphnia to live at temperatures between 25°C and 30°C. The adaptive value of this mutation is entirely dependent on environmental conditions. Genetic Drift Genetic drift refers to changes in the allele frequencies of a gene pool due to chance, as illustrated by the green and brown frogs in Figure 27.14. As you can imagine, genetic drift has greater effects in smaller populations. For exam- 12/21/06 7:33:50 PM Chapter Twenty–Seven Evolution of Life ple, the chance death of one individual in a population of a million will not have an appreciable effect on allele frequencies, but the chance death of one individual in a population of ten could change the frequency of an allele by 10% or even cause its loss altogether (if that individual was the only one with that allele). In nature, two situations, called founder effect and bottleneck effect, lead to small populations whereby genetic drift can drastically affect allele frequencies in a gene pool. The founder effect occurs when a few individuals form a new colony, and only a fraction of the total genetic diversity of the original gene pool is represented in these individuals. The particular alleles carried by the founders is dictated by chance alone. The Amish population of Lancaster, Pennsylvania, is an isolated religious sect descended from a few German founders. Today, as many as one in 14 individuals in this group carries a recessive allele that causes an unusual form of dwarfism (it affects only the lower arms and legs) and polydactylism (extra fingers) (Fig. 27.15). Genetic drift has caused this proportion to be much higher in the Amish than the non-Amish, where the allele is found in only one in 1,000 people. Sometimes a population is subjected to near extinction because of a natural disaster (e.g., earthquake or fire) or because of human interference. The disaster acts as a bottleneck, preventing the majority of genotypes from breeding to form the next generation. For example, a large genetic similarity found in cheetahs is believed to be due to a bottleneck effect. In a skin grafting study, most cheetahs failed to reject skin grafts from unrelated cheetahs because they were so genetically similar. death Gene Flow Gene flow is the movement of alleles between populations, as occurs when individuals migrate from one population to another and breed in that new population. For example, adult plants are not able to migrate, but their gametes are often either blown by the wind or carried by insects. The wind, in particular, can carry pollen for long distances and can therefore be a factor in gene flow among plant populations. Gene flow among populations keeps their gene pools similar. It also prevents close adaptation to a local environment. Nonrandom Mating Nonrandom mating occurs when individuals pair up, not by chance, but according to their genotypes or phenotypes. Inbreeding, or mating between relatives to a greater extent than by chance, is an example of nonrandom mating. Inbreeding decreases the proportion of heterozygotes and increases the proportions of homozygotes at all gene loci. In a human population, inbreeding increases the frequency of recessive abnormalities (see Fig. 27.15). genetic drift Figure 27.14 Genetic drift. Genetic drift occurs when by chance only certain members of a population (in this case, green frogs) reproduce and pass on their alleles to the next generation. The allele frequencies of the next generation’s gene pool may be markedly different from those of the previous generation, particularly in small populations. mad86751_ch27_548-574.indd 561 561 Figure 27.15 Founder effect. A member of the founding population of Amish in Pennsylvania had a recessive allele for a rare kind of dwarfism linked with polydactylism. The percentage of the Amish population now carrying this allele is much higher compared to that of the general population. 12/21/06 7:33:58 PM Charles Darwin’s Theory of Natural Selection Although Charles Darwin is often credited as the first to believe in descent with modification, biologists before him had slowly begun to accept the idea of evolution. JeanBaptiste de Lamarck (1744–1829), a predecessor of Darwin, concluded after studying the succession of life-forms in geological strata, that more complex organisms are descended from less complex organisms. To explain the process of adaptation to the environment, Lamarck proposed of inheritance of acquired characteristics—that the environment can bring about inherited change. One example he gave—and the one for which he is most famous—is that the long neck of a giraffe developed over time because animals stretched their necks to reach food high in trees and then passed gradually longer necks to their offspring (Fig. 27A). This hypothesis for the inheritance of acquired characteristics has never been substantiated. The molecular mechanism of inheritance explains why. Phenotypic changes acquired during an organism’s lifetime do not result in genetic changes that can be passed to subsequent generations. As an example, consider tail cropping in Doberman pincers. All Doberman puppies are born with tails, even though their parents’ tails are most often cropped. That is, tail cropping is a phenotypic change that is not inherited in the DNA. We now know that Lamarck’s ideas, although important for advancing ideas about evolution, were incorrect. Charles Darwin (1809–1882) came to a different conclusion from Lamarck’s after going on a five-year trip as a naturalist aboard the ship the HMS Beagle. He read a book by Charles Lyell, a geologist who suggested the world is very old and has been undergoing gradual changes for many many years. This meant that there was time for evolution to occur. Because the ship sailed in the tropics of the Southern Hemisphere, Darwin encountered different living things that were more abundant and varied than those found in his native England. When Darwin compared the animals of Africa to those of South America, he noted that the African Early giraffes probably had short necks that they stretched to reach food. Their offspring had longer necks that they stretched to reach food. Eventually, the continued stretching of the neck resulted in today’s giraffe. Figure 27A Jean-Baptiste de Lamarck’s proposal of acquired characteristics. ostrich and the South American rhea, although similar in appearance, were actually different animals. He reasoned that they had a different line of descent because they were on different continents. When he arrived at the Galápagos Islands, he began to study the diversity of finches (see Fig. 27.20), whose adaptations could best be explained by assuming they had diverged from a common ancestor. He found such a hypothetical ancestor on the mainland of South America, supporting his theory. With this type of evidence, Darwin concluded that species evolve (change) with time. When Darwin returned home, he spent the next 20 years gathering data to support the principle of biological evolution. His most significant contribution was his theory of natural selection, which explains how populations of a species become adapted to their environment. This theory is explained in Figure 27B. Before formulating the theory, Darwin read an essay on human population growth written by Thomas Malthus. Malthus observed that although humans have a great reproductive potential, many environmental variables, such as availability of food and living space, tend to keep the human population in check with factors such as disease and famine. Darwin applied these ideas to all populations of organisms. A population is all the members of a species living in one particular place. Darwin calculated that a single pair of elephants could have 19 million descendants in 750 year. He realized that other organisms have an even greater reproductive potential than this pair of elephants yet, usually population sizes remain about the same. Darwin decided there is a constant struggle for existence, whereby only certain members of a population survive to reproduce. Those individuals best adapted to their environment produce the greatest number of offspring, and it is their traits that increase in frequency in a population in successive generations. This so-called “survival of the fittest” causes the next generation to be better adapted to the environment than the previous generation. Darwin’s theory of natural selection was nonteleological, meaning that there 562 mad86751_ch27_548-574.indd 562 12/21/06 7:34:09 PM 563 Chapter Twenty–Seven Evolution of Life is no design or purpose in the works or processes of nature. However, rather than believing that organisms strive to adapt themselves to the environment, Darwin concluded that the environment acts on individual phenotypes to select those individuals that are best adapted. These individuals have been “naturally selected” to pass on their characteristics to the next generation. In contrast, the Lamarckian explanation for the long neck of the giraffe was incorrect because ancestors of the modern giraffe were “trying” to reach into the trees to browse on high-growing vegetation. Lamarck’s proposal is teleological because, according to him, the outcome (longer necks) is predetermined. Darwin’s theory of evolution, rather than being progressive or “forward looking” implies that the changing environment does not move toward any predetermined outcome. The critical elements of Darwin’s theory are as follows: Variations. Individual members of a population vary in physical characteristics. To be affected by natural selection, physical variations must be inherited from generation to generation by reproduction rather than being environmentally induced. If there is no variation in a trait in a population, natural selection cannot act. ■ Overproduction and struggle for existence. The members of all populations compete with each other for limited resources. Certain members are able to capture or utilize these resources better than others. ■ Survival of the fittest. Just as humans carry on artificial breeding programs to select which plants and animals will reproduce, natural selection by the environment determines which members of a population survive and reproduce. While Darwin emphasized the importance of survival, modern evolutionists emphasize the importance of unequal reproduction. That is, certain members of the population produce more offspring than others simply because they happen to have a variation or variations that make them better suited to the environment. In a biological sense, fitness is the number of fertile offspring an individual produces throughout its lifetime. ■ Adaptation. The result of natural selection is that populations come to resemble the “best types”—those individuals that produce the most offspring because they are better adapted to the environment. Early giraffes probably had necks of various lengths. ■ mad86751_ch27_548-574.indd 563 Natural selection due to competition led to survival of the longer-necked giraffes and their offspring. Darwin was prompted to publish his findings only after he received a letter from another naturalist, Alfred Russel Wallace, who had come to the same conclusions about evolution. Although both scientists subsequently presented their ideas at the same meeting of the famed Royal Society in London in 1858, only Darwin had outlined his reasoning for the theory in a draft of The Origin of Species by Means of Natural Selection, which he had completed 16 years earlier and eventually published in 1859. This book is still studied by many biologists today. Can natural selection account for the origin of new species and for the great diversity of life? Yes, Darwinian selection is the only accepted scientific theory for the diversity of life. Discussion Questions 1. Currently, a debate is in progress regarding the teaching of intelligent design alongside evolution as a theory for the diversity of life. Explain why the intelligent design idea is teleological. 2. Explain why variation in a trait must be present in order for natural selection to operate. 3. Explain why Lamarck’s idea of “inheritance of acquired characteristics” is incorrect. Eventually, only long-necked giraffes survived the competition. Figure 27B Charles Darwin’s theory of natural selection. 12/21/06 7:34:17 PM Part Six Evolution and Diversity Initial Distribution Survival of Young 564 After Time less than 4 eggs 4 to 5 eggs more than 5 eggs Survival of Young Clutch Size After More Time Survival of Young Clutch Size Clutch Size Figure 27.16 Stabilizing selection. Stabilizing selection occurs when natural selection favors the intermediate phenotype over the extremes. For example, Swiss starlings that lay four to five eggs (usual clutch size) have more surviving young than birds that lay fewer than four eggs or more than five eggs. Natural Selection Natural selection is the process by which some individuals produce more offspring than others. The Science Focus outlines how Charles Darwin explained evolution by natural selection. Here, we restate these steps in the context of modern evolutionary theory. Evolution by natural selection requires: 1. Individual variation. The members of a population differ from one another. 2. Inheritance. Many of these differences are heritable genetic differences. 3. Overproduction. Individuals in a population are engaged in a struggle for existence because breeding individuals in a population tend to produce more offspring than the environment can support. 4. Differential reproductive success. Individuals that are better adapted to their environment produce more offspring than those that are not as well adapted, and consequently, their fertile offspring will make up a greater proportion of the next generation. In biology, the fitness of an individual is measured by the number of fertile offspring produced throughout its lifetime. Gene mutations are the ultimate source of variation because they provide new alleles. However, in sexually reproducing organisms, genetic variation can also result from crossing-over and independent assortment of chromosomes during meiosis and also fertilization when gametes are com- mad86751_ch27_548-574.indd 564 bined. A different combination of alleles can lead to a new and different phenotype. In this context, consider that most of the traits on which natural selection acts are polygenic and thus controlled by more than one gene. Such traits have a range of phenotypes that follow a bell-shaped curve. The three main types of natural selection are stabilizing selection, directional selection, and disruptive selection. Stabilizing selection occurs when an intermediate phenotype is favored. With stabilizing selection, extreme phenotypes are selected against, and individuals near the average are favored. Stabilizing selection can improve adaptation of the population to those aspects of the environment that remain constant. As an example, consider that when Swiss starlings lay four to five eggs, more young survive than when the female lays more or less than this number (Fig. 27.16). Genes determining physiological characteristics, such as the production of yolk, and behavioral characteristics, such as how long the female will mate, are involved in determining clutch size. Through the years, hospital data have shown that human infants born with an intermediate birth weight (3–4 kg) have a better chance of survival than those born with an extreme birth weight—either higher or lower than that range. Stabilizing selection serves to reduce the variability in birth weight in human populations. Directional Selection Directional selection occurs when an extreme phenotype is favored and the distribution curve shifts in that direction (Fig. 27.17). This changes the average phenotype in a population. Such a shift can occur when a population is adapting to a changing environment. For example, the gradual increase in the size of the modern horse, Equus, can be correlated with a change in the environment from forest conditions to grassland conditions. Hyracotherium, the ancestor of the modern horse, was about the size of a dog and was adapted to the forestlike environment of the Eocene epoch of the Paleogene period. This animal could have hidden among the trees for protection, and its low-crowned teeth would have been appropriate for browsing on leaves. Later, in the Miocene and Pliocene epochs, grasslands began to replace the forests. Then the ancestors of Equus were subject to selective pressure for the development of strength, intelligence, speed, and durable grinding teeth. A larger size provided the strength needed for combat, elongated legs ending in hooves gave speed for escaping from enemies, and the durable grinding teeth enabled the animals to feed efficiently on grasses. Nevertheless, the evolution of the horse should not be viewed as a straight line of descent; there were many side branches that became extinct. The evolution of peppered moths discussed previously is another good example of directional selection. Disruptive Selection In disruptive selection, two or more extreme phenotypes are favored over any intermediate phenotype (Fig. 27.18). For example, British land snails 12/21/06 7:34:22 PM Chapter Twenty–Seven Evolution of Life After More Time Body Size Number of Individuals After Time Number of Individuals Number of Individuals Initial Distribution 565 Body Size Body Size a. Hyracotherium Merychippus b. Figure 27.17 Directional selection. Equus Initial Distribution Number of Individuals a. Directional selection occurs when natural selection favors one extreme phenotype, resulting in a shift in the distribution curve. b. For example, Equus, the modern-day horse, which is adapted to a grassland habitat, is much larger than its ancestor, Hyracotherium, which was adapted to a forest habitat. After Time Number of Individuals Banding Pattern After More Time Number of Individuals Banding Pattern Banding Pattern a. b. Figure 27.18 Disruptive selection. a. Disruptive selection favors two or more extreme phenotypes. b. Today, British land snails comprise mainly two different phenotypes, each adapted to a different habitat. Snails with dark shells are more prevalent in forested areas, and light-banded snails are more prevalent in areas with low-lying vegetation. mad86751_ch27_548-574.indd 565 12/21/06 7:34:35 PM 566 Part Six Evolution and Diversity (Cepaeanemoralis) have a wide habitat range that includes grass fields and hedgerows and forested areas. In areas with low-lying vegetation, thrushes feed mainly on snails with dark shells that lack light bands, and in forested areas, they feed mainly on snails with light-banded shells. Therefore, the two different habitats have resulted in two different phenotypes in the population. The agents of evolutionary change are mutations, genetic drift, gene flow, nonrandom mating, and natural selection. These processes cause changes in the allele frequencies of a population. Of these, only natural selection results in adaptation to the environment. Maintenance of Variation You might think that genetic variation, particularly due to deleterious alleles, would eventually disappear because natural selection tends to remove those alleles from a population. But sickle cell disease exemplifies how genetic variation is sometimes maintained in a population. Persons homozygous for the allele that causes sickle cell disease have sickle-shaped red blood cells, which can clog blood vessels and deprive the body of oxygen. Therefore, you would expect this condition to be selected against and eliminated from a population. However, heterozygotes for the sickle cell allele have some sickle-shaped cells, and are also resistant to malaria, a disease caused by a parasite that lives in red blood cells. Malaria is a leading killer in many parts of the world. As a result, the allele for sickle cell disease is maintained in relatively high frequency in regions where there is a high incidence of malaria. A study of the three genotypes and phenotypes involved shows why: Genotype Phenotype Result Hb A Hb A Normal Dies due to malarial infection Hb A Hb S Some sickle cells Lives due to protection from malaria Hb S Hb S Sickle cell disease1 Dies due to sickle cell disease 1All red blood cells sickle shaped The frequency of the sickle cell allele in some parts of Africa is 0.40, while among African Americans, it is only 0.05 due to lowered incidence of malaria in the United States. In Africa, the favored heterozygote keeps the two homozygotes equally present in the population. Maintenance of the same ratio of two or more phenotypes in each generation is called balanced polymorphism. mad86751_ch27_548-574.indd 566 Five agents of evolutionary change are: mutations, genetic drift, gene flow, non-random mating and natural selection. Variation can be maintained in populations through balancing selection or through alleles that cause diseases that may provide an advantage in the heterozygous form. 27.4 Speciation Usually, a species occupies a certain geographical range, within which several subpopulations exist. For our present discussion, species is defined as a group of subpopulations that are capable of interbreeding and are isolated reproductively from other species. The subpopulations of the same species can exchange genes, but different species do not exchange genes. Reproductive isolation of similar species is accomplished by the isolating mechanisms listed in Table 27.2. Prezygotic isolating mechanisms are in place before fertilization, and thus reproduction is never attempted. Postzygotic isolating mechanisms are in place after fertilization, so reproduction may take place, but it does not produce fertile offspring. The Process of Speciation Speciation has occurred when one species gives rise to two species, each of which continues on its own evolutionary pathway. How can we recognize speciation? Whenever reproductive isolation develops between two formerly interbreeding groups of populations, speciation has occurred. One type of speciation, called allopatric speciation, usually occurs when populations become separated by a geographic barrier and gene flow is no longer possible. Figure 27.19 illustrates an TABLE 27.2 Reproductive Isolating Mechanisms Isolating Mechanism Example Prezygotic Habitat isolation Species at same locale occupy different habitats Temporal isolation Species reproduce at different seasons or different times of day Behavioral isolation In animals, courtship behavior differs, or they respond to different songs, calls, pheromones, or other signals Mechanical isolation Genitalia unsuitable for one another Postzygotic Gamete isolation Sperm cannot reach or fertilize egg Zygote mortality Fertilization occurs, but zygote does not survive Hybrid sterility Hybrid survives but is sterile and cannot reproduce F2 fitness Hybrid is fertile, but F2 hybrid has reduced fitness 12/21/06 7:34:36 PM 567 Chapter Twenty–Seven Evolution of Life 1. Members of a northern ancestral population migrated southward. Ensatina eschscholtzi picta A AD EV A N INS RR TA SIE OUN M GE AN S LR TA AIN AS NT CO MOU Ensatina eschscholtzi oregonensis 2. Subspecies are separated by California’s Central Valley. Some interbreeding between populations does occur. It is also possible that a single population could suddenly divide into two reproductively isolated groups without being geographically isolated. The best evidence for this type of speciation, called sympatric speciation, is found among plants, where multiplication of the chromosome number in one plant prevents it from successfully reproducing with others of its kind. Self-reproduction can maintain such a new plant species. Speciation is the origin of a new species. This usually requires geographic isolation followed by reproductive isolation. Adaptive Radiation Ensatina eschscholtzi platensis L RA NT Y CE LLE VA Ensatina eschscholtzi xanthoptica Ensatina eschscholtzi croceater Ensatina eschscholtzi eschscholtzii 3. Evolution has occurred, and in the south two subspecies look quite different from one another. Ensatina eschscholtzi klauberi Figure 27.19 Allopatric speciation. In this example of allopatric speciation, the Central Valley of California is separating a range of populations descended from the same northern ancestral species. Those to the west along the coastal mountains and those to the east along the Sierra Nevada mountains experience gene flow, but gene flow is limited between the eastern populations and the western populations. Members of the most southerly eastern and western populations are quite different in color pattern. example of allopatric speciation that has been extensively studied in California. Apparently, members of an ancestral population of Ensatina salamanders existing in the Pacific Northwest migrated southward, establishing a series of populations. Each population was exposed to its own selective pressures along the coastal and Sierra Nevada mountains. Due to the presence of the Central Valley of California, which is largely dry and thus unsuitable habitat for amphibians, gene flow rarely occurs between eastern and western populations of Ensatina. Genetic differences also increased from north to south, resulting in two distinct forms of Ensatina salamanders in Southern California that differ dramatically in color. mad86751_ch27_548-574.indd 567 One of the best examples of “allopatric” speciation is provided by the finches on the Galápagos Islands, located 600 miles west of Ecuador, South America. The 13 species of finches found there are often called Darwin’s finches because Darwin first realized their significance as an example of how evolution works. These species are believed to be descended from mainland finches that migrated to one of the islands. We can imagine that after the original species on a single island increased, some individuals dispersed to other islands. The islands are ecologically different enough to have promoted divergent feeding habits. This is apparent because, although the birds physically resemble each other in many respects, they have different beaks, each adapted to gathering and eating a different type of food (Fig. 27.20). There are seed-eating ground finches, cactus-eating ground finches, insect-eating tree finches, also with different-sized beaks; and a warbler-type tree finch, with a beak adapted to eating insects and gathering nectar. Among the tree finches is a woodpecker type, which lacks the long tongue of a true woodpecker but makes up for this by using a cactus spine or a twig to ferret out insects. Remarkably, each of these types is found on islands where its beak matches the abundant food type. Therefore, Darwin’s finches are an example of adaptive radiation, or the proliferation of a species by adaptation to different ways of life. The Pace of Speciation Currently, there are two hypotheses about the pace of speciation and, therefore, evolution. One hypothesis is called the phyletic gradualism model, and the other is called the punctuated equilibrium model. Each model gives a different answer to the question of why so few transitional links are found in the fossil record. Traditionally, evolutionists have supported a model called phyletic gradualism, which states that change is very slow but steady within a lineage before and after a divergence 12/21/06 7:34:42 PM Medium ground finch, Geospiza fortis Small tree finch, Camarhynchus parvulus Medium tree finch, Camarhynchus pauper Cactus finch, Geospiza scandens Sharp-beaked ground finch, Geospiza difficilis Large tree finch, Camarhyncus psittacula Vegetarian finch, Platyspiza crassirostris Small ground finch, Geospiza fuliginosa Mangrove finch, Cactospiza heliobates Woodpecker finch, Cactospiza pallida (holding a cactus spine) Warbler finch, Certhidea olivacea Large cactus finch, Geospiza conirostris Large ground finch, Geospiza magnirostris Figure 27.20 The Galápagos finches. Each of these finches is adapted to gathering and eating a different type of food. Note the different sizes and shapes of the beaks in the different species. Tree finches have beaks largely adapted to eating insects and, at times, plants. The woodpecker finch, a tool-user, uses a cactus spine or twig to probe in the bark of a tree for insects. Ground finches have beaks adapted to eating prickly-pear cactus or different-sized seeds. 568 mad86751_ch27_548-574.indd 568 12/21/06 7:35:42 PM Chapter Twenty–Seven Evolution of Life new species 1 transitional link ancestral species new species 2 569 has occurred. Why? Because a new species comes about after reproductive isolation, and reproductive isolation cannot be detected in the fossil record! Only when a new species evolves and displaces the existing species is the new species likely to show up in the fossil record. A model of evolution called punctuated equilibrium has also been proposed (Fig. 27.21b). It says that long periods of stasis, or no visible change, are followed by rapid periods of speciation. With reference to the length of the fossil record (about 3.5 billion years), speciation occurs relatively rapidly, and this can explain why few transitional links are found. Mass extinction events are often followed by rapid (relative to the age of the Earth) periods of speciation. Adaptive radiation is an example of allopatric speciation that is easily observable. Whether speciation occurs slowly or rapidly is being debated. Time a. 27.5 new species 1 ancestral species ancestral species new species 2 Time b. Figure 27.21 Phyletic gradualism compared to punctuated equilibrium. a. Supporters of the phyletic gradualism model believe that speciation takes place gradually and many transitional links occur. b. Supporters of the more recent, punctuated equilibrium model believe that speciation occurs rapidly, with no transitional links. (splitting of the line of descent) (Fig. 27.21a). Therefore, it is not surprising that few transitional links such as Archaeopteryx (see Fig. 27.5) have been found. Indeed, the fossil record, even if it were complete, might be unable to show when speciation mad86751_ch27_548-574.indd 569 Classification Recall that each type of organism is given a scientific name and the scientific name for modern humans is Homo sapiens. The first word, Homo, is the genus, a classification category that contains many species. The second word is the species name, which may describe the organism. The word sapiens refers to a large brain. When species are classified, they are placed in a hierarchy of categories: species, genus, family, order, class, phylum, and kingdom. This text uses one further classification category, called a domain. Phylogeny is the evolutionary relationship among organisms. Ideally, classification reflects phylogeny in that it tells how organisms are related through evolution and common ancestry. Species in the same genus are more closely related than species in separate genera and so forth as we proceed from genus to domain. Each higher classification category is more inclusive than the one below it; therefore, there are many more species within a kingdom than in a phylum, for example. Five-Kingdom System For many years, most biologists favored a five-kingdom classification system consisting of Plantae, Animalia, Fungi, Protista, and Monera. Organisms were placed into these kingdoms on the basis of type of cell (prokaryotic or eukaryotic), level of organization (unicellular or multicellular), and mode of nutrition. In this system, the organisms in the kingdom Monera are distinguished by their structure—they are prokaryotic (lack a membrane-bounded nucleus)—whereas the organisms in the other kingdoms are eukaryotic (have a membrane-bounded nucleus). An evolutionary tree, also called a phylogenetic tree, depicts the relationships among organisms based on how they are classified. Figure 27.22 is an evolutionary tree depicting the five-kingdom system of classification. The tree suggests that 12/21/06 7:35:47 PM 570 Part Six Evolution and Diversity protists evolved from monerans and that fungi, plants, and animals evolved from protists via three separate lines of evolution. In an evolutionary tree, two or more groups that separate from the same juncture share the same common ancestor. The five-kingdom system of classification suggests that fungi, plants, and animals share the same ancestor, presumably an extinct protist known only from the fossil record. Three-Domain System Within the past ten years, new information has called into question the five-kingdom system of classification. The molecule rRNA probably changes slowly during evolution and, indeed, may change when there is a major evolutionary event. Molecular data based on the sequencing of rRNA suggest that there are three domains: Bacteria, Archaea, and Eukarya. Cellular data also support the three-domain system. Bacteria and archaea are both unicellular prokaryotes that lack a membrane-bounded nucleus. However, bacteria and archaea are distinguishable from each other on the basis of lipid and cell wall biochemistry. The biochemical attributes of many archaea allow them to live in very hostile environments, including anaerobic swamps, salty bodies of water, and even hot, acidic environments, such as hot springs and geysers. Molecular and cellular data also suggest that the archaea and eukarya are more closely related to each other than either is to the bacteria. The evolutionary tree depicted in Figure 27.23 reflects this evolutionary relationship. It shows that the archaea and the eukarya share a more recent common ancestor than do all three domains. The kingdoms Protista, Fungi, Plantae, and Animalia, as illustrated in Figure 1.5, are all placed in the domain Eukarya. Exactly how these kingdoms are related is still being determined. Phylogenetics Phylogenetics is the modern way in which organisms are classified and arranged in evolutionary trees. Phylogeneticists arrange species and higher classification categories into clades. Clades may be represented on a diagram called a cladogram. A clade contains a most recent common ancestor and all its descendant species—the common ancestor is presumed and not identified. Figure 27.24 depicts a cladogram for seven groups of vertebrates. Only the lamprey, the so-called “outgroup,” lacks jaws, but the other six groups of vertebrates are in the same clade because they all have jaws, a derived characteristic relative to their ancestors. On the other hand, the vertebrates beyond the shark are all in the same clade because they have lungs, and so forth. Figure 27.24 is somewhat misleading because, although single traits are noted on the tree, phylogeneticists use much more data to arrange groups of organisms into clades. Phylogeneticists are aided in their endeavor by the computer and any and all available data, including morphological data and DNA sequences. In making decisions, they are often guided fungi plants Kingdom Plantae Kingdom Animalia EUKARYA Kingdom Fungi animals protists protists cyanobacteria Kingdom Protista heterotrophic bacteria BACTERIA ARCHAEA Kingdom Monera common ancestor Figure 27.22 Five-kingdom system of classification. Figure 27.23 The three-domain system of All prokaryotes are in the kingdom Monera. The eukaryotes are in kingdoms Protista, Fungi, Plantae, and Animalia. The evolutionary tree shows the lines of descent. Note that the three domain system of classification (Fig. 27.23) is currently preferred. Representatives of each domain are depicted. The phylogenetic tree shows that domain Archaea is more closely related to domain Eukarya than either is to domain Bacteria. mad86751_ch27_548-574.indd 570 classification. 12/21/06 7:36:02 PM 571 Figure 27.24 Cladogram. lamprey shark salamander lizard A cladogram gives comparative information gorilla tiger human about relationships. Organisms in the same clade share the same derived characteristics. Humans and all the other vertebrates shown, except lampreys, are in the same clade as sharks because they all have jaws. However, humans are also alone in a clade because only they are bipedal. This cladogram is simplified because phylogeneticists actually use a great deal more data to construct cladograms. bipedal no tail hair amniotic membrane lungs jaws Class Mammalia mammals Time by the principle of parsimony, which states that the pattern that requires the fewest evolutionary changes is the most likely. Like phylogeneticists, another group of scientists called “traditionalists” consider descent from a common ancestor when grouping organisms, but they also consider the amount of adaptive evolutionary change. For example, traditionalists place crocodiles (class Reptilia) and birds (class Aves) in separate classes (Fig. 27.25a) because of the adaptive advantage of feathers, despite the fact that they agree with phylogeneticists that these groups share a recent common ancestor. However, phylogeneticists place crocodiles and birds in the same group (Archosaurs, Fig. 27.25b) because of the many traits they share. Class Reptilia turtles snakes and lizards Class Aves crocodiles dinosaurs birds early reptiles a. Traditional systematics Organisms are classified into groups based on their evolutionary relationships. Currently a three domain system replaces the previously preferred five kingdom system of classification. Phylogenetics is used to classify groups of closely related organisms, called clades, into evolutionary trees. Mammalia mammals Reptilia Archosaurs turtles crocodiles dinosaurs birds snakes and lizards Summarizing the Concepts 27.1 Origin of Life Chemical reactions are believed to have led to the formation of the first true cell(s). Inorganic chemicals, probably derived from the primitive atmosphere, reacted to form small organic molecules. These reactions occurred in the ocean, either on the surface or in the region of hydrothermal vents deep within. After small organic molecules such as glucose, amino acids, and nucleotides arose, they polymerized to form the macromolecules. Amino acids joined to form proteins, and nucleotides joined to form nucleic acids. Perhaps RNA was the first nucleic acid. The RNAfirst hypothesis is supported by the discovery of ribozymes, RNA enzymes. The protein-first hypothesis is supported by the observation that amino acids polymerize abiotically when exposed to dry heat. Once a plasma membrane developed, the protocell came into being. Eventually, the DNADRNADprotein system evolved, and a true cell came into being. 27.2 Evidence of Evolution The fossil record and biogeography, as well as studies of comparative anatomy, development, and biochemistry, all provide evidence of evo- mad86751_ch27_548-574.indd 571 early reptiles Time b. Cladistic systematics Figure 27.25 Traditional versus cladistic view of reptilian phylogeny. a. According to traditionalists, crocodiles and birds are in separate classes. b. According to phylogeneticists, crocodiles and birds share a recent common ancestor and should be in the same clade. lution. The fossil record gives clues about the history of life in general and allows us to trace the descent of a particular group. Biogeography shows that the distribution of organisms on Earth can be influenced by a combination of evolutionary and geological processes. Comparing the anatomy and the development of organisms reveals homologous 12/21/06 7:36:14 PM Evolution of Antibiotic Resistance Through Darwin’s theory of natural selection, we have come to understand why bacteria become resistant to the antibiotics we use to treat patients. Some people refer to the use of antibiotics as “artificial selection,” because humans are involved. Nonetheless, the process is the same as natural selection—antibiotics kill bacteria that are susceptible, but bacterial populations within a single patient are usually so large that resistant individuals are likely to survive and reproduce. Through time, the frequency of resistant individuals increases in the population to the point that a certain antibiotic may no longer be effective. Keep in mind that a patient infected with a resistant strain might require several antibiotics. Ever since the introduction of antibiotics, resistance has often evolved soon after (Table 27A). An extreme example is methicillin, an antibiotic that it only took one year for bacteria to evolve resistance against! This is the type of “accelerated evolution” you learned about at the beginning of this chapter. Anitbacterial resistance creates the need for even newer antibiotics. The development of a single new antibiotic is estimated to cost between $400 and $500 million. Antibiotic resistance adds $30 billion to annual medical costs in the United States alone! Some strains of tuberculosis (or TB), a disease caused by bacteria in the genus Mycobacterium, are resistant to multiple antibiotics. TB is spread though the air from one person to another, usually when an infected person coughs or sneezes. TB is the most common infectious disease today, infecting about one-third of the world’s population, or about 2 billion people. You may not realize it, but TB kills 2–3 million people worldwide each year—right up there with AIDS and malaria! In extreme cases, infections can spread through the lungs, causing lesions and even holes, and eventually leading to death in untreated patients. When a patient tests positive for TB, he or she is usually put on a six-month course of the antibiotic isoniazid. Many patients feel better after a few weeks on the drug, and some discontinue its use. This selects for resistant strains of TB. Our understanding of evolutionary biology has helped doctors treat patients with TB. In New York City, for example, patients are treated with what is called “direct observation therapy,” in which a doctor or nurse actually watches a patient take the medicine. In addition, the initial diagnosis of TB includes testing whether the strain is drug resistant. This way, doctors can treat patients with an effective drug regimen, instead of allowing their infection to persist and spread to other people. In patients with strains that are resistant to isoniazid, four drugs are recommended for treatment. Multidrug-resistant strains of TB can be very difficult and costly to treat; usually an 18-month course of multiple antibiotics is necessary, and treatment for strains of TB resistant to methicillin alone can cost $50,000 per person! TABLE 27A 1. In New York City, state health officials have the power to quarantine TB patients who do not take their medicine. That is, they can essentially lock them up for as long as needed (often up to 18 months) to treat their illness. However, some of the medications can have serious side effects. What do you think about this policy? 2. Many people in less-developed countries die from TB, not because their disease is incurable, but simply because they do not have health insurance and cannot afford the medications. Should we in the United States pay more for our medications so that pharmaceutical companies can provide them to lower-income people at a reduced cost or for free? 3. Antibiotics kill bacteria only, not viruses. Knowing what you know about antibiotic resistance and natural selection, when would you prescribe antibiotics if you were a doctor? Dates of Antibiotic Discovery and Resistance Antibiotic Discovery/ Introduction Resistance Penicillin 1928/1943 1946 Sulfonamides 1930s 1940s Streptomycin 1943/1945 1959 Chloramphenicol 1947 1959 Tetracycline 1948 1953 Erythromycin 1952 1988 Vancomycin 1956 1988/1993 Methicillin 1960 1961 Ampicillin 1961 1972 Cefotaxime/ceftazidime 1981/1985 1983/1984/1988 structures among those that share common ancestry. All organisms have certain biochemical molecules in common, and these chemical similarities indicate the degree of relatedness. 27.3 The Process of Evolution Discussion Questions Microevolution is a process that involves a change in allele frequencies within the gene pool of a sexually reproducing population. The Hardy-Weinberg principle states that gene pool frequencies arrive at an equilibrium that is maintained generation after generation unless disrupted by mutations, genetic drift, gene flow, nonrandom mating, or natural selection. Any change from the initial allele frequencies in the gene pool of a population signifies that evolution has occurred. 27.4 Speciation Speciation is the origin of new species. This usually requires geographic isolation, followed by reproductive isolation. The evolution of several species of finches on the Galápagos Islands is an example of speciation caused by adaptive radiation because each one has a different way of life. Currently, there are two hypotheses about the pace of speciation. Traditionalists support phyletic gradualism—slow, steady 572 mad86751_ch27_548-574.indd 572 12/21/06 7:36:21 PM 573 Chapter Twenty–Seven Evolution of Life change leading to speciation. In contrast, a more recent model, called punctuated equilibrium, proposes that long periods of stasis are interrupted by rapid speciation. 27.5 Classification Classification involves assigning species to a hierarchy of categories: kingdom, phylum, class, order, family, genus, and species, and in this text, domain. The five-kingdom system of classification recognizes these kingdoms: Monera (the bacteria), Protista (algae, protozoans), Fungi, Plantae, and Animalia. The more recent which is three-domain system (Bacteria, Archaea, and Eukarya), based on molecular data, is currently preferred. Both bacteria and archaea are prokaryotes. Members of the kingdoms Protista, Fungi, Plantae, and Animalia are eukaryotes. Phylogeneticists classify and diagram the evolutionary relationships among organisms. They use as many characteristics as possible to put species in clades, which are represented on portions of a diagram called a cladogram. A clade contains a most recent common ancestor and all its descendant species, which share the same derived characteristics relative to their ancestors. Testing Yourself Choose the best answer for each question. 1. The atmosphere in which life arose lacked a. carbon. c. oxygen. b. nitrogen. d. hydrogen. 2. The RNA-first hypothesis for the origin of cells is supported by the discovery of a. ribozymes. c. polypeptides. b. proteinoids. d. nucleic acid polymerization. 3. Protocells probably obtained energy as a. photosynthetic autotrophs. c. heterotrophs. b. chemoautotrophs. d. None of these are correct. 4. All true cells are able to a. replicate DNA. c. absorb nutrients. b. synthesize sugars. d. export minerals. 5. DNA genes may have arisen from RNA genes via a. DNA polymerase. c. reverse transcriptase. b. RNA polymerase. d. DNA ligase. 6. Fossils that serve as transitional links allow scientists to a. determine how prehistoric animals interacted with each other. b. deduce the order in which various groups of animals arose. c. relate climate change to evolutionary trends. d. determine why evolutionary changes occur. 7. Carbon dating cannot be used to determine the age of dinosaur fossils because a. levels of atmospheric carbon were very low when dinosaurs were alive. b. dinosaurs contained very low levels of carbon. c. dinosaur fossils contain very low levels of carbon. d. the half-life of radioactive carbon is too short. 8. Marine animals experience mass extinction approximately every 26 million years. Scientists believe this pattern is due to meteorites that reach Earth because a. our solar system shifts its location in the Milky Way in a 26-million-year pattern. mad86751_ch27_548-574.indd 573 b. the sun moves far away from Earth every 26 million years. c. Earth moves into an asteroid belt every 26 million years. d. the moon moves close to Earth every 26 million years. 9. Which of the following groups of organisms might be found on multiple continents? See Table 27.1, and remember that the continents began to move apart about 250 million years ago. a. primitive primates b. reptiles c. birds d. Both b and c are correct. e. All of these are correct. 10. The flipper of a dolphin and the fin of a tuna are a. homologous structures. b. homogeneous structures. c. analogous structures. d. reciprocal structures. 11. Which of the following is not an example of a vestigial structure? a. human tailbone b. ostrich wings c. pelvic girdle in snakes d. dog kidney 12. Which of the following is sure to be a population? a. grizzly bears of the Rocky Mountains b. mosquitoes of the United States c. barracuda in the Caribbean Sea d. sequoia grove in Sequoia National Park e. dandelions of Pennsylvania 13. The frequency of a rare disorder expressed as an autosomal recessive trait is 0.0064. Using the Hardy-Weinberg principle, determine the frequency of carriers for the disease in this population. a. 0.147 d. 0.020 b. 0.080 e. 0.846 c. 0.920 14. Which of the following generally results in a gain in genetic variability? a. genetic drift b. mutation c. founder effect d. bottleneck For questions 15–19, match the description with the appropriate term in the key. Key: a. mutation d. bottleneck b. natural selection e. gene flow c. founder effect f. nonrandom mating 15. The Northern elephant seal went through a severe population decline as a result of hunting in the late 1800s. As a result of a hunting ban, the population has rebounded but is now homozygous for nearly every gene studied. 16. A small, reproductively isolated religious sect called the Dunkers was established by 27 families that came to the United States from Germany 200 years ago. The frequencies for blood group alleles in this population differ significantly from those in the general U.S. population. 17. Turtles on a small island tend to mate with relatives more often than turtles on the mainland. 18. Within a population, plants that produce an insect toxin are more likely to survive and reproduce than plants that do not produce the toxin. 19. The gene pool of a population of bighorn sheep in the southwest U.S. is altered when several animals cross over a mountain pass and join the population. 12/21/06 7:36:21 PM 574 Part Six Evolution and Diversity 20. People who are heterozygous for the cystic fibrosis gene are more likely than others to survive a cholera epidemic. This heterozygote advantage seems to explain why homozygotes are maintained in the human population, and is an example of a. disruptive selection. c. high mutation rate. b. balanced polymorphism. d. nonrandom mating. 21. The creation of new species due to geographic barriers is called a. isolation speciation. d. sympatric speciation. b. allopatric speciation. e. symbiotic speciation. c. allelomorphic speciation. 22. Which of the following models is supported by the observation that few transitional links are found in the fossil record? a. phyletic gradualism c. Both a and b are correct. b. punctuated equilibrium d. None of these are correct. 23. Organisms have been placed in the five-kingdom classification system based on a. level of organization and mode of nutrition. b. genetic diversity and mode of nutrition. c. genetic diversity and level of organization. d. reproductive traits and mode of nutrition. e. reproductive traits and genetic diversity. 24. The three-domain classification system has recently been developed based on a. b. c. d. e. mitochondrial biochemistry and plasma membrane structure. cellular and rRNA sequence data. plasma membrane and cell wall structure. rRNA sequence data and plasma membrane structure. nuclear and mitochondrial biochemistry. Understanding the Terms adaptive radiation 567 allopatric speciation 566 analogous structure 556 Archaea 570 autotroph 552 Bacteria 570 biogeography 556 bottleneck effect 563 clade 570 cladogram 570 class 569 common ancestor 570 continental drift 556 directional selection 564 disruptive selection 564 domain 569 Eukarya 570 evolution 552 family 569 fitness 564 fossil 553 founder effect 563 gene flow 563 mad86751_ch27_548-574.indd 574 gene pool 559 genetic drift 562 genus 569 heterotroph 552 homologous structure 556 kingdom 569 kingdom Animalia 570 kingdom Fungi 570 kingdom Plantae 570 kingdom Protista 570 liposome 552 microevolution 559 microsphere 551 mutation 562 natural selection 564 nonrandom mating 563 order 569 phyletic gradualism 567 phylogenetics 570 phylogeny 569 phylum 569 population 559 postzygotic isolating mechanism 566 prezygotic isolating mechanism 566 protein-first hypothesis 551 proteinoid 551 protocell 550 punctuated equilibrium 569 RNA-first hypothesis 551 speciation 566 species 566, 569 stabilizing selection 564 sympatric speciation 567 transitional link 553 vestigial structure 557 Match the terms to these definitions: a._______________ Process by which populations become adapted to their environment. b._______________ Type of natural selection in which an extreme phenotype is favored, usually in a changing environment. c._______________ An evolutionary model that proposes periods of rapid change dependent on speciation followed by long periods of stasis. d._______________ Structure that is similar in two or more species because of common ancestry. e._______________ Movement of genes from one population to another via sexual reproduction between members of the populations. Thinking Critically 1. Viruses such as HIV are rapidly replicated and have very high mutation rates; thus, evolution of the virus can be observed in a single infected person. Using what you have learned in this chapter, explain why HIV is so hard to treat, even though multiple drugs to treat HIV have been developed. 2. If the conditions for the Hardy-Weinberg principle are rarely, if ever, met in nature, why is it such an important idea? 3. You observe a wasting disease in cattle that you know is genetically caused and thus heritable. The disease is fatal in young cattle. The allele frequency for the gene that causes the disease is 0.05 in the United States, but 0.35 in South America. Explain why such a difference in allele frequencies might exist. 4. Why are homologous structures, as opposed to analogous ones, used to determine the evolutionary relationships of species and to reconstruct phylogenies? 5. Why do scientists continue to devise experimental systems that mimic the conditions of the early Earth, despite the fact that it is difficult to do and there is no way to know for sure what the conditions on Earth were billions of years ago? Are you aware of any such experiments and their results? ARIS, the Inquiry into Life Website ARIS, the website for Inquiry into Life, provides a wealth of information organized and integrated by chapter. You will find practice quizzes, interactive activities, labeling exercises, flashcards, and much more that will complement your learning and understanding of general biology. www.aris.mhhe.com 12/21/06 7:36:21 PM