_{Evolution on a}
Large Scale

Chapter

16

Outline

16.1 Macroevolution

• Macroevolution involves the origin of species, the history of species, and the extinction of species.246

• New species come about when populations are reproductively isolated from other, similar populations.248

• Allopatric speciation results from geographic barriers; sympatric speciation relies on reproductive barriers in a single population.250

• Adaptive radiation is the rapid development of several new species from a single original species; each new species is adapted to a unique way of life.251

16.2 The History of Species

• A dated fossil record allows us to trace the history of species on planet Earth.253

• The fossil record supports two proposed models for the pace of evolutionary change: the gradualistic model and the punctuated equilibrium model.254

• Continental drift and meteorite impacts have contributed to several episodes of mass extinction during the history of life.254

16.3 Classification of Species

• The field of systematics encompasses both taxonomy (the naming of organisms) and classification (placing species in the proper categories).256

• The fossil record, homology, and molecular data are used to decide the evolutionary relatedness of species.257–59

• Cladistics is a new school of systematics that is expanding its influence today.259

• The three-domain system recognizes domains Bacteria, Archaea, and Eukarya. The domain Eukarya contains the kingdoms Protista, Fungi, Plantae, and Animalia.261

Iguanas are a type of lizard known for a very long tail, which can be up to three times their body length. The green iguana, the second largest lizard in the world, is common throughout the South American continent. Its claws enable it to climb trees, where it feeds on succulent leaves and soft fruits along riverbanks. It is an excellent swimmer and uses rivers to travel to new feeding areas.

Islands to the west and northeast of South America don’t have green iguanas. Instead, the Galápagos Islands to the west have one type of land iguana and one type of marine iguana. Marine iguanas, black in color, are unique to the Galápagos Islands—they occur no place else on planet Earth. These air-breathing reptiles can dive to a depth exceeding 10 meters (33 feet) and remain submerged for more than 30 minutes. They use their claws to cling to the bottom rocks while feeding on algae.

Hispaniola,¹ an island to the northeast of South America, has a type of iguana called the rhinoceros iguana, which has three horny bumps on its snout and is dark brown to black in color. This iguana lives mainly in dry forests with rocky, limestone habitats and feeds on a wide variety of plants.

The Theory of Evolution can be used to explain the occurrence of the unique iguanas on the Galápagos Islands and Hispaniola. It can be hypothesized that ancestral green iguanas swam or hitchhiked on floating driftwood from South America to the Galápagos to the west and to Hispaniola to the northeast. After arrival, a green iguana population, cut off from the others of its species, evolved into these new and different species. Speciation is the topic of this chapter.

_{16.1 Macroevolution}

Chapter 15 considered evolution on a small scale—that is, microevolution. In this chapter, we turn our attention to evolution on a large scale—that is, macroevolution. The history of life on Earth is a part of macroevolution. Macroevolution requires speciation, the splitting of one species into two or more new species. Speciation involves the gene pool changes that we studied in Chapter 15.

Species originate, adapt to their environments, and then may become extinct (Fig. 16.1). Without the origin and extinction of species, life on Earth would not have an ever-changing history.

Defining Species

Before we consider the origin of species, we first need to define a species. Appearance is not always a good criterion. The members of different species can look quite similar, while the members of a single species can be diverse in appearance. The biological species concept offers a way to define a species that does not depend on appearance: The members of a species interbreed and have a shared gene pool, and each species is reproductively isolated from every other species. For example, the flycatchers in Figure 16.2 are members of separate species because they do not interbreed in nature.

According to the biological species concept, gene flow occurs between the populations of a species but not between populations of different species. The red maple and the sugar maple are found over a wide geographical range in the eastern half of the United States, and each species is made up of many populations. However, the members of each species’ population rarely hybridize in nature. Therefore, these two types of plants are separate species. In contrast, the human species has many populations that certainly differ in physical appearance (Fig. 16.3). We know, however, that all humans belong to one species because the members of these populations can produce fertile offspring.

The biological species concept is useful, as we shall see, but even so, it has its limitations. For example, it applies only to sexually reproducing organisms and cannot apply to asexually reproducing organisms. Then, too, sexually reproducing organisms are not always as reproductively isolated as we would expect. Some North American orioles live in the western half of the continent and some in the eastern half of the continent. Yet even the two most genetically distant oriole species, as recognized by analyzing their mitochondrial DNA, will hybridize where they meet in the middle of the continent.

There are other definitions of species aside from the biological definition. Later in this chapter, we will define species as a category of classification below the rank of genus. Species in the same genus share a recent common ancestor. A common ancestor is an ancestor for two or more different groups. Your grandmother is the common ancestor for you and your cousins. Similarly, there is a common ancestor for all species of roses.

Reproductive Barriers

As mentioned, for two species to be separate, they must be reproductively isolated—that is, gene flow must not occur between them. Reproductive barriers are isolating mechanisms that prevent successful reproduction from occurring (Fig. 16.4). In evolution, reproduction is successful only when it produces fertile offspring.

Prezygotic (before the formation of a zygote) isolating mechanisms are those that prevent reproduction attempts and make it unlikely that fertilization will be successful if mating is attempted. Habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gamete isolation make it highly unlikely that particular genotypes will contribute to the gene pool of a population.

Habitat isolationWhen two species occupy different habitats, even within the same geographic range, they are less likely to meet and attempt to reproduce. This is one of the reasons that the flycatchers in Figure 16.2 do not mate, and that the red maple and sugar maple do not exchange pollen. In tropical rain forests, many animal species are restricted to a particular level of the forest canopy, and in this way they are isolated from similar species.

Temporal isolationTwo species can live in the same locale, but if each reproduces at a different time of year, they do not attempt to mate. For example, Reticulitermes hageni and R. virginicus are two species of termites. The former has mating flights in March through May, whereas the latter mates in the fall and winter months.

Behavioral isolationMany animal species have courtship patterns that allow males and females to recognize one another (Fig. 16.5). Male fireflies are recognized by females of their species by the pattern of their flashings; similarly, male crickets are recognized by females of their species by their chirping. Many males recognize females of their species by sensing chemical signals called pheromones. For example, female gypsy moths secrete chemicals from special abdominal glands. These chemicals are detected downwind by receptors on the antennae of males.

Mechanical isolationWhen animal genitalia or plant floral structures are incompatible, reproduction cannot occur. Inaccessibility of pollen to certain pollinators can prevent cross-fertilization in plants, and the sexes of many insect species have genitalia that do not match, or other characteristics that make mating impossible. For example, male dragonflies have claspers that are suitable for holding only the females of their own species.

Gamete isolationEven if the gametes of two different species meet, they may not fuse to become a zygote. In animals, the sperm of one species may not be able to survive in the reproductive tract of another species, or the egg may have receptors only for sperm of its species. In plants, only certain types of pollen grains can germinate so that sperm successfully reach the egg.

Postzygotic (after the formation of a zygote) isolating mechanisms prevent hybrid offspring (reproductive product of two different species) from developing or breeding, even if reproduction attempts have been successful.

Zygote mortalityA hybrid zygote may not be viable, and so it dies. A zygote with two different chromosome sets may fail to go through mitosis properly, or the developing embryo may receive incompatible instructions from the maternal and paternal genes so that it cannot continue to exist.

Hybrid sterilityThe hybrid zygote may develop into a sterile adult. As is well known, a cross between a horse and a donkey produces a mule, which is usually sterile—it cannot reproduce (Fig. 16.6). Sterility of hybrids generally results from complications in meiosis that lead to an inability to produce viable gametes. A cross between a cabbage and a radish produces offspring that cannot form gametes, even though the diploid number is 18, an even number, most likely because the cabbage chromosomes and the radish chromosomes could not align during meiosis.

F₂ fitnessIf hybrids can reproduce, their offspring are unable to reproduce. In some cases, mules are fertile, but their offspring (the F₂ generation) are not fertile.

Models of Speciation

The introduction to this chapter suggests that the green iguana of South America may be the common ancestor for both the marine iguana on the Galápagos Islands (to the west of South America) and the rhinoceros iguana on Hispaniola (an island to the northeast of South America). If so, how could it have happened? Green iguanas are strong swimmers, so by chance a few could have migrated to these islands, where they formed populations separate from each other and from the parent population back in South America. Each population continued on its own evolutionary path as new mutations, genetic drift, and natural selection occurred. Eventually, reproductive isolation developed and there were three species of iguanas. A speciation model based on geographic isolation of populations is called allopatric speciation because allo means different and patria loosely means homeland.

Figure 16.7 features an example of allopatric speciation, which has been extensively studied in California. Members of an ancestral population of Ensatina salamanders existing in the Pacific Northwest migrated southward, establishing a range of populations. Each population was exposed to its own selective pressures along the coastal mountains and along the Sierra Nevada mountains. Due to the barrier created by the Central Valley of California, no gene flow occurred between the eastern populations and the western populations. Genetic differences increased from north to south, resulting in two distinct forms of Ensatina salamanders in southern California that differ dramatically in color and interbreed only rarely.

With sympatric speciation, a population develops into two or more reproductively isolated groups without prior geographic isolation. The best evidence for this type of speciation is found among plants, where it can occur by means of polyploidy, an increase in the number of sets of chromosomes to 3n or above. The presence of sex chromosomes makes it difficult for polyploidy speciation to occur in animals. In plants, hybridization between two species can be followed by a doubling of the chromosome number. Such polyploid plants are reproductively isolated by a postzygotic mechanism; they can reproduce successfully only with other similar polyploids, and backcrosses with their parents are sterile. Therefore, three species instead of two species result. Figure 16.8 shows that the parents of our present-day bread wheat had 28 and 14 chromosomes, respectively. The hybrid with 21 chromosomes is sterile, but polyploid bread wheat with 42 chromosomes is fertile because the chromosomes can align during meiosis.

Adaptive Radiation

One of the best examples of speciation through adaptive radiation is provided by the finches on the Galápagos Islands, which are often called Darwin’s finches because Darwin first realized their significance as an example of how evolution works. During adaptive radiation, many new species evolve from a single ancestral species. The many species of finches that live on the Galápagos Islands are believed to be descended from a single type of ancestral finch from the mainland (Fig. 16.9). The populations on the various islands were subjected to the founder effect involving genetic drift, genetic mutations, and the process of natural selection. Because of natural selection, each population became adapted to a particular habitat on its island. In time, the various populations became so genotypically different that now, when by chance they reside on the same island, they do not interbreed and are therefore separate species. There is evidence that the finches use beak shape to recognize members of the same species during courtship. Rejection of suitors with the wrong type of beak is a behavioral prezygotic isolating mechanism.

Similarly, inhabiting the Hawaiian Islands is a wide variety of hone-y-creepers, all descended from a common goldfinchlike ancestor that arrived from Asia or North America about 5 million years ago. Today, honeycreepers have a range of beak sizes and shapes for feeding on various food sources, including seeds, fruits, flowers, and insects.

_{16.2 The History of Species}

The history of the origin and extinction of species on Earth is best discovered by studying the fossil record. Fossils are the traces and remains of past life or any other direct evidence of past life. Traces include trails, footprints, burrows, worm casts, or even preserved droppings. Most fossils consist of the remains of hard parts, such as shells, bones, or teeth, because these are usually not consumed or destroyed (Fig. 16.10). Paleontology is the science of discovering and studying the fossil record and, from it, making decisions about the history of species.

The Geological Timescale

Because life-forms have evolved over time, the strata (layers of sedimentary rock, see Fig. 14.2a) of the Earth’s crust contain different fossils. By studying the strata and the fossils they contain, geologists have been able to construct a geological timescale (Table 16.1). It divides the history of life on Earth into eras, then periods, and then epochs, and describes the types of fossils common to each of these divisions of time. Notice that in the geological timescale, only the periods of the Cenozoic era are divided into epochs, meaning that more attention is given to the evolution of primates and flowering plants than to the earlier evolving organisms. Despite an epoch assigned to modern civilization, humans have only been around about .04% of the history of life.

In contrast, prokaryotes existed alone for some 2 billion years before the eukaryotic cell and multicellularity arose during Precambrian time. Some prokaryotes became the first photosynthesizers to add oxygen to the atmosphere. The presence of oxygen may have spurred the evolution of the eukaryotic cell and multicellularity during the Precambrian. All major groups of animals evolved during what is sometimes called the Cambrian explosion. The fossil record for Precambrian time is meager, but the fossil record for the Cambrian period is rich. The evolution of the invertebrate external skeleton accounts for this increase in the number of fossils. Perhaps this skeleton which impedes the uptake of oxygen couldn’t evolve until oxygen was plentiful. Or perhaps the external skeleton was merely a defense against predation.

The origin of life on land is another interesting topic. During the Paleozoic era, plants are present on land before animals. Nonvascular plants preceded vascular plants, and among these, cone-bearing plants (gymnosperms) preceded flowering plants (angiosperms). Among vertebrates, the fishes are aquatic, and the amphibians invaded land. The reptiles (e.g., dinosaurs) are ancestral to both birds and mammals. The number of species in the world has continued to increase until the present time, despite the occurrence of five mass extinctions and one significant mammalian extinction during the history of life on Earth.

The Pace of Speciation

Many evolutionists believe as Darwin did that evolutionary changes occur gradually. In other words, they support a gradualistic model to explain the pace of evolution. Speciation probably occurs after populations become isolated, with each group continuing slowly on its own evolutionary pathway. These evolutionists often show the evolutionary history of groups of organisms by drawing the type of evolutionary tree shown in Figure 16.11a. Note that in this diagram, an ancestral species has given rise to two separate species, represented by a slow change in plumage color. The gradualistic model suggests that it is difficult to indicate when speciation has occurred because there would be so many transitional links. In some cases, it has been possible to trace the evolution of a group of organisms by finding transitional links.

More often, however, species appear quite suddenly in the fossil record, and then they remain essentially unchanged phenotypically until they undergo extinction. Some paleontologists have therefore developed a punctuated equilibrium model to explain the pace of evolution. The model says that a period of equilibrium (no change) is punctuated (interrupted) by speciation. Figure 16.11b shows the type of diagram paleontologists prefer to use when representing the history of evolution over time. This model suggests that transitional links are less likely to become fossils and less likely to be found. Speciation most likely involves only an isolated subpopulation at one locale. Only when this new subpopulation expands and replaces existing species is it apt to show up in the fossil record.

The differences between these two models are subtle, especially when we consider that the “sudden” appearance of a new species in the fossil record could represent many thousands of years because geological time is measured in millions of years.

Mass Extinctions of Species

As we have noted, most species exist for only a limited amount of time (measured in millions of years), and then they die out (become extinct). Mass extinctions are the disappearance of a large number of species or a higher taxonomic group within a relatively short period of time. The geological timescale in Table 16.1 shows the occurrence of five mass extinctions: at the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. Also, there was a significant mammalian extinction at the end of the Pleistocene epoch. While many factors contribute to mass extinctions, two possible types of events (continental drift and meteorite impacts) are believed to be particularly significant.

Continental drift—the movement of continents—is believed to have contributed to several extinctions. You may have noticed that the coastlines of several continents are mirror images of each other. For example, the outline of the west coast of Africa matches that of the east coast of South America. Also, the same geological structures are found in many of the areas where the continents at some time in the past touched. A single mountain range runs through South America, Antarctica, and Australia, for example. But the mountain range is no longer continuous because the continents have drifted apart. The reason the continents drift is explained by a branch of geology known as plate tectonics, which is based on the fact that the Earth’s crust is fragmented into slablike plates that float on a lower, hot mantle layer (Fig. 16.12). The continents and the ocean basins are a part of these rigid plates, which move like conveyor belts.

The loss of habitats is a significant cause of extinctions, and continental drift can lead to massive habitat changes. We know that 250 million years ago (mya), at the time of the Permian mass extinction, all the Earth’s continents came together to form one large continent called Pangaea (Fig. 16.13a). The result was dramatic environmental change that profoundly affected life. Marine organisms suffered as the oceans were joined, and the amount of shoreline, where many marine organisms live, was reduced. They didn’t recover until some continents drifted away from the poles and warmth returned (Fig. 16.13b). Terrestrial organisms were affected as well because the amount of interior land, which tends to have a drier and more erratic climate, increased. Immense glaciers developing at the poles withdrew water from the oceans and chilled even once-tropical land.

The other event that is believed to have contributed to mass extinctions is the impact of a meteorite as it crashed into the Earth. The result of a large meteorite striking Earth could have been similar to that of a worldwide atomic bomb explosion: A cloud of dust would have mushroomed into the atmosphere, blocking out the sun and causing plants to freeze and die. This type of event has been proposed as a primary cause of the Cretaceous extinction that saw the demise of the dinosaurs. Cretaceous clay contains an abnormally high level of iridium, an element that is rare in the Earth’s crust but more common in meteorites. A layer of soot has been identified in the strata alongside the iridium, and a huge crater that could have been caused by a meteorite was found in the Caribbean–Gulf of Mexico region on the Yucatán peninsula.

_{16.3 Classification of Species}

The many recognized species on Earth are classified according to their presumed evolutionary relationship. Taxonomy is the branch of biology concerned with identifying, naming, and classifying organisms. Classification begins when organisms are given a scientific name. For example, Lilium buibiferum is a type of lily. The first word, Lilium, is the genus in a classification category that can contain many species. The second word, the specific epithet, refers to one species within that genus. Notice that the scientific name is in italics; the genus is capitalized, while the specific epithet is not. The species is designated by the full binomial name—in this case, Lilium buibiferum. However, the genus name can be used alone to refer to a group of related species. Also, the genus can be abbreviated to a single letter if used with the specific epithet (e.g., L. buibiferum) and if the full name has been given previously.

Today, taxonomists use the following categories of classification: species, genus, family, order, class, phylum, and kingdom. Recently, a higher taxonomic category, the domain, has been added to this list. There can be several species within a genus, several genera within a family, and so forth. In this hierarchy, the higher the category, the more inclusive it is (Fig. 16.14). Also, the organisms that fill a particular classification category are distinguishable from other organisms by sharing a structural, chromosomal, or molecular feature that distinguishes one group from another. Organisms in the same -domain have general characteristics in common; those in the same species have quite specific characteristics in common.

In most cases, each category of classification can be subdivided into three additional categories, as in superorder, -order, suborder, and infraorder. Considering these, there are more than 30 categories of classification.

Classification and Phylogeny

Taxonomy and classification are a part of the broader field of systematics, which is the study of the diversity of organisms at all levels of orga-ni-za-tion. One goal of systematics is to determine phylogeny, the evolutionary history of a group of organisms. When we say that two species (or genera, families, etc.) are closely related, we mean they share a recent common ancestor.

Using traditional systematics, Figure 16.15 shows how the classification of groups of organisms allows us to construct a phylogenetic tree, a diagram that indicates common ancestors and lines of descent (lineages). In order to classify organisms and to construct a phylogenetic tree, it is necessary to determine the characteristics, or simply, characters, of the various groups, called taxa (sing., taxon). A primitive character is present in the common ancestor and in all taxa belonging to that group. A derived character is found only in a particular line of descent. Whether a character is primitive or derived depends on the group being considered. For example, a primitive character for primates may be a derived character for mammals. Different lineages diverging from a common ancestor are expected to have different derived characters. Thus, all the animals in the family Cervidae have antlers, but they are highly branched in red deer and palmate (having the shape of a hand) in reindeer.

Tracing Phylogeny

Systematists gather all sorts of data in order to discover the evolutionary relationships between species. They rely heavily on a combination of data from the fossil record, homology, and molecular data to determine the correct sequence of common ancestors in any particular group of organisms. If you can determine the common ancestors in the history of life, you know how evolution occurred and can classify organisms accordingly.

HomologyHomology is character similarity that stems from having a common ancestor. Comparative anatomy, including embryological evidence, provides information regarding homology. Homologous structures are related to each other through common descent. The forelimbs of vertebrates are homologous because they contain the same bones organized in the same general way as in a common ancestor (see Fig. 14.14). However, a horse has but a single digit and toe (the hoof), while a bat has four lengthened digits that support its membranous wings.

Deciphering homology is sometimes difficult because of convergent evolution and parallel evolution. Convergent evolution is the acquisition of the same or similar characters in distantly related lines of descent. Similarity due to convergence is termed analogy. The wings of an insect and the wings of a bat are analogous. You may recall from Chapter 14 that analogous structures have the same function in different groups, but organisms with these structures do not have a common ancestor. Both cactuses and spurges are adapted similarly to a hot, dry environment, and both are succulent, spiny, flowering plants. However, the details of their flower structure indicate that these plants are not closely related. Parallel evolution is the acquisition of the same or a similar character in two or more related lineages without it being present in a common ancestor. A similar banding pattern is found in several species of moths, for example. It is sometimes difficult to tell if features are primitive, derived, convergent, or parallel.

Molecular DataSpeciation occurs when mutations bring about changes in the base-pair sequences of DNA. Systematists, therefore, assume that the more closely species are related, the fewer changes will be found in DNA base-pair sequences. Because molecular data are straightforward and numerical, they can sometimes sort out relationships obscured by inconsequential anatomical variations or convergence. Software breakthroughs have made it possible to analyze nucleotide sequences quickly and accurately using a computer. Also, these analyses are available to anyone doing comparative studies through the Internet, so each investigator doesn’t have to start from scratch. The combination of accuracy and availability of past data has made molecular systematics a standard way to study the relatedness of groups of organisms today.

All cells have ribosomes, which are essential for protein synthesis. Further, the genes that code for ribosomal RNA (rRNA) have changed very slowly during evolution because any drastic changes lead to malfunctioning cells. Therefore, it is believed that comparative rRNA sequencing provides a reliable indicator of the similarity between organisms. Ribo-somal RNA sequencing helped investigators conclude that all living things can be divided into the three domains.

One study involving DNA differences produced the data shown in Figure 16.16. Although the data suggest that chimpanzees are more closely related to humans than to other apes, in most classifications, humans and chimpanzees are placed in different families; humans are in the family Hominidae, and chimpanzees are in the family Pongidae. In contrast, the rhesus monkey and the green monkey, which have more numerous DNA differences, are placed in the same family (Cercopithecidae). To be consistent with the data, shouldn’t humans and chimpanzees also be in the same family? Traditional systematists, in particular, believe that since humans are markedly different from chimpanzees because of adaptation to a different environment, it is justifiable to place humans in a separate family.

Cladistic Systematics

Cladistics is a school of systematics that was introduced in 1950 and has become increasingly influential. The goal is to produce testable hypotheses about the evolutionary relationships among a series of organisms. Cladistics depends on distinguishing primitive from shared derived characters to classify organisms and arrange taxa in a type of diagram called a cladogram. A cladogram traces the evolutionary history of the group being studied. Let’s see how it works.

The first step when constructing a cladogram is to draw up a table that summarizes the characters of the taxa being compared (Fig. 16.17a). Although this table concerns anatomical characters only, cladists use all types of data, including molecular data, to arrive at evolutionary relationships. At least one, but preferably several, species are studied as an outgroup, a group that is not part of the study group. In this example, lancelets are in the outgroup, and selected vertebrates (eels, newts, snakes, and lizards) are in the study group. Any character found in both the outgroup and the study group is a shared primitive character presumed to have been present in an ancestor common to both the outgroup and the study group. Lancelets (in the outgroup) have a supporting rod called a notochord as an embryo, and so do all vertebrates (in the study group). Therefore, a notochord is a primitive character shared by all taxa in Figure 16.17a.

Cladists are always guided by the principle of parsimony, which states that the least number of assumptions is the most probable. Thus, they construct a cladogram that minimizes the number of assumed evolutionary changes, or leaves the fewest number of derived characters unexplained. Therefore, any character in the table found in scattered taxa (in this case, four bony limbs and a long cylindrical body) is not used to construct the cladogram because we would have to assume that these characters evolved more than once among the taxa of the study group. The other characters are designated as shared derived characters—that is, they are homologies shared by only certain taxa of the study group.

The Cladogram

In a cladogram, a clade is an evolutionary branch that includes a common ancestor together with all its -de-scendant species. A clade includes the taxa that have shared derived characters. The cladogram in Figure 16.17b has three clades in the study group that differ in size because the first includes the other two, and so forth. Based on Figure 16.17a, the common ancestor at the root of the tree has one primitive character—a notochord when an embryo—and all taxa in the cladogram share this character. Figure 16.17a also tells us that all the taxa in the study group belong to a clade that has vertebrae; only newts, snakes, and lizards are in a clade that has lungs and a three-chambered heart; and only snakes and lizards are in a clade that has an amniotic egg and internal fertilization. Therefore, this is the sequence in which these characters must have evolved during the evolutionary history of vertebrates.

A cladogram is objective—it lists the characters used to construct the cladogram. Cladists regard a cladogram as a hypothesis that can be tested and either corroborated or refuted on the basis of additional data. Therefore, they consider a cladogram to be more scientific than a traditional phylogenetic tree.

Traditionalists Versus Cladists

Traditional systematists use a wider range of information than do cladists, and they make value judgments about which characters are more indicative of phylogeny. Traditionalists believe that organisms adapted to a new environment need not be classified with the common ancestor from which they evolved. In the traditional phylogenetic tree shown in Figure 16.18a, birds and mammals are placed in different classes because it is quite obvious to the most casual observer that mammals (having hair and mammary glands) and birds (having feathers) are quite different in appearance from reptiles (having scaly skin). The traditionalist tree shows that mammals and birds have a different reptilian ancestor.

Cladists prefer the cladogram in Figure 16.18b. All the animals shown are in one clade because they all evolved from a common ancestor that laid eggs. Mammals can be placed in their own class because they alone have hair and mammary glands and three middle ear bones. Not so for reptiles: The only thing that dinosaurs, crocodiles, snakes, lizards, and turtles have in common is that they are not birds or mammals. On the other hand, crocodiles and birds share common derived anatomical and behavioral characters not present in snakes and lizards. On this basis, cladists place crocodiles and birds in the same taxon. They agree that birds and crocodiles are adapted to different ways of life, but this observation is not the type of data used to construct a cladogram. Most traditionalists today regard adaptations as valuable data, and they place birds in a separate taxon from reptiles. They also place humans in a different family from all the apes, even though molecular data suggest that humans shared a common ancestor with chimpanzees.

Classification Systems

Classification systems change over time. Until recently, most biologists utilized the five-kingdom system of classification, which contains kingdoms for the plants, animals, fungi, protists, and monerans. Organisms were placed into these kingdoms based on type of cell (prokaryotic or eukaryotic), level of organization (unicellular or multicellular), and type of nutrition. In the five-kingdom system, the monerans were distinguished by their structure—they were prokaryotic (lack a membrane-bounded nucleus)—whereas the organisms in the other kingdoms were eukaryotic (have a membrane-bounded nucleus). The kingdom Monera contained all prokaryotes, which according to the fossil record evolved first.

Within the past ten years, new information has called into question the five-kingdom system of classification. Molecular and cellular data suggest that there are two groups of prokaryotes—the bacteria and the archaea—and that these groups are fundamentally different from each other. In fact, they are so different that they should be assigned to separate domains, a category of classification that is higher than the kingdom category. The bacteria arose first, and they were followed by the archaea and then the eukarya (Fig. 16.19). The archaea and eukarya are more closely related to each other than either is to the bacteria. Systematists, using the three-domain system of classification, are in the process of sorting out what kingdoms belong within domain Bacteria and domain Archaea. Domain Eukarya contains kingdoms for protists, animals, fungi, and plants. Later in this text, we will study the individual kingdoms that occur within the domain Eukarya. The protists do not share one common ancestor, and some suggest that the kingdom should be divided into many different kingdoms. How many kingdoms is still a matter of debate among systematists, illustrating that taxonomy can be subjective until data are available and an objective conclusion is acceptable to most systematists.

The Chapter in Review

Summary

16.1 Macroevolution

Macroevolution is evolution on a large scale because it considers the history of life on Earth. Macroevolution begins with speciation, the origin of new species. Without speciation and the extinction of species, life on Earth would not have a history.

Defining Species

The biological concept of species

• recognizes a species by its inability to reproduce with members of another group.

• is useful because species can look similar, and members of the same species can have different appearances.

• has its limitations. Hybridization does occur between some species, and it is only relevant to sexually reproducing organisms.

Reproductive Barriers

Prezygotic and postzygotic barriers keep species from reproducing with one another.

• Prezygotic isolating mechanisms involve habitat isolation, temporal isolation, and behavioral isolation.

• Postzygotic isolating mechanisms prevent hybrid offspring from developing or breeding if reproduction has been successful.

Models of Speciation

Allopatric speciation and sympatric speciation are two models of speciation.

• The allopatric speciation model proposes that a geographic barrier keeps groups of populations apart. Meanwhile, prezygotic and postzygotic isolating mechanisms develop, and these prevent successful reproduction if these two groups come into future contact.

• The sympatric speciation model proposes that a geographic barrier is not required.

16.2 The History of Species

The fossil record, as outlined by the geological timescale, traces the history of life in broad terms. It has been possible to absolutely date fossils by using radioactive dating techniques.

The fossil record

• can be used to support a gradualistic model: Two groups of organisms arise from an ancestral species and gradually become two different species.

• can be used to support a punctuated equilibrium model: A period of equilibrium (no change) is interrupted by speciation within a relatively short period of time.

• shows that at least five mass extinctions and one significant mammalian extinction have occurred during the history of life on Earth. Two major contributors to mass extinctions are the loss of habitats due to continental drift and the disastrous results from a meteorite impacting Earth.

16.3 Classification of Species

Systematics

• assigns every species a scientific name, which immediately indicates its genus. From that beginning, species are also assigned to a family, order, class, phylum, kingdom, and domain according to their molecular and structural characters.

• attempts to determine phylogeny, the evolutionary history of a group of organisms. Systematics relies on the fossil record, homology, and molecular data to determine relationships among organisms.

• has different schools. Cladists use shared primitive and shared derived characters to construct cladograms. In a cladogram, a clade consisting of a common ancestor and all the species derived from a common ancestor, the species have shared derived characteristics. Traditionalists are not opposed to making value judgments (particularly about adaptations to new environments) in constructing phylogenetic trees, but cladists use only objective data when constructing cladograms.

Classification Systems

• The five-kingdom system recognized the kingdoms for the monerans, protists, animals, fungi, and plants.

• The three-domain system uses molecular data to designate three evolutionary domains: Bacteria, Archaea, and Eukarya:

• Domains Bacteria and Archaea contain prokaryotes.

• Domain Eukarya contains kingdoms for the protists, animals, fungi, and plants.

Thinking Scientifically

1. Use the data from the following table to fill in the phylogenetic tree for vascular plants. Plants with vascular tissue have transport tissue.

2. The Hawaiian Islands are located thousands of kilometers from any mainland. Each island arose from the sea bottom and was colonized by plants and animals that drifted in on ocean currents or winds. Each island has a unique environment in which its inhabitants have evolved. Consequently, most of the plant and animal species on the islands do not exist anywhere else in the world.

In contrast, on the islands of the Florida Keys, there are no unique or indigenous species. All of the species on those islands also exist on the mainland. Suggest an explanation for these two different patterns of speciation.

Testing Yourself

Choose the best answer for each question.

1. A biological species

a. always looks different from other species.

b.
always has a different chromosome number from that of other species.

c. is reproductively isolated from other species.

d. never occupies the same niche as other species.

For questions 2–9, indicate the type of isolating mechanism described in each scenario.

Key:

a. habitat isolation

b. temporal isolation

c. behavioral isolation

d. mechanical isolation

e. gamete isolation

f. zygote mortality

g. hybrid sterility

h. low F₂ fitness

2. Males of one species do not recognize the courtship behaviors of females of another species.

3. One species reproduces at a different time than another species.

4. A cross between two species produces a zygote that always dies.

5. Two species do not interbreed because they occupy different areas.

6. A hybrid between two species produces gametes that are not viable.

7. Two species of plants do not mate with each other because they are visited by different pollinators.

8. The sperm of one species cannot survive in the reproductive tract of another species.

9. The offspring of two hybrid individuals exhibit poor vigor.

10. Complete the following diagram illustrating allopatric speciation by using these phrases: genetic changes (used twice), geographic barrier, species 1, species 2, species 3.

11. Transitional links are least likely to be found if evolution proceeds according to the

a. gradualistic model.

b. punctuated equilibrium model.

12. Which is the scientific name of an organism?

a. Rosa rugosa

b. Rosa

c. rugosa

d. rugosa rugosa

e. Both a and d are correct.

13. Which of these statements best pertains to taxonomy? Species

a. always have three-part names, such as Homo sapiens sapiens.

b. are always reproductively isolated from other species.

c. always share the most recent common ancestor.

d. always look exactly alike.

e. Both c and d are correct.

14. Which of the following groups are domains? Choose more than one answer if correct.

a. bacteria

b. archaea

c. eukarya

d. animals

e. plants

15. Which pair is mismatched?

a. homology—character similarity due to a common ancestor

b. molecular data—DNA strands match

c. fossil record—bones and teeth

d. homology—functions always differ

e. molecular data—DNA and RNA data

16. One benefit of the fossil record is

a. that hard parts are more likely to fossilize.

b. that fossils can be dated.

c. its completeness.

d. that fossils congregate in one place.

e. All of these are correct.

17. The discovery of common ancestors in the fossil record, the presence of homologies, and molecular data similarities help scientists decide

a. how to classify organisms.

b. the proper cladogram.

c. how to construct phylogenetic trees.

d. how evolution occurred.

e. All of these are correct.

18. In cladistics,

a.
a clade must contain the common ancestor plus all its descendants.

b. derived characters help construct cladograms.

c. data for the cladogram are presented.

d. the species in a clade share homologous structures.

e. All of these are correct.

19. Answer these questions about the following cladogram.

a.
This cladogram contains how many clades? How are they
designated in the diagram?

b.
What character is shared by all animals in the study group? What characters are shared by only snakes and lizards?

c.
Which animals share a recent common ancestor? How do you know?

20. Allopatric, but not sympatric, speciation requires

a. reproductive isolation.

b. geographic isolation.

c. spontaneous differences in males and females.

d. prior hybridization.

e. rapid rate of mutation.

21. Which kingdom is mismatched?

a. Protista—domain Bacteria

b. Protista—multicellular algae

c. Plantae—flowers and mosses

d. Animalia—arthropods and humans

e. Fungi—molds and mushrooms

Go to www.mhhe.com/maderessentials for more quiz questions.

Bioethical Issue

Paleoanthropologists often use cladistics to create phylogenetic trees based on bone and tooth measurements. According to developmental biologists, though, bone shape and arrangement are influenced by both the environment and complex genetic interactions. Therefore, some evolutionary biologists are now suggesting that skeletal anatomy cannot be used to produce cladograms that make biological sense. Cladograms based on skull and tooth measurements do not coincide with those based on DNA base-pair sequence similarities. In order to determine the most valuable traits for cladistic analyses, researchers need to determine the molecular pathways involved in the development of the human body. This is an area that would require millions of dollars of research funding.

Would you support the use of government funding for research into the genetics and biology of body development, for the purpose of better understanding our phylogenetic tree? If not, would you support the idea if the research had the potential to advance our knowledge of related medical problems, such as osteoporosis?

Understanding the Terms

adaptive radiation251

allopatric speciation250

analogous structure257

analogy257

binomial name256

cladistics259

class256

common ancestor247, 256

convergent evolution257

domain256

domain Archaea261

domain Bacteria261

domain Eukarya261

evolutionary tree254

family256

five-kingdom system261

fossil253

genus256

homologous structure257

kingdom256

macroevolution246

mass extinction254

order256

paleontology253

parallel evolution257

phylogenetic tree256

phylogeny256

phylum256

postzygotic isolating
mechanism249

prezygotic isolating
mechanism248

speciation246

species256

specific epithet256

sympatric speciation250

systematics256

taxon (pl., taxa)256

taxonomy256

three-domain system261

traditional systematics256

Match the terms to these definitions:

a. _______________ Evolution of several new species from a single ancestral species.

b. _______________ Science of discovering and studying the fossil record.

c. _______________ Disappearance of a large number of species in a short period of time.

d. _______________ Scientific name that includes genus and species.

e. _______________ Diagram that tells the evolutionary history of a group of organisms.

f. _______________ Most inclusive of the classification categories in the three-domain system.

g. _______________ Anatomical structure indicative of relatedness through common descent.

¹ Hispaniola is divided into Haiti and the Dominican Republic.

The unique marine iguana feeds at sea and lives to the west of South America, only in the Galápagos Islands.

The common green iguana, a strong swimmer native to South America, could be ancestral to other types of iguanas.

The rhinoceros iguana, with bumps on its snout, lives to the northeast of South America, only on Hispaniola.

Figure 16.1Dinosaurs.

Triceratops (left) and Tyrannosaurus rex (right) were dinosaurs that lived about 65 million years ago when flowering plants were already flourishing. The prevailing forms of life on Earth change as new species come into existence, flourish, and then decline or become extinct.

Figure 16.3Human populations.

The Massai of East Africa (a) and the Eskimos that live near the Arctic Circle (b) are both members of the species Homo sapiens because Massai can reproduce with Eskimos and produce fertile offspring.

Figure 16.2Three species of flycatchers.

Although these flycatcher species are nearly identical in appearance, we know they are separate species because they are reproductively isolated—the members of each species reproduce only with one another. Each species has a characteristic song and its own particular habitat during the mating season as well.

Figure 16.4Reproductive barriers.

Prezygotic isolating mechanisms prevent mating attempts or a successful outcome should mating take place. No zygote is ever formed. Postzygotic isolating mechanisms prevent offspring from reproducing—that is, the hybrids are unable to breed successfully.

Check Your Progress

1. Define a species according to the biological species concept.

2.
Describe the limitations of the biological species concept.

3. Contrast prezygotic isolating mechanisms with postzygotic isolating mechanisms.

Answers: 1.A biological species is a group of organisms that can interbreed. The members of this species are reproductively isolated from members of other species.2. The biological species concept cannot be applied to asexually reproducing organisms. In addition, reproductive isolation may not be complete, so some breeding between species may occur.3. Prezygotic isolating mechanisms do not allow zygotes to be formed. Postzygotic mechanisms allow zygotes to be formed, but the zygotes either die or become offspring that are infertile.

Figure 16.5Prezygotic isolating mechanism.

An elaborate courtship display allows the blue-footed boobies of the Galápagos Islands to select a mate. The male lifts his feet in a ritualized manner that shows off their bright blue color.

Figure 16.6Postzygotic isolating mechanism.

Mules are horse-donkey hybrids. Mules are infertile due to a difference in the chromosomes inherited from their parents.

Figure 16.7Allopatric 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. The limited contact between the populations on the west and those on the east allow genetic changes to build up so that two populations, both living in the south, do not reproduce with one another and therefore can be designated as separate species.

Figure 16.8Sympatric speciation.

In this example of sympatric speciation, two populations of wild wheat hybridized many years ago. The hybrid is sterile, but chromosome doubling allowed some plants to reproduce. These plants became today’s bread wheat.

Check Your Progress

1. Contrast allopatric speciation with sympatric speciation.

2.
Explain why Darwin’s finches are an example of adaptive radiation.

Answers:1. Allopatric speciation occurs when a geographic barrier stops gene flow, while sympatric speciation produces reproductively isolated groups without geographic isolation.2. One mainland finch species was probably ancestral to all existing species on the Galápagos Islands. Each new species evolved in response to the unique environment on the island it inhabits.

Figure 16.9Darwin’s finches.

Each of Darwin’s finches is adapted to gathering and eating a different type of food. Tree finches have beaks largely adapted to eating insects and, at times, plants. Ground finches have beaks adapted to eating the flesh of the prickly-pear cactus or different-sized seeds.

Figure 16.10Fossils.

a. A fern leaf from 245 million years ago (mya) remains because it was buried in sediment that hardened to rock. b. This midge (40 mya) became embedded in amber (hardened resin from a tree).
c. The shells of ammonites (135 mya) left a mold filled in by minerals. d. Most fossils, such as this early insectivore mammal (47 mya) are remains of hard parts because they do not decay as the soft parts do. e. These tree trunks (190 mya) are petrified because minerals have replaced their soft parts. f. Signs of ancient life, such as these dinosaur footprints (135 mya) are traces of past life.

Figure 16.11Pace of evolution.

a. According to the gradualistic model, new species evolve slowly from an ancestral species. b. According to the punctuated equilibrium model, new species appear suddenly and then remain largely unchanged until they become extinct.

Figure 16.12Plate tectonics.

The Earth’s surface is divided into several solid tectonic plates floating on the fluid magma beneath them. At rifts in the ocean floor, two plates gradually separate as fresh magma wells up and cools, enlarging the plates. Mountains, including volcanoes, are raised where one plate pushes beneath another at subduction zones. Where two plates slowly grind past each other at a fault line, tension builds up, which is released occasionally in the form of earthquakes.

Figure 16.13Continental drift.

a. About 250 mya, all the continents were joined into a supercontinent called Pangaea. b. By 65 mya, all the continents had begun to separate. This process is continuing today. North America and Europe are separating at a rate of about 2 centimeters per year.

Figure 16.14Taxonomy hierarchy.

A domain is the most inclusive of the classification categories. The plant kingdom is in the domain Eukarya. In the plant kingdom are several phyla, each represented here by lavender circles. The phylum Anthophyta has only two classes (the monocots and eudicots). The class Monocotyledones encompasses many orders. In the order Orchidales are many families; in the family Orchidaceae are many genera; and in the genus Cypripedium are many species—for example, Cypripedium acaule. (This illustration is diagrammatic and doesn’t necessarily show the correct number of subcategories.)

Figure 16.15Classification and phylogeny.

a. The classification and phylogenetic tree for a group of organisms is ideally constructed to reflect their phylogenetic history. A species is most closely related to other species in the same genus, more distantly related to species in other genera of the same family, and so forth, on through order, class, phylum, and kingdom.
b. A diagrammatic tree showing the location of common ancestors, which are said to have primitive (ancestral) characters. Following cladogenesis (a divergence), each line of descent has its own derived (evolved) characters.

Check Your Progress

1. List the categories of classification, in order, from the largest category to the smallest.

2.
Contrast homologous structures with analogous structures.

3.
What is a phylogenetic tree?

Answers:1. Domain, kingdom, phylum, class, order, family, genus, species.
2. Homologous structures are similar in anatomy because they are derived from a common ancestor. Analogous structures have the same function in different groups and do not share a common ancestry.3. A phylogenetic tree is a diagram that indicates common ancestors and lines of descent.

Figure 16.16Molecular data.

The relationships of certain primate species are based on a study of their genomes. The length of the branches indicates the relative number of DNA base-pair differences between the groups. These data, along with knowledge of the fossil record for one divergence, make it possible to suggest a date for the other divergences in the tree.

Check Your Progress

1. What types of data are used to determine evolutionary relationships?

2. Explain why the sequences of ribosomal RNA genes are used for evolutionary studies.

Answers:1. Homology and molecular data are used.2. All organisms have genes that code for ribosomal RNA. Because these genes are essential, they cannot tolerate mutations very well and accumulate them slowly. Similarities in DNA sequence for these genes can be used to determine who is related to whom.

Check Your Progress

1. Describe how cladists classify organisms.

2.
Explain how cladists employ the principle of parsimony.

3. Contrast traditionalists with cladists.

Answers:1. Cladistics uses only shared derived characters to determine that several taxa share a common ancestor and belong to a clade.2. Cladists use the minimum number of assumptions to construct a cladogram.
3. Traditionalists use a wider range of information than do cladists and make value judgments about the importance of certain characters.

Figure 16.18Traditional versus cladistic view of reptilian phylogeny.

Traditionalists are more likely to use value judgments than are cladists. a. Although crocodiles and birds share many characteristics, traditionalists place birds in a different class because the evolution of feathers is such a significant event in the history of animals.
b. Cladists make no value judgments and place crocodiles and birds in the same class on the basis of shared derived characters. The colors show that the placement of common ancestors is the same in both diagrams.

Figure 16.19Three-domain system.

In this system, the prokaryotes are in the domains Bacteria and Archaea. The eukaryotes are in the domain Eukarya, which contains four kingdoms for the protists, animals, fungi, and plants.

Figure 16.17Constructing a cladogram.

a. First, a table is drawn up that lists characters for all the taxa. An examination of the table shows which characters are primitive (notochord aqua) and which are derived (lavender, orange, and yellow). The shared derived characters distinguish the taxa. b. In a cladogram, the shared derived characters are sequenced in the order they evolved and are used to define clades. A clade contains a common ancestor and all the species that share the same derived characters (homologies). Four bony limbs and a long cylindrical body were not used in constructing the cladogram because they are in scattered taxa.

Acadian flycatcher, Empidonax virescens

least flycatcher,
Empidonax minimus

Traill’s flycatcher, Empidonax trailli

Table 16.1

The Geological Timescale: Major Divisions of Geological Time and Some of the Major Evolutionary Events That Occurred

Era	Period	Epoch	MYA	Plant and Animal Life
		Holocene	0–0.01	AGE OF HUMAN CIVILIZATION; Destruction of tropical rain forests accelerates extinctions.
		Significant Mammalian Extinction
Cenozoic*	Neogene	Pleistocene	0.01–2	Modern humans appear; modern plants spread and diversify.
		Pliocene	2–6	First hominids appear; modern angiosperms flourish.
		Miocene	6–24	Apelike mammals, grazing mammals, and insects flourish; grasslands spread; and forests contract.
		Oligocene	24–37	Monkeylike primates appear; modern angiosperms appear.
	Paleogene	Eocene	37–58	All modern orders of mammals are present; subtropical forests flourish.
		Paleocene	58–65	Primates, herbivores, carnivores, insectivores are present; angiosperms diversify.
		Mass Extinction: Dinosaurs and Most Reptiles
	Cretaceous		65–144	Placental mammals and modern insects appear; angiosperms spread and conifers persist.
Mesozoic	Jurassic		144–208	Dinosaurs flourish; birds and angiosperms appear.
		Mass Extinction
	Triassic		208–250	First mammals and dinosaurs appear; forests of conifers and cycads dominate land; corals and molluscs dominate seas.
		Mass Extinction
	Permian		250–286	Reptiles diversify; amphibians decline; and gymnosperms diversify.
	Carboniferous		286–360	Amphibians diversify; reptiles appear; and insects diversify. Age of great coal-forming forests.
		Mass Extinction
Paleozoic	Devonian		360–408	Jawed fishes diversify; insects and amphibians appear; seedless vascular plants diversify and seed plants appear.
	Silurian		408–438	First jawed fishes and seedless vascular plants appear.
		Mass Extinction
	Ordovician		438–510	Invertebrates spread and diversify; jawless fishes appear; nonvascular plants appear on land.
	Cambrian		510–543	Marine invertebrates with skeletons are dominant and invade land, and marine algae flourish.
Precambrian time			600	Oldest soft-bodied invertebrate fossils
			1,400–700	Protists evolve and diversify.
			2,000	Oldest eukaryotic fossils
			2,500	O2 accumulates in atmosphere
			3,500	Oldest known fossils (prokaryotes)
			4,500	Earth forms.

* Many authorities divide the Cenozoic era into the Tertiary period (contains Paleocene, Eocene, Oligocene, Miocene, and Pliocene) and the Quaternary period (contains Pleistocene and Holocene).