Chap 16 - Bio

Iguanas as Examples of Evolutionary Change

guanas are a type of lizard known for a very long tail, which can be up to

three times their body length. The green iguana is commo'i'rthroughout 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.

Green iguanas do not inhabit islands to the west and northeast of South America. Instead, the Galapagos 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 Galapagos Islands-they occur no place else on 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, an island to the northeast of South America, is inhabited by

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,

lmestone habitats and feeds on a wide variety of plants.

The theory of evolution explains the occurrence of the unique iguanas

on the Galapagos Islands and Hispaniola. It is hypothesized and supported by various data that ancestral iguanas swam or hitchhiked on floating drift

wood from South America to the Galapagos to the west and to Hispaniola to

the northeast. After arrival, these iguana populations were cut off from other

iguana populations, and this allowed them to evolve into these new and dif

ferent species. The origin of species and how they are classified and studied

are the topics of this chapter.

Evolution on a Large Scale

OUTLINE

16.1 Speciation and Macroevolution 266

16.2 The Fossil Record 272

16.3 Systematics 277

BEFORE YOU BEGIN

Before beginning this chapter, take a few moments to

review the following discussions.

Section 14.2 What roles do the fossil records and the

study of comparative anatomy have in understanding

evolutionary change?

Section 15.1 How does natural selection act as the

mechanism of evolutionary change?

Section 15.2 What is microevolution?

265

266 PARTTHREE Evolution

Figure 16.1 Dinosaurs.

Artwork of a Mesozoic landscape, Cretaceous period,

including saltasaurus, brachiosaurus, mononykus,

protoceratops, nyctosaurus, styracosaurus, tarbosaurus,

maiasaura, stegoceras, evoplocephalus, deltatheridium, and

ichthyornis (145 to 65 MYA). All of these species are now extinct.

16.1 Speciation and Macroevolution

Learning Outcomes

Upon completion of this section, you should be able to

1. Explain how scientists define a biological species and describe the

limitation of this definition.

2. List five types of prezygotic isolating mechanisms and explain how

they prevent members of two different species from reproducing.

3. List three types of postzygotic isolating mechanisms and explain

how they prevent members of two different species from reproducing .

4. Contrast allopatric speciation with sympatric speciation and explain

how each method may result in the creation of a new species.

5. Explain how new species may arise by adaptive radiation.

Chapter 15 considered evolution on a small scale-that is, microevolution, small changes over a short period of time. In this chapter, we turn our attention to evolution on a large scale-that is, macroevolution, large changes over a very long period of time. The history of life on Earth is a part of macroevolu

tion. 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, evolve adaptations to their environments, and then may become extinct (Fig. 16.1). Without the origin and extinction of species, life on

Earth would not have the ever-changing history that we find in the fossil record.

)

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 si ngle species can be

diverse in appearance. Although many definitions have been proposed, the

.ion, tion era

olu

two lied

ay

·on )rd.

ies.

e

be

~he

hio/ogical species concept offers a testable 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 geographic range

m 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

m nature. Therefore, these two types of plants are separate species. In contrast,

the human species has many populations, which certainly differ in physical

appearance (Fig. 16.3). We know, however, that all humans belong to one spe

cies 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 organ

Isms and cannot apply to asexually reproducing

organisms. Then, too, sexually reproducing

organisms are not always as reproductively

1solated as we would expect. Some North

~merican orioles live in the western half of

the continent, some in the eastern half, 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

1pecies 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

1hare a recent common ancestor. Empidonax trailli

CHAPTER 16 Evolution on a Large Scale

Empidonax virescens

Least flycatcher,

Empidonax minimus

Figure 16.2 Three 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 habitat

during the mating season as well.

Figure 16.3 Human populations.

a. b.

The Maassai of East Africa (a) and the Kuna Indians from the San Bias Islands of Panama

(b) are both members of the species Homo sapiens because the Maassai and Kuna can

reproduce and produce fertile offspring.

268 PART THREE Evolution

Connections and Misconceptions

How can we determine if an organism that does not reproduce sexually is a distinct species?

Many organisms either do not or

very rarely reproduce sexually.

For example, there are species of

mosses that reproduce sexually only

every 200 to 300 years! To determine

if two populations of asexual organ

isms are distinct species, scientists

rely on DNA analysis, morphological

studies, and a close examination of the organisms' ecology

to determine whether the two populations would reproduce

naturally. Often, scientists have to revisit the classification of

a species as research unveils new information.

A common ancestor is a single ancestor for two or more different groups. For example, your father's mother is the common ancestor for you, your siblings. and your paternal 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 barri· ers are isolating mechanisms that prevent successful reproduction (Fig. 16.4). In evolution, reproduction is successful only when it produces fertile offspring.

Prezygotic (before the formation of a zygote) isolating mechanisms prevent reproductive attempts and make it unlikely that fertilization will be sue· cessful 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 isolation When 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 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; in this way, they are isolated from similar species.

Temporal isolation Two species can live in the same locale, but if they reproduce at different times 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 .

. ,.

Prezygotlc Isolating mechanisms Postzygotic isolating mechanisms

Premating

Habitat isolation Species at same locale occupy different habitats.

Temporal isolation Species reproduce at different seasons or different times of day.

Behavioral isolation In animal species, courtship behavior differs, or individuals respond to different songs, calls, pheromones, or other signals.

Figure 16.4 Reproductive barriers.

Mating

Mechanical isolation Genitalia between species are unsuitable for one another.

Gamete isolation Sperm cannot reach or fertilize egg.

Fertilization

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.

Prezygotic isolating mechanisms prevent mating attempts or a successful outcome, should mating take place-for example, between two species of orioles. No zygote is

ever formed if these mechanisms are successful. Postzygotic isolating mechanisms prevent offspring from reproducing- that is, if a hybrid oriole should result, it would be

unable to breed successfully.

Behavioral isolation Many anima! species have courtship patterns that allow males and females to recognize one another (Fig. 16.5). Female fireflies recognize males of their species by the pattern of the males' flashings;

similarly, female crickets recognize males of their species by the males' chirping. Many males recognize females of their species by sensing chemical signals, called pheromones. For example, female gypsy moths secrete chemicals from abdominal glands. These chemicals are detected downwind by receptors on the antennae of males.

Video Flirting Flies

Mechanical isolation When 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 isolation Even 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. Also, in each type of flower, only certain 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 develop

ing or breeding, even if reproduction attempts have been successful.

Zygote mortality The hybrid zygote may not be viable, 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 sterility The 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, which 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 cannot align during meiosis. F2funess If 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 uggests that iguanas of South America may be the common ancestor for both the marine iguana on the Galapagos Islands (to the west of South America) and the rhinoceros iguana on Hispaniola (the Caribbean island containing the countries of Haiti and the Dominican Republic). 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 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


Figure 16.5 Prezygotic isolating mechanism.

An elaborate courtship display allows the blue-footed boobies of thj

Galapagos Islands to select a mate. The male lifts his feet in a ritualiz

manner that shows off their bright blue color.

d'donkey

Figure 16.6 Postzygotic isolating mechanism.

Mules are horse-donkey hybrids. Mules are infertile due to a differenc

the chromosomes inherited from their parents.


were three species of iguanas. A speciation model based on geographic

isolation of populations is called allopatric speciation because al/o means different and patria loosely means homeland.

Ensatina eschscholtzii Figure 16.7 features an example of allopatric speciation thai

has been extensively studied in California. Members of an ancestral

population of Ensatina salamanders existing in the Pacific Northwe11

oregonensis

Western migration southward establishes these populations.

Ensatina eschscholtzii xanthoptica

Eastern migration southward establishes these populations.

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, limited 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. One of the best examples to eas·

ily illustrate this type of speciation is found among

plants, where it can occur by means of polyploid': an increase in the number of sets of chromosome'

Ensatina eschscholtzii to 3n or above. The presence of sex chromosome1

makes it difficult for polyploidy speciation 10

occur in animals. In plants, hybridization between

two species can be followed by a doubling of the

chromosome number. Such polyploid planls

are reproductively isolated by a postzygotic

mechanism; they can reproduce successfully on!)

croceater

with other similar polyploids, and backcrosses

with their parents are sterile. Therefore, three species

instead of two species result. Figure 16.8 shows thai

the parents 9f our present-day bread wheat had 28

and 14, chromosomes, respectively. The

~:wr:::::c::=--.. hybrid with 21 chromosomes is ster-

Ensatina eschscholtzii eschschoftzii

In the end, two reproductively isolated populations cannot mate in Southern California.

Ensatina eschscholtzii klauberi

ile, but polyploid bread wheat wilh

42 chromosomes is fertile because

the chromosomes can align during meiosis.

Adaptive Radiation Figure 16.7 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. The limited contact between the populations on the

west and those on the east allow genetic changes to build up to such an

extent that members of the two southern populations rarely reproduce

with each other and are designated as subspecies.

A clear example of speciation through adaptive radiation is pro

vided by the finches on the Galapagos Islands, which are often called Darwin's

Figure 16.8 Sympatric 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.

X

Wild wheat 2n= 28

Triticum turgidum

__..... Sterile hybrid 2n= 21

Wild wheat 2n= 14

Triticum taushii

Doubling of chromosome number

Bread wheal 2n=42

Triticum aestivum

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 Galapagos Islands are hypothesized to be descendants of a single type of ancestral finch from the mainland (Fig. 16.9). The population 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 Video

species during courtship. Rejection of suitors with the wrong Finches Adaptive

type of beak is a behavioral prezygotic isolating mechanism. Radiation

Similarly, inhabiting the Hawaiian Islands is a wide variety of honeycreepers, all descended from a common goldfinch- like ancestor that arrived from Asia or North America about 5 MYA. Today, honeycreepers have a range of beak sizes and hapes (see Fig. I .11) for feeding on various food sources, including seeds, fruits, flowers, and insects.

Check Your Progress 1~.1

0 Define species according to the biological species concept.

f) Describe the limitations of the biological species concept.

Categorize the different types of reproductive barriers as being either

a prezygotic or postzygotic barrier, and give an example of each.

Compare and contrast allopatric speciation with sympatric

speciation. Give an example of each.

0 Relate adaptive radiation to Darwin's finches.

Woodpecker finch

Grasping beaks

Warbler finch

Probing beaks

CHAPTER 16 Evolution on a Large Scale 2 1


Are there examples of polyploid species in animal!

In general, polyploidy is rarer in

animals than in plants. However,

there are examples of polyploid

insects and fish, and polyploidy also

appears to occur frequently in the

amphibians, specifically in salaman

ders. In 1999, scientists reported a

polyploid rat species (Typanoctomys

barrerae) in Argentina, but later genetic analysis refuted tf

claim. Most geneticists believe that polyploidy in mamm<

is unlikely due to the well-defined role of mammalian s•

chromosomes and the balance between the number of autt

somes and sex chromosomes.

Connecting the Concepts

For more information on the concepts presented in this

section, refer to the following discussions.

Section 9.4 provides background information on how

nondisjunction causes changes in chromosome number.

Section 14.1 examines the role that the Galapagos finches

played in establishing natural selection as the mechanism

for evolutionary change.

Cactus ground finch

Figure 16.9 Darwin's finches.

Each of Darwin's finches is adapted to gathering anc

eating a different type of food. Tree finches

have beaks largely adapted to eating

insects and, at times, plants.

Sharp-beaked ground finch

Crushing beaks

Ground finches have beaks

adapted to eating the

flesh of the prickly

pear cactus or

different-sizec

seeds.


e.

16.2 The Fossil Record

Learning Outcomes


1. Describe how the geological timescale is divided into eras, periods,

and epochs.

2. Contrast the gradualistic model of evolution with the punctuated

equilibrium model of evolution.

3. Explain some of the ways in which mass extinctions of organisms

may have occurred.

The history of the origin and extinction of species on Earth is best discovered

by studying the fossil record (Fig.16.10). Fossils are the traces and remains of

past life or any other direct evidence of past life. 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, geo logi~~s 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, Animation humans have only been around about r oo GeologicaiHistory

~11!8 of the Earth .04% of the history of life.

In contrast, prokaryotes existed alone for some 2 billion years

before the eukaryotic cell and multicellularity arose during Precam

brian 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

Figure 16.10 Fossils.

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. 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. d. This dinosaur footprint (135 MYA) is a sign of

ancient life. e. This tree trunk (190 MYA) is petrified because minerals have

replaced the original tissues.

CHAPTER 16 Evolution on a Large Scale 2:

Table 16.1 The Geological Timescale: Major Divisions of Geological Time and Some of the Major Evolutionary Events That Occurred

Era Period

Quaternary

Cenozoic

Tertiary

Cretaceous

Jurassic Mesozoic

Triassic

Permian

Carboniferous

Devonian

Paleozoic

Silurian

Ordovician

Cambrian

Precambrian time

Epoch MYA Plant and Animal Life

Holocene 0-0.01 AGE OF HUMAN CIVILIZATION; Destruction of ecosystems accelerates extinctions.

SIGNIFICANT MAMMALIAN EXTINCTION

Pleistocene 0.01 - 2

Pliocene 2-6

Miocene 6-24

Oligocene 24- 37

Eocene 37-58

Paleocene 58- 65

MASS EXTINCTION:

65- 144

144-208

MASS EXTINCTION :

208-250

Modern humans appear; modern plants spread and diversify.

First hominids appear; modern angiosperms flourish.

Ape-like mammals, grazing mammals, and insects

flo"'''"' 9""''"d' 'pceod; ood foce>t' mn~ Monkey-like primates appear; modern

., angiosperms appear.

All modern orders of mammals are present; subtropical forests flourish.

Primates, herbivores, carnivores, insectivores are prese.nt;. angiosperms diversify.

50 % OF ALL SPECIES, DINOSAURS, AND

Placental mammals ill"!d modern insects appear; angiosperms spread .and conifers persist.

Dinosaurs flourish; birds and angiosperms appear. ,

48 % OF ALL SPECIES,

First mammals and dinosaurs appear; forests of conifers and cycads dominate land; corals and molluscs dominate seas.

MASS EXTINCTION (THE "GREAT DYING " ): 83 % OF ALL SPECIES ON

250-286

286-360

MASS EXTINCTION:

360-408

408-438

Reptiles diversify; amphibians decline; and gymnosperms diversify.

Amphibians diversify; reptiles appear; and insect;,~ diversify. Age of great coal-forming forests. ~

OVER 50 % OF COASTAL MARINE SPECIES ,

Jawed fishes diversify; insects and amphibians appear; seedless vascular plants diversify and seed plants appear.

First jawed fishes and seedless vascular plants appear.

MASS EXTINCTION : OVER 57 % OF MARINE SPECIES

438-510

510-543

600

1,400-700

2,000

2,500

3,500

4,500

Invertebrates spread and diversify; jawless fishes appear; nonvascular plants appear on land.

Marine invertebrates with skeletons are dominant and invade land, and marine algae flourish.

Oldest soft-bodied invertebrate fossils.

Protists evolve and diversify.

Oldest eukaryotic fossils.

0 2 accumulates in atmosphere.

Oldest known fossils (prokaryotes).

Earth forms.

CORALS



What is the Burgess Shale?

The Burgess Shale is the name

for a rock formation in the Cana

dian Rocky Mountains near the

Burgess Pass. Around 525 MYA,

this region was located along

the coast. It is believed that an

earthquake caused a landslide that almost instantly buried

much of the marine life living in the shallow coastal waters.

Unlike many fossil beds, the Burgess Shale contains the

remains of soft-shelled organisms, such as worms and sea

cucumbers, as well as other organisms from the Cambrian

explosion-a period of rapid diversification in marine life

around 545 MYA. Over 60,000 unique types of fossils have

been found in the Burgess Shale (including the trilobite fossil

shown here), making this fossil bed one of our most valuable

assets for studying the early evolution of life in the oceans.

Figure 16.11 Paceofevolution.

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.

a. Gradualistic model

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 were 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 were aquatic, and the amphibians invaded land. The reptiles, including dinosaurs and birds, shared

an amniote ancestor with the mammals. The number of species in the world has continued to increase until the pre ent time, despite the occurrence of five mas1 extinctions, including one significant mammalian extinction during the histof)

of life on Earth.

The Pace of Speciation Darwin theorized that evolutionary changes occur gradually. In other words, he sup

ported 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. The gradualism model often shows the evolutionaf)

history of groups of organisms by drawing the type of evolutionary tree shown in

Figure 16.11a. In this diagram, note that 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 ap(.>ear quite suddenly in the fossil record, and then they remain essentially unchan'ged phenotypically until they undergo extinc·

tion. Some paleontologists have therefore developed a punctuated equilibrium model to explain the pace of evolution. The model says that a period of equilib-

Time

Change

Time

Change

b. Punctuated equilibrium model

I,

I.

c d j

j

j

s

rium (no change) is punctuated (interrupted) by speciation. Figure l6.llb 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 specie in the fossil record

could represent many thousands of years because geological time is measured

in millions of years.

Mass Extinctions of Species As researchers 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 taxo

nomic 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) have particular significance.

Continental drift-the movement of continents-has contributed to sev

eral 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 touched at one

time. 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 slab-like 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.

Continental plates meet along a fault line, shift, and cause earthquakes.

Continental plate is folded into

Earth's


Figure 16.12 Plate tectonics.

The Earth's surface is divided into several solid tectonic plates flo;

on the fluid magma beneath them. At rifts in the ocean floor, two

gradually separate as fresh magma wells up and cools, enlarging j

plates. Mountains, including volcanoes, are raised where one plat

pushes beneath another at subduction zones. Where two plates s

grind past each other at a fault line, tension builds up, which is rel1

occasionally in the form of eathquakes.

Oceanic plate sinks beneath continental plate and melts into magma again.

-~ _ _.:.:=:d~laC::"'1 Hot magma rises to the surface and cools.


a.

Pangaea:

North America

-o

"'

Eurasia

South 1- Africa America Q -1

~A{ India

Antarctica

Late Paleozoic, 250 MYA

... '\

Equator

Australia

North America ... Eurasia

b.

' South America

' Most modern continents had formed by the end of the Mesozoic, 65 MYA

I ' Africa

I

.if" Jl' Antarctica

Figure 16.13 Continental drift.

t

f Equator

India

t Australia

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.


For more information on the material in this section, refer to the following discussions.

Section 14.2 describes how the fossil record is used as evidence of evolutionary change.

Sections 18.1 and 19.1 provide an overview of the evolution ofthe plants and animals, respectively.

Section 32.1 examines some of the influences that humans are having on the diversity of life.

The loss of habitat 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 supercontinent called Pangaea (Fig. 16.13a). The result was dramatic environmental change through the shifting of wind patterns, ocean currents, and most importantly the amount of available shallow marine habitat. Marine organisms suffered as the oceans merged, and the amount of shoreline, where many marine organisms lived, was drastically reduced. Species diversity did not recover until some continents drifted away from the poles,

shorelines increased, 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 known 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 mete

orites. A layer of soot has been identified iri 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 Yucatan peninsula .


What is the "Sixth Mass Extinction Event"?

Many ecologists now support the concept that we are currently involved in the Earth's sixth mass extinction event. However, unlike the first five

major events, this one is caused not by geological or astronomical events but by human actions. Pollution, land use, invasive species, and global

climate change associated with the burning of fossil fuels are all recognized as contributing factors. The exact rate of species loss can be difficult to

determine, but international agencies report that the current loss of species is between 100 and 1,000 times faster than the pre-human rates recorded by

the fossil record.

Check Your Progress-16.2

0 Summarize how the fossil record is the best evidence for macroevolution.

Explain the phrase "pu nctuated equilibrium model."

Relate the mass extinctions of species with the types of events that can cause them.

Discuss some of the major evolutionary events that occurred during different periods of geological time to form the geological timescale (Table 16.1).

16.3 Systematics Learning Outcomes


1. List the hierarchical levels of Linnaean classification from the

most inclusive to the least inclusive and explain how this type of

classification is useful to biologists.

2. Explain what information can be learned from a phylogenetic tree and

list some of the types of information that is used in constructing them.

3. Contrast a homologous structure with an analogous structure.

4. Define cladistics and explain how this method may be used to study

the evolutionary relationships between groups of organisms.

5. List the three domains of living organisms, and describe the general

characteristics of organisms contained within each domain.

I field of biology, but especially systematics, are dedicated to mlanding the evolutionary history of life on Earth. Systemat-1\ very analytical and relies on a combination of data from the sil record and comparative anatomy and development, with emphasis today on molecular data, to determine phylogeny,

Domain Bacteria

evolutionary history of a group of organisms. Classification is a part of tematics because ideally organisms are classified according to our present mtanding of evolutionary relationships.

nnaean Classification Taxonomy is the branch of biology concerned with identifying, nam

. and classifying organisms. A taxon (pl., taxa) is an organism or a poforganisms at a particular level in a classification system. The mial system of nomenclature assigns a two-part name to each of organism. For example, the plant in Figure 16.14 has been ed Cypripedium acaule. This name means that the plant is

the genus Cypripedium and that the specific epithet is acaule.

,e that the scientific name is in italics and only the genus is nalized. The genus can be abbreviated to a single letter if the name has been given previously and if it is used with a specific

. For example, C. acaule is an acceptable way to designate plant. The name of an organism usually tells you something about

· . In this instance, the genus name, Cypripedium, refers to the of the flower, and the specific epithet, acaule, says that the flower

independent stem. Why do organisms need scientific names? And why do scientists use Latin,

common names, to describe organisms? There are several reasons. First, name will vary from country to country because clifferent countries use

languages. Second, even people who speak the same language sometimes ilifferent common names to describe the same organism-for example, bowfin,

16.14 Taxonomy hierarchy.

CHAPTER 16 Evolution on a Large Scale 277

Kingdom Plantae

Monocotyledones

Family

Asparagales

Orchidaceae

Species

Cypripedium

Cypripedium acaule

is the most inclusive of the classification categories. The plant kingdom is in the domain Eukarya.ln the plant kingdom are several phyla, each represented here by

I The phylum Anthophyta has only two classes (the monocots and eudicots). The class Monocotyledones encompasses many orders. In the order Orchid ales

lllln,,f,miilio<·· in the family Orchidaceae are many genera; and in the genus Cypripedium are many species-for example, Cypripedium acaule. (This illustration is

i and doesn't necessarily show the correct number of subcategories.)


Ovis aries (sheep)

Ovis

Bos taurus (cattle)

Cervus elaphus (red deer)

Figure 16.15 Classification and phylogeny.

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.

Rangifer tarandus (reindeer)

Species

Genus

Family

Order

grindle, choupique, and cypress trout are all the same

common fish , Amia calva. Furthermore, between

countries the same common name is sometimes given

to different organisms. A robin in England is very dif· ferent from a robin in the United States, for example.

Latin, on the other hand, is a universal language that not

too long ago was well known by most scholars, many

of whom were physicians or clerics. When scientists throughout the world use a scientific binomial name.

they know they are speaking of the same organism.

Today, taxonomists use several categories of clas ification created by Swedish biologist Carl Linnaeus in the 1700s to show varying levels of similar

ity: species, genus, family, order, class, phylum,

and kingdom. More recently, a higher taxonomic cat

egory, 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).

Therefore species in the same domain have general traits

in common, while those in the same genus have quite specific traits in common.

In most cases, each category of classification can be subdivided into three additional categories, as in superorder, order, ·suborder, a.nd infraorder.

Considering these, there are more than 30 categories of classification.

Phylogenetic Trees 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). The common ancestor at the base of the tree

has traits that are shared by all the other groups in the tree. For example, the

Artiodactyla are characterized by having hoofs with an even number of toes. On

the other hand, notice that the Cervidae have antlers but the Bovidae have no

antlers. Finally, among the Cervidae, the antlers are highly branched in the red

deer but palmate (having the shape of a hand) in reindeer. As the lineage moves

from common ancestor to common ancestor, the traits become more pecific to

just particular groups of animals. It is this progression in

specificity that allows classification categories to serve as a

basis for depicting a phylogenetic tree.

Tracing Phylogeny

Animation Phylogenetic Trees

While Figure 16.15 makes use of morphological data, systematists today use

a multitude of data in order to discover the evolutionary relationships between

species. They rely heavily on a combination of fossil record data, morpho

logical data, and molecular data to determine the correct sequence of common

ancestors in any group of organisms.

Morphological data include homologies, which are similarities among

organisms that stem from having a common ancestor. Comparative anatomy,

including embryological evidence and fossil data, provides information regard

ing homology. Homologous structures are related to each other through com

mon 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. 14.15). This is the case even though 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. Convergent evolution is the acquisition of the same or similar traits 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 recent common ancestor. Analogous structures arise because of adaptations to the same ~pe of environment. Both cactuses and spurge 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.

Speciation 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. Computer software breakthroughs have made it possible to analyze nucleotide sequences quickly and accurately. Also, these analyses are available to anyone doing comparative studies through the Internet, so each mvestigator doesn't have to start from scratch. The combination of accuracy and availability of vast amounts of data, even entire genomes, has made molecular systematics a standard way to study the relatedness of organisms.

All cells have ribosomes essential for protein synthesis, and the gene !hat code for ribosomal RNA (rRNA) have changed very slowly during evolution because drastic changes lead to malfunctioning cells. Therefore, comparauve rR A sequencing provides a reliable indicator of the similarity between organisms. Ribosomal RNA sequencing helped investigators conclude that all tving things can be divided into the three domains.

One study involving DNA differences produced the data shown in Figure 16.16. Notice the close relationship between chimpanzees and humans.

0 Galago Capuchin Green monkey Rhesus monkey

~ 10 ~

0 Cl <t VI 20 :v ~ ..... 0 VI 30 c: ~ :ll

40 I I


Gibbon Chimpanzee Human

I I

I

I

hips 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

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.


03 Qi Q)

"E () ~ ~

c Qi "' "' .!!! Q) c ~ Q) c "'

Notochord in embryo

Vertebrae

Lungs

Three-chambered heart

Internal fertilization

Amniotic membrane in egg

Four bony limbs

Long cylindrical body

a.

b.

Figure 16.17 Constructing a cladogram.

a. First, a table is drawn up, listing characters for all the taxa. An

examination of the table shows which characters are ancestral

(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.

This relationship has recently been recognized by the designation of a new subfamily, Homininae, that includes not only chimpanzees and humans but also gorilla . Molecular data indicate that gorillas and chimpanzees are more

closely related to humans than they are to orangutans. Below the taxon subfamily, humans and chimpanzees are placed together in their own tribe, a rarely used classification category.

Cladistics and Cladograms Cladistics is a way to trace the evolutionary history of a group by using shared traits derived from a common ancestor to determine relationships. These traits are then used to construct phylogenetic trees called cladograms. A cladogram depicts the evolutionary hi tory of a group based on the available data.

The first step in constructing a cladogram is to draw up a table that summarizes the traits of the species being compared. At least one but preferably several species are considered an outgroup. The outgroup is oot part of the study group, also called the ingroup. In Figure 16.17a, lancelets are the outgroup because, unlike the pecies in the ingroup, they are not vertebrate . Any trait, such as a notochord, found in both the outgroup and the in group is a

shared ancestral trait, presumed to have been present in a common ancestor to both the outgroup and ingroup. Ancestral traits are not shared derived traits and therefore are not used to construct a cladogram. They merely help us determine which traits will be used to construct the cladogram.

A rule that many cladists follow is 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 that leaves the fewest number of derived traits unexplained. Therefore, any trait in the table found in scattered species (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 traits evolved more

than once among the species of the study group. The other difference are designated as shared derived traits-that is , they are homologies shared by only certain species of the study group. Combining the data regarding shared derived traits will tell us how the members of the ingroup are related to one another.

The Cladogram A cladogram contains everal clades; each clade includes a common ancestor (at the circles) and all of its descendant species. The cladogram in Figure 16. l7b has three clades (l-3), which differ in size because the first includes the other two and so forth. All the species 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. (An amniotic egg has a sac thai surrounds and protects the embryo-fish and amphibian eggs do not have this sac.) Following the princip le of parsimony, this is the sequence in which these traits must have evolved during the evolutionary history of vertebrates. Any

other arrangement of species would produce a less parsimonious evolutionary sequence-that is, a tree that would be more complicated.

A cladogram is objective-it lists the traits used to construct the cladogram. Cladists typically use much morphological, fossil, and molecular data to construct a cladogram. Still, cladists regard a cladogram as a hypothesis. Whether our tree is consistent with the one, true evolutionary history of life can be tested, and modifications can be made on the basis of additional data.

Linnaean Classification Versus Cladistics Figure 16.18 illustrates the types of problems that arise when trying to reconcile Linnaean classification with the principles of cladistics. Figure 16.18, which is based on cladistics, shows that birds are in a clade with crocodiles, with which they share a recent common ancestor. This ancestor had a gizzard. An examination of the skulls of crocodiles and birds would show other derived traits that they share. Birds have scaly skin and share this ancestor with other reptiles as well. However, Linnaean classification places birds in their own group, separate from crocodiles and from reptiles in general. In many other instances, Linnaean classification is not consistent with new understandings about phylogenetic relationships. Therefore, some cladists have proposed a different system of classification, called the International Code of Phylogenetic Nomenclature, or PhyloCode, which sets forth rules for the naming of clades. Other biologists are hoping to modify Linnaean classification to be consistent with the principles of cladistics.

Two major problems may be unsolvable: ( I) Clades are hierarchical, as are Linnaean categories. However, there may be more clades than Linnaean laxonomic categories, and it is therefore difficult to equate clades with taxons. (2) The taxons are not necessarily equivalent in the Linnaean system. For example, the family taxon within Kingdom Plantae may not be equivalent lo the family taxon in Kingdom Animalia. Because of such problems, some cladists recommend abandoning Linnaeus altogether .

•

mammals turtles

Figure 16.18 Cladistic classification .


~"'--amniotic egg, internal fertilization

Taxonomic designations are based on evolutionary history. Each taxon includes a common ancestor

and all of its descendants.


Figure 16.19 Three-do main 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.

cyanobacteria

Bacteria • Prokaryotic, unicellular

organisms • Lack a membrane-bounded

nucleus • Reproduce asexually • Heterotrophic by absorption • Autotrophic by

chemosynthesis or by photosynthesis

• Move by flage lla

common ancestor

The Three-Domain System Classification systems change over time. Historically, 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 nucleu ). The kingdom Monera contained all prokaryotes, which evolved first, according to the fossil record.

eukaryotes

plants

animals

Eukarya • Eukaryotic, unicellular to

multicellular organisms • Membrane-bounded

nucleus • Sexual reproduction • Phenotypes and nutrition

are diverse • Each kingdom has

specializations • Flagella, if present, have a

9 + 2 organization

prokaryotes

protists

Archaea • Prokaryotic, unicellular

organisms • Lack a membrane-bounded

nucleus • Reproduce asexually • Many are autotrophic by

chemosynthesis; some are heterotrophic by absorption

• Unique rRNA base sequence

• Distinctive plasma membrane and cell wall chemistry

Sequencing the genes for rRNA has provided new information that calls

into question the five-kingdom system of classification. Aside from molecular

data, cellular data also suggest that there are two groups of prokaryotes, named

the Bacteria and the Archaea. These two groups are so fundamentally different

from each other they have been assigned to separate domains, a category of

classification that is higher than the kingdom category. The Bacteria arose first,

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

v;ithin the domain Eukarya. The protists do not share one common ancestor, and

some suggest that the kingdom should be divided into many different kingdoms.

The number of kingdoms is still being determined among

systematists, illustrating that classification can be changed

as new data become available.


Animation

For more information on the topics presented in this section, refer to the

following discussions.

Section 14.2 provides an overview of how comparative anatomy is used to

study evolution.

Section 17.3 examines the evolution of the prokaryotic domains of life.

Figure 19.4 illustrates the phylogenetic tree of animal evolution.

Check Your Progress 16.3

0 List the categories of classification in order from smallest to largest.

G Name the types of data used to determine evolutionary relationships.

Contrast homologous structure with analogous structure.

Explain why the sequencing of ribosomal RNA (rRNA) is done for

evolutionary studies.

0 Explain why the Linnaean classification is difficult to reconcile with the principles of cladistics.

0 Explain the following: If Band Dare fishes, what other animal

(designated by a letter) in this cladogram is also a fish? Explain.

A B c 0


- -------------


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~~ connect~e I BIOLOGY

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The Chapter in Review Summary

16.1 Speciation and 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 produce viable fertile offspring 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 extant (living or not extinct) 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 in future contact.

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

16.2 The Fossil Record 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.

• also supports the 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, including one significant mammalian extinction, have occurred during the history of life on Earth. Two major contributors to mass extinctions are the loss of habitat due to continental drift and the disastrous results from a meteorite impacting Earth.

16.3 Systematics • Systematics is the study of the evolutionary relationships among

all organisms, past and present. Systematics includes classification. In the Linnaean system of classification, every organism is assigned a scientific name, which indicates its genus and specific epithet. Species are also assigned to a family, order, class, phylum, kingdom, and domain according to their molecular and structural similarities as well as evolutionary relationships to other species.

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

• Cladists use shared derived characters to construct cladograms. In a cladogram, a clade consisting of a common ancestor and all the species derived from that ancestor, the species have shared derived characteristics.

• Linnaean classification has come under severe criticism because it does not always follow the principles of cladistics in the grouping of organisms.

Classification Systems

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

~ . Bactena

• Domains Bacteria and Archaea contain prokaryotes.

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

Key Terms adaptive radiation 271

allopatric speciation 270

analogous structure 279

analogy 279

clade 280

cladistics 280

cladogram 280

class 278

common ancestor 268

convergent evolution 279

domain 278

domain Archaea 283

domain Bacteria 283

domain Eukarya 283

evolutionary tree 274 family 278

five-kingdom system 282 fossil 272

genus 278

homologous structure 278

kingdom 278

macroevolution 266

mass extinction 275

order 278

paleontology 272

phylogenetic tree 278

phylogeny 277

phylum 278

postzygotic isolating

mechanism 269

prezygotic isolating

mechanism 268

speciation 266

species 278

sympatric speciation 270

systematics 277 taxon (pl., taxa) 277

taxonomy 277

three-domain system 283

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 e. gamete isolation

b. temporal isolation f. zygote mortality

c. behavioral isolation g. hybrid sterility

d. mechanical isolation h. low F2 fitness

2. Females of one species do not recognize the courtship behaviors of males of another species.

3. One species reproduces at a different time of year 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 hybridize because they are visited by different pollinators.

B. 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.

a.

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

a. gradualistic model.

b. punctuated equilibrium model.

12. Which of the following is the scientific name of an organism?

a. Rosa rugosa d. rugosa rugosa

b. Rosa e. Both a and d are correct.

c. rugosa

13. Which of these statements best pertains to taxonomy?

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

b. Species are always reproductively isolated from other species.

c. Species share ancestral traits but may have their own unique derived traits.

d. Species always look exactly alike.

e. Both c and dare correct.

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

a. bacteria d. animals

b. archaea e. plants

c. eukarya

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 determine

a. how to classify organisms.

b. the proper cladogram.

c. how to construct phylogenetic trees.

d. how evolution occurred.



18. In cladistics, a. a clade must contain the common ancestor plus all its

descendants. b. shared derived characters help construct cladograms.

c. the principle of parsimony states that the simpler hypothesis is preferred.

d. the species in a clade share homologous structures.


19. Answer these questions about the following cladogram.

amniotic egg, internal fertilization

lungs, three-chambered heart

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 the most 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- single-celled algae

c. Plantae-flowers and mosses d. Animalia- arthropods and humans

e. Fungi- molds and mushrooms 22. Many new species evolving in various environments from a

common ancestor is called

a. cladistics. b. the gradualistic model of evolution.

c. adaptive radiation.

d. convergent evolution.

e. the PhyloCode.

Thinking Scientifically 1. Using as many terms as necessary (from both X andY axes), fill in

the proposed phylogenetic tree for vascular plants.

Ferns Conifers Ginkgos Monocots Eud1cots

vascular tissue X X X X X

produce seeds X X X X

naked seeds X X

needle-like leaves X

fan-shaped leaves X

enclosed seeds X X

one embryonic leaf X

two embryonic leaves X

a. d. e. g. h.

vascular 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.

Bioethicallssue Funding for Phylogenies Reconstructing evolutionary relationships can have its benefits. For example, an emerging virus is apt to jump to a related species rather than to an unrelated species. We now know, for example, that the HIV virus jumped from the chimpanzee-with which we share 95% of our DNA sequence-to humans. Chimpanzees don't become ill from the virus; therefore, studying their immune system might help us develop strategies to combat AIDS in humans.

Cladists want to acquire more information about how genetic and environmental factors influence bone development. Cladograms based on bone shape and arrangement do not match up well with those based on DNA base-pair sequencing. If we knew more about bone development under different environmental conditions, we might be able to discover the reason they don't match and develop evolutionary (phylogenetic) trees in which all have confidence. To acquire the necessary data could cost millions of dollars in research funding.

Should the public be willing to fund all types of research or only research that has immediate medical benefits, as was described for HIV research? Would you be willing to fund research that helps us understand our evolutionary past, even if the medical benefit is not immediately known? Do you think public education in cladistic research and its benefits would be beneficial?

Chap 16 - Bio

Documents

new species

different species

iguana populations

biological species

origin of species

type of marine iguana

galapagos islands

rhinoceros iguana