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Evidence for Evolution
After you have finished reading this chapter, you should be able to:
Explain
how homologous structures, fossils, comparative anatomy,
embryology, and biochemistry provide evidence for evolution.
Describe
the differences between relative dating and absolute dating of
fossils.
Compare and contrast microevolution and macroevolution.
Nothing in biology makes sense except in the light of evolution.
Theodosius Dobzhansky
Introduction
In the mid-19th century, Charles Darwin first proposed a theory to
explain the ways species change through natural selection. Since that
time, a tremendous body of evidence that supports this theory has been
developed. Now, in the 21st century, the theory of evolution by natural
selection is accepted by biologists in much the same way that other scientists accept the theory of gravitation and the atomic theory as valid
explanations for events that occur in the natural world.
No single idea explains the enormous diversity and complexity of life
on Earth more powerfully than the theory of evolution proposed by Darwin. Evidence that supports this theory includes fossils (see Figure 2-1),
the shapes and structures of living organisms, the chemicals all living
things are made of, and the distribution of species on Earth today.
■■ CREATING A FAMILY TREE
Biologists try to understand the relationships that exist among different
species. Often, simple diagrams can be used to represent these relation24
Figure 2-1
Fossilized
trilobites.
Time
ships. (See Figure 2-2.) Suppose that A, B, C, and D represent four living
species. The letters E, F, and G represent ancestral forms of these species
that are most likely extinct. In this case, organisms B and C are more
closely related because they evolved from
their common ancestor, E, most recently. B Present
A B
C
D
and C are both equally related to A, with a
more distant common ancestor, F. Organism
D is the least closely related to the others
E
because it evolved from their common ancestor G the longest time ago.
F
An example might make this clearer. Suppose a group of biologists is studying the relaG
tionships that exist among horses, cats,
whales, and humans. The biologists want to
Past
develop a family tree for these animals. They
Figure 2-2 A diagram
need to determine which of these animals
used to represent the
evolved from common ancestors and at what relationships among
time in the past this occurred. This taskLIVING
different
species. BIOLOGY, 2e/fig. 2-2
ENVIRONMENT
requires the biologists to study the similarities
and differences that exist in these animals. The biologists would have to
learn as much as possible about the animals’ ancestors, which are usually, but not always, extinct species.
s/s
26 Evolution
■■ HOMOLOGOUS AND ANALOGOUS STRUCTURES
On way to determine the relationships among organisms is to find a characteristic they share that they inherited from a common ancestor. Such a
feature is called a homologous structure. For example, similarities exist
in the bones of the forelimbs of five very different animals. The wing of
a bat, the flipper of a whale, the leg of a cat, the arm of a human, and the
wing of a bird—although they appear to be quite different—are all made
up of the same types of bones. These bones are attached to each other
and to other bones in similar ways. The forelimbs of these five animals are
homologous structures and indicate that, long ago, these different animals evolved from a common ancestor. (See Figure 2-3.)
Human
Cat
Bat
Whale
Bird
Figure 2-3 The bones in each forelimb are homologous.
Using similarities alone to conclude that organisms evolved from a
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-3 s/s
common ancestor can sometimes lead to mistakes. For example, the penguin, shark, and sea lion are all excellent swimmers. (See Figure 2-4.)
These very different animals have fins or flippers and streamlined bodies that enable them to propel themselves through the water. However,
other evidence shows that these animals have very different ancestors.
Because, over time, their ancestors became adapted to living in water,
the living species appear similar in ways, even though they are not
closely related. This process is called convergent evolution. The features
shared by organisms through convergent evolution are called analogous
structures.
Determining if organisms are related requires looking at other evidence besides homologous structures. For example, the first vertebrates
(animals with backbones) to live on land had five digits on their fore-
Chapter 2 / Evidence for Evolution 27
Penguin
Shark
Sea lion
Figure 2-4 The similarity in the streamlined bodies of the penguin, shark, and sea
lion is an example of convergent evolution.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-4 s/s
limbs. Today, terrestrial vertebrates such as humans, crocodiles, and toads
also have five digits that they inherited from a common ancestor. However, the feet of pigs, camels, and deer have only two digits. But we know
that humans are more closely related to pigs, camels, and deer than to
crocodiles or toads. This means that having five digits, like the more distant ancestors of terrestrial vertebrates, is a primitive trait. Over time,
vertebrates that had fewer digits evolved, and this modified trait found in
pigs, camels, and deer is called an advanced trait.
Learning which traits are primitive and which are advanced is important for biologists as they construct family trees that show the evolutionary relationships among organisms.
■■ EVIDENCE FROM FOSSILS
Fossils are traces or remains of dead organisms that have been preserved
by natural processes. There are many ways fossils can be formed. Although
it is usually only the hard parts of organisms—such as bones, shells, or
teeth—that become fossils, occasionally the complete remains of an
organism are found. In ancient times, organisms were sometimes trapped
in the sticky plant sap that oozed from certain kinds of trees. Later, the sap
turned into clear yellow-orange amber, a substance that looks like rock.
Amber samples collected from around the world contain mosquitoes, spiders, leaves, and even small vertebrates such as lizards that are many millions of years old. Ice has preserved the remains of woolly mammoths,
elephantlike animals that have been extinct for thousands of years. (See
Figure 2-5 on page 28.)
A common way fossils are formed is through the gradual replacement
of the organism’s remains by other substances. In this method of fossil formation, the organism is usually buried in sediments, its tissues slowly
28 Evolution
Figure 2-5 Woolly mammoth.
replaced by minerals dissolved in underground water. In time, these minerals harden to form an exact copy
of the original organism. In fact, the
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-5 s/s
remains of dinosaurs found by scientists are usually not the dinosaur’s
actual bones but instead are copies of them made from minerals in this
manner. (See Figure 2-6.) Tree trunks, too, have been turned into petrified
wood by this process.
Fossils can also be formed if the original plant or animal creates an
impression in soft mud or clay. The mud hardens and the remains of the
organism disappear, leaving an empty mold. Later, when this mold fills
with minerals, it forms a cast. This process shows only the original external shape of the organism and not the internal structure, as do some other
methods of fossil formation.
A similar method is the creation of fossil imprints of animal footprints
in clay or mud. Today, you can find dinosaur tracks in rock that was once
the soft mud in which the animals walked. More than 3.5 million years
ago, in eastern Africa, a volcano spread a layer of ash over the land. A
Figure 2-6 Dinosaur skull
—a type of fossil.
Chapter 2 / Evidence for Evolution 29
light rain dampened the ash. At least 20 types of animals, including
giraffes, a horse, birds, a wild cat, and at least three ancient humans,
walked through the ash. The ash dried, baked by the heat of the sun. In
time, the ash hardened into stone. The animals left footprints behind that
today provide evidence of their presence long ago. These footprints were
discovered by accident. A group of scientists working in the area were
relaxing by throwing dried elephant dung at each other. When one scientist slipped and fell, he got a close-up view of the strange marks on the
ground. After further investigation, the team of scientists made one of
the most important discoveries of our time—the fossil footprints of early
human ancestors, two adults and a youngster, who walked side by side,
3,600,000 years ago.
■■ THE AGE OF FOSSILS
Fossils can be used as evidence for evolution only if the age of the fossils can be determined. For many years, scientists could determine only
whether one fossil was older or younger than another—that is, their age
relative to one another. Relative age is determined by observing the layers of the rock in which the fossils are found. Remains of plants and animals are normally found in sedimentary rock. This type of rock forms,
often at the bottom of lakes or seas, when layers of sediments such as
sand, mud, or clay are deposited. Eventually the lower layers are compressed by the layers settling down above them. The compressed lower
layers become cemented into rock. (See Figure 2-7.) In a series of undisturbed layers, the oldest are found at the bottom and the most recent at
the top. Therefore, the ages of fossils found in the layers can be related
to the ages of the layers. Because this method determines only if one
fossil is younger or older than another, it is called relative dating. (See
Figure 2-8 on page 30.)
Stream carrying
sediments
Ocean
Sedimentary
rocks
Figure 2-7 Formation of sedimentary rocks.
30 Evolution
Today, scientists can determine the age of fossils much more accurately than by the position of
rock layers. Very precise methods have been developed that determine when the fossil was formed.
This method of dating is called absolute dating.
Absolute dating techniques usually depend on
radioactive isotopes, atoms of elements that
release energy in the form of radiation at a uniform rate. As energy is released in a process called
radioactive decay, the radioactive isotope changes
into a different element. The process of radioactive decay continues. Over time, fewer atoms of
the original isotope and more of the new element,
called the decay product, are present. By measuring the amount of each element present in a
sample, the length of time since the fossil was
formed can be calculated.
Several methods use radioactive elements to
measure the age of rocks and the fossils they conFigure 2-8 Fossils in
tain. One method uses the ratio of the radioactive
a series of layers of
element uranium to its decay product, lead, in a
sedimentary rock.
sample. Another method calculates the ratio of
radioactive potassium to its decay product, argon. These methods can
LIVING ENVIRONMENT
2e/fig.
2-8formed
s/s
date fossilsBIOLOGY,
and rocks
that
many millions of years ago.
Another method of absolute dating uses isotopes of carbon. This
method measures the ratio of radioactive carbon-14 to carbon-12. When
an organism is alive, it takes in fixed amounts of both carbon-12 and
carbon-14. When it dies, the carbon-14 in the organism begins to change
to its decay product, nitrogen, while the carbon-12 remains unchanged.
Thus, the ratio of carbon-14 to carbon-12 changes. How long ago an
organism died can be determined by finding the ratio of carbon-14 to
carbon-12 in a sample from the organism. Radiocarbon dating can be
used to date the remains of living things such as logs and bones that are
less than 50,000 years old.
■■ EVIDENCE FROM COMPARATIVE ANATOMY
A structure that has little or no function in an organism, but is clearly
related to a more fully developed structure in another organism, is known
as a vestigial structure. It is thought that such a structure did serve a
Chapter 2 / Evidence for Evolution 31
Backbone
Rib
Python
Leg
bones
Figure 2-9 The skeletons of some
snake species have vestigial leg bones.
function in an ancestral organism. The appendix, a vestigial structure
LIVING ENVIRONMENT
BIOLOGY, 2e/fig.
2-9 s/ssac
(rev.attached
10/13/03) to the place where the small
present in humans,
is a small
and large intestines meet. In appearance, the appendix is a smaller copy
of the cecum, which is a large pouch found in plant-eating mammals such
as rabbits. In a rabbit, the cecum contains microorganisms that digest the
plant materials the animal eats. The fact that a similar organ in one
species is still useful is evidence that humans evolved from an ancestor
that once had this larger, functional structure. There are many examples
of vestigial structures found in organisms alive today. These structures
support the idea that species have evolved over time as a result of natural selection. For example, although snakes have no legs, there are tiny leg
bones in the skeletons of some snake species. These vestigial structures
show that snakes evolved from animals that had legs. (See Figure 2-9.)
■■ EVIDENCE FROM COMPARATIVE EMBRYOLOGY
Figure 2-10 illustrates five animals at very early stages in their development. Although all of these embryos resemble one another, you might be
Salamander
Chicken
Gill slits
Yolk
sac
Pig
Gill slits
Monkey
Gill slits
Human
Gill slits
Yolk
sac
Figure 2-10 The early embryos of these five animals show some similarities.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-10 s/s (rev. 10/13/03)
Gill slits
32 Evolution
surprised to learn that these show the embryos of a salamander, a chicken,
a pig, a monkey, and a human. These diagrams provide evidence that all
vertebrates follow a common plan in their early stages of embryological
development. We are now learning that this is due to their having similar sets of genes, the molecular instructions for life; and this similarity
comes from having common ancestors.
Check Your Understanding
How do organisms’ similarities in anatomy and embryology provide
evidence for evolution?
■■ EVIDENCE FROM COMPARATIVE BIOCHEMISTRY
The chemistry of living things, called biochemistry, provides some of
the strongest evidence that organisms evolved from common ancestors
long ago. All organisms store the genetic information that is passed from
one generation to the next in DNA molecules in a manner that is almost
exactly the same. This genetic code shows that all organisms are related
in fundamental ways.
Biochemistry also shows how different species are related to each
other. Proteins, a type of molecule in all living things, are made up of
smaller units called amino acids. Cytochrome c is a protein that helps
organisms get energy. The 100 amino acids that make up the cytochrome
c of humans and chimpanzees are identical. There are six differences in
the amino acids that make up the cytochrome c of humans and monkeys. The cytochrome c of humans and kangaroos differs in 14 amino
acids. Biologists now recognize that a small number of amino acid differences means that the two species are more closely related in evolutionary terms. On the other hand, a large number of differences means
the two species are more distantly related.
More evidence for evolution has been discovered by molecular biologists who have uncovered the specific genes that direct the body development of very diverse animals. Called homeotic genes, these genes are like
molecular architects. Within them are stretches of DNA, or homeobox
sequences, which act like switches to turn the genes on and off. Importantly, the homeobox sequence in a fruit fly’s genes is almost identical
to that in a mouse’s genes—further evidence for a common ancestor
long ago. These common homeobox genes in diverse organisms help to
explain the similarities in early embryonic development, as shown in
Figure 2-10.
Chapter 2 / Evidence for Evolution 33
1 DNA molecules from
Species A, B, and C.
2 DNA molecules are
broken apart by heating.
Species Species Species
A
B
C
3 Hybrid DNA molecules
form after mixing and
cooling. Species A & B
are more closely related
than Species A & C.
A+A
A+B
A+C
Figure 2-11 DNA hybridization.
DNA molecules
store a species’ genetic information in long sequences
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-11 s/s (rev. 10/13/03),(10/20/03)
of subunits called nucleotides. For example, the DNA in each human cell
that contains all the genetic information for a human has about one billion nucleotides in a specific order. The nucleotides that are present, and
the order in which they are found, identify the species. By matching a
DNA sequence from one individual with the DNA sequence from another,
scientists can determine if the sequences belong to the same, closely
related, or distantly related species. Again, the greater the similarity, the
closer the species are related.
The most common method used to compare DNA sequences is called
DNA hybridization. (See Figure 2-11.) DNA molecules from two different
individual organisms are mixed together to form “hybrid” DNA. Wherever the molecules from the two individuals are similar, they attach to
each other. Heat can be used to separate the hybrid DNA molecules. The
greater the similarity, the greater the numbers of attachments and the
more difficult it is to once again separate the two molecules. Biologists
therefore measure the temperature required to separate the hybrid DNA
molecules. The higher the temperature, the greater the number of attachments; hence the more closely related the species are.
Many evolutionary puzzles have now been solved using this technique
of DNA hybridization. For example, ornithologists, scientists who study
birds, disagreed on whether the flamingo was more closely related to
storks or to ducks and geese. DNA hybridization studies have shown that
flamingos are indeed closely related to storks, both having evolved most
recently from a common ancestor. (See Figure 2-12 on page 34.)
A more famous controversy involved the relationship of the giant
panda to the red panda. (See Figure 2-13 on page 35.) It has been known
for a long time that the smaller red panda is a member of the raccoon family. Was the giant panda, whose anatomy and behavior closely resemble
34 Evolution
?
?
Flamingo
Stork
Duck
Figure 2-12 DNA hybridization shows that flamingos are closely related to storks.
that of the red panda, also a member of the same family and not a bear,
LIVING
ENVIRONMENT
BIOLOGY,
2e/fig.
2-12
s/s panda first became
as was commonly
thought?
From 1869,
when
the
giant
known to the Western world, until recently, this question remained unanswered. DNA hybridization has now shown that the giant panda is indeed
a bear. The two pandas have developed by convergent evolution to resemble one another in some ways. This makes sense, because they are both
native to the same mountainous environment in China, and both eat similar plants.
■■ EVIDENCE FOR MICROEVOLUTION
Evolution occurs all the time within a species—on a small scale—as inheritable traits in a population change through natural selection from one
generation to another. This kind of change is known as microevolution.
When it occurs over a short period of time, this process does not produce
significant enough change to result in a new species. However, by studying examples of microevolution, biologists can learn about the process of
evolution that does lead to significant changes, new species, and eventually entirely new groups of organisms. Change on a large scale over a long
enough period of time is known as macroevolution.
You may have observed moths fluttering around a bright light outdoors on a summer night. One species of moth, the peppered moth, has
been carefully studied in England for more than a century. This study
provides one of the best-known examples of microevolution. When the
peppered moth was first studied, most were a light color. The moths were
well camouflaged when they rested on trees and rocks covered with lightcolored lichens. (See Figure 2-14 on page 36.) In 1845, a dark-colored peppered moth was observed for the first time, in the northern English city
of Manchester. At that time, soot and smoke produced by coal-burning
Chapter 2 / Evidence for Evolution 35
Figure 2-13 DNA
hybridization has
shown that the red
panda (above) and
the giant panda are
similar as a result
of convergent
evolution.
factories had begun to pollute the air. The trees and rocks became dark
with soot, and the lichens began to die. The light-colored moths were
easily seen against the new, darker backgrounds on which they rested.
Increasingly, the light-colored moths became the easy prey of insecteating birds. By 1900, most of the peppered moths in Manchester were
dark-colored. Why did this happen? The darker moths were better camouflaged against the tree trunks and rocks blackened by the soot from the
factories. The process of dark-colored moths replacing light-colored moths
is known as industrial melanism. It has been observed with many species
36 Evolution
Light-colored
peppered moth
Soot-covered
bark of tree
Figure 2-14 The
light-colored and
dark-colored
peppered moth
populations vary due
to microevolution.
Dark-colored
peppered moth
since then, not only in Manchester but also in similar industrial cities in
the United States, such as Pittsburgh, Pennsylvania. It is important to
LIVING ENVIRONMENT
BIOLOGY,
2e/fig.
2-14they
s/s were
understand that the light-colored
moths did not
change
color;
replaced by dark-colored moths through natural selection. The peppered
moths with a dark color were the ones least likely to be preyed upon and
so most likely to survive to pass on their genetic traits. It has been discovered that the dark color was a mutation that occurred infrequently in
the population, due to the genetic variation that occurs naturally within
every species. Natural selection, acting on the peppered moth, in the
process of microevolution, changed the moth population from the lightcolored to the dark-colored variety. Interestingly, but not surprisingly, as
air pollution controls have been implemented over the years, the number of light-colored moths is once again increasing. Another example of
microevolution that you can now recognize is the appearance of drugresistant bacteria, as described in Chapter 1.
■■ EVIDENCE FOR MACROEVOLUTION
The large-scale changes of macroevolution led to the development of new
adaptive features, new species, and new groups of species. Examples of
new features that resulted from macroevolution are the legs on amphibians (such as a salamander), the egg of a turtle, and the large brain of
primates. An entirely new group of species that arose through macroevolution was the flowering plants. Macroevolution has also led to the disappearance of species, for example, the extinction of dinosaurs.
Naturalists living in Europe and England were accustomed to observing
relatively small numbers of species. People thought that each species
Chapter 2 / Evidence for Evolution 37
always existed exactly as it currently appeared, living in the same place it
was currently living. However, as a result of wide-ranging explorations
around the globe, such as Darwin’s voyage on the Beagle, it came to be realized that many more species of organisms existed than anyone could have
possibly imagined. For example, tropical rain forests revealed a diversity of
life previously unimagined. In just one hectare (10,000 square meters) of
a tropical rain forest, more than 100 species of trees and up to 40,000
species of insects can be found! The rain forests that surround the Amazon
River in South America are home to more than 1600 bird species. As Darwin traveled up the west coast of South America, he was impressed by the
gradual changes in the kinds of plants and animals that live there. These
changes corresponded to the gradually changing environments. This wide
diversity of species strongly supported the idea that over a very long period
of time, organisms had evolved, adapting to many different environments
in many different ways. It was becoming difficult to argue that each of these
many different species had remained unchanged from its original form.
Another observation led Darwin to his theory of evolution. Darwin
realized that there was a relationship between the types of plants and animals and the regions they live in. The study of the relationship of living
things to their distribution in particular areas is called biogeography. Biogeography raises many questions that can logically be answered only by
the theory of evolution. For example:
◆ Why are rabbitlike animals distributed throughout the world? (See
Figure 2-15.) The European rabbit, the Patagonian hare of South America (related to rats), and the Australian banded hare (related to kangaroos) are similar in appearance and behavior. It is most likely that these
animals evolved, from unrelated ancestors, over millions of years in
Figure 2-15 Rabbitlike
animals are found
throughout the world.
38 Evolution
Mitochondrial Eve Meets Mungo Man
In 1987, a group of scientists thought they could use a better method to
study human evolution. Instead of studying bits and pieces of fossil
remains, they decided to study the genes contained within the cells of
living people. These genes, passed from generation to generation, have
stored within them a history of our origins. The molecular biologists
decided to examine the DNA located in our cells’ mitochondria. Unlike
ordinary DNA—the genetic material in the nuclei of our cells that we get
from both parents—mitochondrial DNA (mtDNA) in our cells comes only
from our mother. From one generation to the next, mtDNA never gets
mixed with the DNA in the genes we get from our father.
The researchers collected mtDNA from women living in many parts of the
world. By studying the similarities and differences in mtDNA in these
women, the researchers were able to look back in time to study the origins
of human history. Their startling conclusion was that the molecular
evidence indicated that all humans alive today are the descendants of a
single female who lived in Africa about 200,000 year ago. Some people
began to call this person “Mitochondrial Eve.”
Since 1987, scientists have disagreed widely on the results. Some scientists
claim that the computer program the researchers used for their analysis was
not used correctly. Others think that Mitochondrial Eve lived only 150,000
years ago. Still others point to evidence showing that modern humans
evolved much earlier—and in several parts of the world, not only in Africa.
In 2001, scientists determined that mtDNA from a human fossil found years
earlier in Mungo, Australia, showed no linkage to any humans living today.
Therefore, “Mungo Man,” as the fossil is called, could not have descended
from Mitochondrial Eve. “Put the gloves on, Mitochondrial Eve, because
Mungo Man has stepped into the ring,” began a recent article on the topic,
showing how the debate still continues. This kind of open discussion is
what science is all about—questions are asked and answered; and then,
when more evidence is found, even more questions arise. For now, the
answer to this question about our ancestry remains undecided.
similar environments but in different areas of Earth. They adapted in
some of the same ways to fill similar roles in their respective environments. Hence, today these animals superficially resemble each other.
(This is, in fact, another example of convergent evolution.)
◆ Why are the species found in South American rain forests more closely
related to those species that live in South American deserts than to
species found in Asian or African rain forests? Could it be that all the
South American animals evolved from common ancestors, adapting to
Chapter 2 / Evidence for Evolution 39
the rain forest or desert environments in their own particular ways,
while the African and Asian species evolved independently, adapting
to their own regions?
◆ Darwin’s questions about biogeography were most clearly answered
when he investigated the Galápagos Islands off the coast of Ecuador.
(See Figure 2-16.) Why did each of these 13 islands have its own
unique species of birds and reptiles, very similar to those on the other
islands and to those on the mainland? If each island were to be given
its own unique species in some special process, why weren’t they fundamentally different from each other? It is much more logical to
accept that ancestors of these animals arrived from the South American mainland after the islands—which had formed from volcanoes
on the ocean’s bottom—appeared above the water’s surface. On each
isolated island, the ancestral birds and reptiles evolved independently,
in response to the slightly different environments they lived in.
These questions, and many others, can be explained only by recognizing that organisms have changed over time in response to particular
environmental pressures.
One final type of evidence was used by Darwin to support his theory
P
MEXICO
AN
AMA
VENEZUELA
Cocos
Island
Malpelo
Island
COLOMBIA
Pacific
Ocean
ECUADOR
´
Galapagos
Islands
PERU
Figure 2-16 On the Galápagos Islands, the ancestral birds and reptiles evolved
independently in response to their environments.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-16 s/s (rev. 10/13/03),(10/20/03)
40 Evolution
of evolution. Darwin argued that a particular adaptation that exists not
only in its most advanced form but also in a wide range of intermediate
forms suggests that evolution has occurred. An example he used in this
argument—the eye—is also one that those who object to the theory of
evolution sometimes use. They have argued: How could an organ as
remarkable and highly developed as the eye be the product of a long series
of small evolutionary changes? How could an eye have evolved from
something else?
In On the Origin of Species, Darwin showed that there are many animals that have organs sensitive to light in a variety of ways. For example,
a sea star has small gel-filled depressions that may concentrate light at a
particular spot on its body. (See Figure 2-17.) These depressions cannot
form an actual picture of a sea star’s surroundings. Yet for the sea star, this type of “eye” is sufficient to aid the animal’s survival and gives it an
advantage over animals that do not have even
this structure. (Likewise, other invertebrates have
a variety of “eyes” that have differing abilities to
see or perceive different levels of light.) On the
other hand, it is possible that this type of adaptation in an ancestral species was improved on
by natural selection. This improvement led to a
Figure 2-17 The sea
structure in a later species that eventually could
star has eyespots at the
“see,” as our eye does today. Darwin described
tip of each arm.
this gradual selection of structures (“transparent
layers”) to perceive light. “We must suppose,” Darwin stated, “that there
is a power, represented by natural selection or the survival of the fittest,
always intently watching each slight alteration in the transparent layers;
and carefully preserving each which, under varied circumstances, in any
way or in any degree, tends to produce a distinctive image.”
LABORATORY INVESTIGATION 2
How Have Animals
Evolved Over Time?
INTRODUCTION
Much evidence to support the theory of evolution is found in Earth’s fossil record. In this investigation, you will make a model that shows how a
series of imaginary fossil organisms may have evolved.
MATERIALS
“Creature Sheet” handout (from the Teacher’s Manual), scissors, glue or
tape, unlined paper
PROCEDURE
1. Cut out each imaginary creature from the Creature Sheet.
2. Select one creature that seems to represent the oldest fossil, the ancestor of all the others. (Hint: Its form will be simpler than the others.)
3. Determine which creature appears to have evolved next. Continue
arranging the drawings to show a pattern of evolution from one form
to the next. There are several possible ways to arrange them. You may
arrange your creatures in a straight line or in a set of branched lines
similar to the branches of a tree. The pattern can branch into two or
more directions. You should be able to support the order you place the
organisms in with some logical reasons.
4. Glue or tape the creatures to the unlined paper in the order in which
you have arranged them. Draw arrows that show the direction of evolutionary change.
INTERPRETIVE QUESTIONS
1. Did you choose a linear or branching diagram, or a combination of
the two types? Explain your choice.
2. What shared characteristics did you use to determine the main
sequences or branches of your pattern?
3. Imagine what kind of environment the organisms at the end of each
branch or sequence of the pattern would live in. How might they
move? What might they eat?
Chapter 2 / Evidence for Evolution 41
42 Evolution
■■ CHAPTER 2 REVIEW
Answer these questions on a separate sheet of paper.
VOCABULARY
The following list contains all of the boldfaced terms in this chapter. Define
each of these terms in your own words.
absolute dating, advanced trait, analogous structures, biochemistry,
biogeography, convergent evolution, decay product, DNA
hybridization, embryos, fossils, genes, homeobox sequence,
homologous structure, industrial melanism, macroevolution,
microevolution, primitive trait, radioactive isotopes, relative dating,
sedimentary rock, vestigial structure
PART A—MULTIPLE CHOICE
Choose the response that best completes the sentence or answers the question.
1. Which organism has the forelimb with the most “primitive” trait?
a. bird b. human c. horse d. whale
2. Examples of homologous structures are a. a dog’s tail and a
fish’s tail b. a bee’s eye and a fish’s eye c. a porpoise’s flipper
and a hawk’s wing d. a crab’s claw and a penguin’s wing.
3. Evidence for evolution is shown by a. homologous structures
b. fossils c. biochemistry d. all of these.
4. Scientists who study birds are called a. ornithologists
b. ichthyologists c. aviaries d. biogeographers.
5. The streamlined bodies and powerful tails of sharks, dolphins, and
ichthyosaurs (ocean-dwelling relatives of the dinosaurs) illustrate
a. divergent evolution b. convergent evolution c. retention of
ancestral traits d. development of vestigial organs.
6. Which of the following is not compared in order to clarify the
evolutionary relationships among related organisms?
a. analogous structures b. embryo development c. genes
d. DNA hybridization
7. Fossils are usually found in a. radioactive isotopes
b. igneous rock c. trap rock d. sedimentary rock.
8. DNA from a giant panda hybridizes more readily with DNA from
a brown bear than with DNA from a red panda. This indicates that
a giant panda
Chapter 2 / Evidence for Evolution 43
9.
10.
11.
12.
13.
14.
15.
a. shares numerous analogous structures with a brown bear
b. is more closely related to a brown bear than to a red panda
c. is more closely related to a red panda than to a brown bear
d. lives in a similar environment to a brown bear.
Which of the following statements is false? a. In late nineteenthcentury Manchester, natural selection resulted in the peppered
moths turning from light gray to dark gray. b. Light-colored
moths had dark-colored offspring because the moths needed to
blend in with sooty tree bark. c. The technical name for the color
change from light to dark as a result of pollution is industrial
melanism. d. The peppered moths’ change in color is an example
of microevolution.
The older a specimen of sedimentary rock, a. the greater the
amount of radioactive isotopes in it b. the closer to the surface it
is found in undisturbed rock layers c. the greater the amount of
decay products present d. the more likely it is to contain moldand-cast fossils.
Examples of vestigial structures include a. a seal’s flipper and a
penguin’s wing b. a duckbill platypus’s bill and a duck’s bill
c. a bat’s foot and a human’s foot d. a human’s appendix and a
whale’s hind limbs.
In three layers of undisturbed sedimentary rock, a. the middle
layer contains the oldest fossils b. the top layer contains the
youngest fossils c. the fossils in the bottom layer are younger
than the fossils in the middle layer d. the fossils in the bottom
layer have usually been destroyed by geologic activity.
Homeobox gene sequences provide evidence in support of
evolution because a. they evolved only recently b. mammals
have these genes but no other animals do c. similar homeobox
gene sequences are found in very different organisms d. fruit flies
have a very ancient form of these genes.
The study of the relationship of plants and animals to their
regional distribution is called a. ornithology b. paleontology
c. biogeography d. melanism.
Sea urchins and jellyfish are both round, have no head or tail, and
live in the ocean; but their embryos develop in radically different
ways. They probably a. are descended from the same ancestor
b. should be placed in the same group c. have numerous
vestigial structures d. are not very closely related.
44 Evolution
PART B—CONSTRUCTED RESPONSE
Use the information in the chapter to respond to these items.
16. Scientists analyzed the protein cytochrome c in different organisms
in order to construct this evolutionary diagram. Explain how such
biochemical data provide evidence for evolution.
Candida (yeast)
Saccharomyces (yeast)
Neurospora
Moth
Screwworm
Tuna
Turtle
Penguin
Chicken
Duck
Pigeon
Kangaroo
Rabbit
Pig
Donkey
Horse
Dog
Monkey
Human
23.4
9.6
17.4
2.1
28.1
9.9
5.7
6.5
15.2
17.2
1.1 4.9
1 .5
1
1.2 3.3
3.1
1.6
1.1 1.3
4.6
2.7
.6 1.4 1.1
.1
2.9
.9
1.4
3.0 6.9
.2
.8 1.7
0
5
5.4
25
10
15
20
Average minimum number of mutations
30
17. Use the diagram to answer the following questions.
a. Is a turtle most closely related to a dog, tuna, penguin, horse, or
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-Q16 s/s
silkworm moth?
b. What organism is most closely related to a horse?
c. Which group of organisms is least closely related to the other
groups—fungi, insects, fish, reptiles, birds, or mammals?
d. What can you conclude about the turtle’s placement on this
“family tree”?
18. Describe three ways in which fossils are formed.
19. Describe and give specific examples of evidence for evolution from
fossils, comparative embryology, comparative biochemistry,
comparative anatomy, microevolution, and biogeography.
20. Compare and contrast relative dating and absolute dating.
Chapter 2 / Evidence for Evolution 45
PART C—READING COMPREHENSION
Base your answers to questions 21 through 23 on the information below and
on your knowledge of biology. Source: Science News (February 22, 2003):
vol. 163, p. 126.
Farming Sprouted in Ancient Ecuador
People living in the lowlands of what’s now southwestern Ecuador
began to grow squash between 10,000 and 9,000 years ago, about the
same time that residents of Mexico’s southern highlands domesticated
the vegetable (SN: 5/24/97, p. 322), according to a study in the Feb. 14
Science.
The comparably ancient roots of plant cultivation in these two regions
indicate that “in South America, there was no single center of agricultural origins,” conclude Dolores R. Piperno of the Smithsonian Tropical
Research Institute in Balboa, Panama, and Karen E. Stothert of the University of Texas at San Antonio.
In the soil of two prehistoric sites in Ecuador, the scientists isolated
and studied microscopic crystals from squash rinds that had been uncovered there. These ancient crystals were the same size as those in the
squash’s modern domesticated form, but not those in its present-day
wild counterpart. Piperno and Stothert were able to date tiny bits of carbon that were trapped inside the ancient crystals as they formed.
21. State what has been learned about farming that went on in Ecuador
and Mexico approximately 10,000 years ago.
22. Explain what conclusion can be made about agriculture in South
America.
23. Explain how scientists have learned about plants that were grown
by people in Ecuador long ago.