Download Speciation Through Isolation

SECTION 11.5
11.5 Speciation Through Isolation
. Plan and Prepare
Objectives
• Explain how isolation of populations
can lead to speciation.
• Describe how populations can
become isolated.
Section Resources
VOCABULARY
reproductive isolation
speciation
behavioral isolation
geographic isolation
temporal isolation
MAIN IDEAS
The isolation of populations can lead to speciation.
Populations can become isolated in several ways.
Connect to Your World
>
Online Teacher Resources
PowerPresentation
Teacher Toolkit
. Teach
New species can arise when populations
If you travel through two different cities, towns, or even neighborhoods, you’ll notice
differences in the way people live. When groups of people are separated, ideas and
resources are not shared, and so these groups of people may become more different.
Similarly, genes cannot flow between populations that are isolated from each other,
and they are more likely to become different. What happens if no gene flow occurs
between two populations? This is one way that new species can arise.
Online Student Resources
Study Guide (English and Spanish)
PowerNotes
Reinforcement Worksheet
Section Self-Check
Interactive Reader
Activate Prior Knowledge Have
students think about the potential for
speciation in dog breeds. Ask
• How do we know that a threefoot-tall Irish wolfhound and a
six-inch-high Chihuahua are the
same species? They are capable of
mating and producing fertile
offspring.
• At what point would the two breeds
become separate species? unable to
mate or produce fertile offspring Tell students that at that
point, speciation has occurred.
KEY CONCEPT
are isolated.
MAIN IDEA
The isolation of populations can lead
to speciation.
If gene flow between two populations stops for any reason, the populations
are said to be isolated. As these populations adapt to their environments, their
gene pools may change. Random processes such as mutation and genetic drift
can also change gene pools. All of these changes add up over many generations. With time, the two isolated populations become more and more genetically different. Individuals in one population may also begin to look and
behave differently from individuals in the other population.
CONNECT TO
GENETICS
Fruit flies (Drosophila
melanogaster) are very common
in genetic research, as you may
recall from the Genetics unit.
Their popularity is based on
several factors: they are easy to
breed, they are common, and
they have well-understood
genetic structures.
Reproductive isolation occurs when members of different populations
can no longer mate successfully. Sometimes members of the two populations
are not physically able to mate with each other. In other cases, they cannot
produce offspring that survive and reproduce. Reproductive isolation between
populations is the final step of becoming separate species. The rise of two or
more species from one existing species is called speciation.
FIGURE 5.1 illustrates a recent experiment that shows how one mutation can
result in reproductive isolation. Scientists studied the ds2 gene of fruit flies.
This gene affects how well fruit flies can deal with cold temperatures. Fruit
flies living in tropical areas, where competition for food is high, have a tropical
allele. Fruit flies living in cooler regions, where there is less competition for
food, have a temperate allele. The ds2 gene also affects chemical scents called
pheromones. Fruit flies use these scents to attracts mates of their own species.
Science Trivia
• The common fruit fly, Drosophila
melanogaster, is one of 900 fruit fly
species.
• This fly has an estimated 13,600
genes on four chromosomes and
over 3000 known mutations.
• It takes 11 days for the fly’s fertilized
egg to develop into an adult that is
capable of reproducing.
324 Unit 4: Evolution
Differentiated Instruction
324 Unit 4: Evolution
English Learners
Untitled-284 324
Have students form into home groups of
three and number off. Make all 1s responsible
for the concept of behavioral isolation, 2s for
geographic isolation, and 3s for temporal
isolation. Ask each student to start a cluster
diagram with the key concept of the section.
Have students form into “expert” groups to
complete their part of the cluster diagram
and then return to the home group to
construct a complete version.
Teacher Toolkit, Section C, Jigsaw Reading;
Cluster Diagram
5/5/2011 2:17:07 PM
FIGURE 5.1 Reproductive Isolation
ds2
1
ds2
Tropical fruit flies have a
tropical ds2 allele.
ds2
Temperate fruit flies have a
temperate ds2 allele.
A Synthesize Explain why fruit flies with a specific ds2 allele
(tl) ©Royalty-Free/Corbis; (tc) ©Image Plan/Corbis; (insets) ©Oliver Meckes/Nicole Ottawa/Photo Researchers, Inc.
(tl) ©Royalty-Free/Corbis; (tc) ©Image Plan/Corbis; (insets) ©Oliver Meckes/Nicole Ottawa/Photo Researchers, Inc.
prefer to mate with fruit flies that have the same allele.
TEACH FROM VISUALS
ds2
Reproductive isolation occurs when members of
isolated populations are no longer able to mate
with each other successfully.
2
ds2
ds2
ds2
FIGURE 5.1 Have students read the
captions. Ask
• Why didn’t the scientists bring
together flies from the original
locations? They wanted to be sure
the flies would mate.
• What type of isolation produced
populations of tropical and temperate flies? geographic
• What type of isolation kept lab flies
from mating? behavioral
ds2
ds2
ds2
Scientists used lab fruit flies that are genetically similar.
They developed a technique that allowed them to
replace the ds2 gene in each lab fruit fly with either the
tropical or temperate allele.
ds2
ds2
ds2
ds2
ds2
ds2
ds2
ds2
Take It Further
Laboratory males that received the tropical allele were
attracted to females that received the tropical allele.
Males that received the temperate allele were attracted
to females that received the temperate allele.
When the two different ds2 alleles
became selected for in the different
climates, mating behaviors were also
affected. This is because the ds2 gene
codes for both temperature tolerance
and mating behaviors. The two populations of fruit flies were first isolated
geographically. Then, because of how
the allele mutation affected mating
behaviors, the populations also became
isolated reproductively.
This experiment shows how speciation may have occurred in natural
fruit fly populations. Fruit flies migrating north from Africa to areas where
there is less competition for food faced colder temperatures. A mutation in
the ds2 gene may have produced the temperate allele. This allele allows fruit
flies to survive in cooler climates. Because the ds2 gene also affects pheromones, mating behaviors changed. Fruit flies with the temperate allele and
fruit flies with the tropical allele mated together less and less often. Eventually,
these populations became reproductively isolated.
B Summarize Why is reproductive isolation considered to be the final stage
in speciation?
MAIN IDEA
Populations can become isolated in several ways.
Several kinds of barriers can prevent mating between populations, leading
to reproductive isolation. These include behavioral, geographic, and
temporal barriers.
Web
Word Origins A pheromone is a
chemical that is secreted by an organism
and affects others of the same species,
often for the purpose of attracting a
mate. The Greek root is pherein,
meaning “to carry.” Students may better
remember the meaning pheromone if
they think of the word ferry, which also
means “to carry” and is derived from the
Middle English word ferein—probably
derived from the same Greek root.
HMDScience.com
PREMIUM CONTENT
Speciation in Action
Behavioral Barriers
Chemical scents, courtship dances of birds, and courtship songs of frogs are
sexual signals used to attract mates. Changes in these signals can prevent
mating between populations. Behavioral isolation is isolation caused by
differences in courtship or mating behaviors. Over 2000 species of fireflies are
isolated in this way. Male and female fireflies produce patterns of flashes that
attract mates of their own species. For example, Photuris frontalis emits one
flash every second, P. hebes emits one flash every 2 seconds, and P. fairchildi
produces a double flash every 5.5 seconds.
Answers
Chapter 11: The Evolution of Populations
Untitled-284 325
Vocabulary
Below Level
Pre-AP
Have students use sequence diagrams to
summarize how behavioral, geographic, and
temporal isolation can lead to reproductive
isolation and speciation.
Tell students that when the Isthmus of 5/5/2011
Panama formed about three million years ago,
it separated the Pacific Ocean and Caribbean
Sea. The Panama Canal has reestablished a
connection. Have students write a few
paragraphs addressing the question of
whether long-isolated marine species could
possibly interbreed if they were to migrate
from one body of water to the other.
Teacher Toolkit, Section C, Sequence
Diagram
325
2:17:10 PM
A Synthesize Fruit flies with the
temperate ds2 allele produce a specific
pheromone that only attracts mates
that have the same ds2 allele. The same
is true for the tropical ds2 allele.
B Summarize When populations can no
longer mate successfully with each other,
they are considered to be separate
species. Until this occurs, other forms of
isolation can be overcome as long as the
two populations can still interbreed and
have healthy, fertile offspring.
Teacher Toolkit, Section C, Quick-Write
Chapter 11: The Evolution of Populations 325
NORTH
AMERICA
TEACH FROM VISUALS
20 °N
FIGURE 5.2 Have students find the
Isthmus of Panama on the map. Tell
them that the isthmus formed about
three million years ago from the shifting
of Earth’s tectonic plates. Ask
• How did the formation of the
isthmus affect marine species? They
became geographically isolated
• How did the formation of the
isthmus affect the land species of
North and South America? Land
populations were no longer
geographically isolated.
• What might it mean for marine
species that there is now a canal
connecting the Caribbean Sea and
the Pacific Ocean? Some species
may be able to migrate.
Answers
ATLANTIC
OCEAN
N
W
10 °N
0°
E
0
Isthmus of Panama
0
S
Assess Use the Section Self-Check
or Section Quiz, both available at
­HMDScience.com.
Reteach Give students hypothetical
examples of populations becoming
isolated. Have them identify the type of
isolation each example illustrates.
11.5 FORMATIVE ASSESSMENT 326
1,000 miles
Equator
SOUTH
AMERICA
PACIFIC
OCEAN
10 °S
110 °W
100 °W
90 °W
80 °W
Although snapping shrimp in the Atlantic
and Pacific oceans look similar, they are
distinct species that have evolved through
geographic isolation.
MapQuest
McDougal-Littel,
Biology Program '05
Isthmus of Panama
Temporal
Barriers
bhspe-041105-004
R E A D I N G TO O L B OX
VOCABULARY
Temporal comes from the Latin
word tempus, meaning “time.”
A Compare and Contrast Behavioral
isolation involves differences in
courtship or mating behaviors; temporal
isolation involves differences in the
timing of courtship or mating behaviors.
. Assess and Reteach
500
500 1,000 kilometers
Barriers can also involve timing. Temporal isolation exists when timing
2nd proof date: 9/27/05
prevents
reproduction between populations. Some members of a population
may show signs of courtship at different times if there is a lot of competition
for mates. Reproductive periods may change to a different time of the year or a
different part of the day. These differences in timing can lead to speciation.
For example, two tree species that grow on the Monterey peninsula in
California are very closely related. However, they have different pollination
periods. The Monterey pine sheds its pollen in February, while the Bishop
pine sheds its pollen in April. These pine species have likely evolved through
temporal isolation.
A Compare and Contrast What are the differences and similarities between
behavioral isolation and temporal isolation?
SELF-CHECK Online
11.5
HMDScience.com
Formative Assessment
REVIEWING
MAIN IDEAS
1. How can reproductive isolation lead
to speciation?
2. What are three types of barriers that
can lead to reproductive isolation?
PREMIUM CONTENT
CRITICAL THINKING
3. Apply Why are the flash patterns
of fireflies considered to be
behavioral isolation?
4. Synthesize How did
geographic isolation affect the
diversity Darwin observed in
Galápagos finches?
CONNECT TO
SCIENTIFIC PROCESS
5. What could have been used
as a control group in the fruit
fly experiment described in
Figure 5.1?
Unit 4: Evolution
1.If populations cannot mate successfully,
3.The flash-pattern mating behaviors of
genetic differences may accumulate in the
Untitled-284 326 fireflies are not related to the timing of
populations. Over time, the populations
mating, so this is considered behavioral
may change so much that they become
isolation rather than temporal isolation.
different species.
4.The geographic isolation of the finches on
2.behavioral barriers, geographic barriers, and
different islands meant that each island’s
temporal barriers
environment selected for traits that were
beneficial on that particular island. Over
time, genetic differences accumulated in
the isolated populations, leading to many
distinct finch species.
326 Unit 4: Evolution
The most commonly studied type of isolation
is geographic isolation. Geographic isolation
involves physical barriers that divide a population
into two or more groups. These barriers can
include rivers, mountains, and dried lakebeds.
As shown in FIGURE 5.2, the formation of the
Isthmus of Panama created a barrier for many
marine species. Marine organisms could no
longer easily cross between the Atlantic and
Pacific oceans. Over time, the isolated populations
became genetically different. Several species of
snapping shrimp have evolved through
geographic isolation. These species appear
almost identical to one another. However, when
males and females from opposite sides of the
isthmus are placed together, they snap at each
other instead of courting. Because they will no
longer mate, these shrimp are classified as
different species.
©Fred Bavendam/Minden Pictures
. Teach continued
Geographic Barriers
FIGURE 5.2 GEOGRAPHIC BARRIER
5.Flies bred in the laboratory with their
original ds2 genes could have been used
as a control group, because they would
not have received any kind of treatment.
The effects of ds2 insertions on the two
treatment groups could have been compared against this group.
5/5/2011 2:17:12 PM