(tl) ©

Roya

lty-F

ree/

Corb

is; (

tc) ©

Imag

e Pl

an/C

orbi

s; (i

nset

s) ©

Oliv

er M

ecke

s/Ni

cole

Otta

wa/

Phot

o Re

sear

cher

s, In

c.

Speciation Through IsolationVOCABULARY

reproductive isolationspeciationbehavioral isolationgeographic isolationtemporal isolation

KEY CONCEPT New species can arise when populations are isolated.

MAIN IDEAS The isolation of populations can lead to speciation.

Populations can become isolated in several ways.

Connect to Your World 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.

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 genera-tions. With time, the two isolated populations become more and more geneti-cally different. Individuals in one population may also begin to look and behave differently from individuals in the other population.

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.

CONNECT TO

GENETICSFruit 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.

> VIRGINIA STANDARDS

BIO.7.d The student will investigate and understand how populations change through time. Key concepts include emergence of new species.

324 Unit 4: Evolution

11.5

Untitled-284 324 5/5/2011 2:17:07 PM

Web

(tl) ©

Roya

lty-F

ree/

Corb

is; (

tc) ©

Imag

e Pl

an/C

orbi

s; (i

nset

s) ©

Oliv

er M

ecke

s/Ni

cole

Otta

wa/

Phot

o Re

sear

cher

s, In

c.

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 phero-mones, 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.

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.

Behavioral BarriersChemical 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.

FIGURE 5.1 Reproductive Isolation

1 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.

2 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.

ds2ds2

Synthesize Explain why fruit flies with a specific ds2 allele prefer to mate with fruit flies that have the same allele.

Tropical fruit flies have a tropical ds2 allele.

Temperate fruit flies have a temperate ds2 allele.

Reproductive isolation occurs when members of isolated populations are no longer able to mate with each other successfully.

HMDScience.com

PREMIUM CONTENT

Speciation in Action

ds2

ds2

ds2

ds2

ds2

ds2

ds2

ds2

ds2 ds2

ds2 ds2

ds2ds2

ds2ds2

Chapter 11: The Evolution of Populations 325

Untitled-284 325 5/5/2011 2:17:10 PM

Differentiated Instruction

EnglIsh lEarnErsHave 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

Objectives• Explain how isolation of populations

can lead to speciation.• Describe how populations can

become isolated.

section resourcesOnline student resources

Study Guide (English and Spanish)PowerNotesReinforcement WorksheetSection Self-CheckInteractive Reader

Online Teacher resourcesPowerPresentationTeacher Toolkit

activate Prior Knowledge Have students think about the potential for speciation in dog breeds. Ask• How do we know that a three-

foot-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 off-spring Tell students that at that point, speciation has occurred.

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.

. Plan and Prepare

. Teach

324 Unit 4: Evolution

sECTIOn 11.5

(tl) ©

Roya

lty-F

ree/

Corb

is; (

tc) ©

Imag

e Pl

an/C

orbi

s; (i

nset

s) ©

Oliv

er M

ecke

s/Ni

cole

Otta

wa/

Phot

o Re

sear

cher

s, In

c.

Speciation Through IsolationVOCABULARY

reproductive isolationspeciationbehavioral isolationgeographic isolationtemporal isolation

KEY CONCEPT New species can arise when populations are isolated.

MAIN IDEAS The isolation of populations can lead to speciation.

Populations can become isolated in several ways.

Connect to Your World 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.

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 genera-tions. With time, the two isolated populations become more and more geneti-cally different. Individuals in one population may also begin to look and behave differently from individuals in the other population.

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.

CONNECT TO

GENETICSFruit 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.

> VIRGINIA STANDARDS

BIO.7.d The student will investigate and understand how populations change through time. Key concepts include emergence of new species.

324 Unit 4: Evolution

11.5

Untitled-284 324 5/5/2011 2:17:07 PM

Web

(tl) ©

Roya

lty-F

ree/

Corb

is; (

tc) ©

Imag

e Pl

an/C

orbi

s; (i

nset

s) ©

Oliv

er M

ecke

s/Ni

cole

Otta

wa/

Phot

o Re

sear

cher

s, In

c.

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 phero-mones, 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.

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.

Behavioral BarriersChemical 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.

FIGURE 5.1 Reproductive Isolation

1 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.

2 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.

ds2ds2

Synthesize Explain why fruit flies with a specific ds2 allele prefer to mate with fruit flies that have the same allele.

Tropical fruit flies have a tropical ds2 allele.

Temperate fruit flies have a temperate ds2 allele.

Reproductive isolation occurs when members of isolated populations are no longer able to mate with each other successfully.

HMDScience.com

PREMIUM CONTENT

Speciation in Action

ds2

ds2

ds2

ds2

ds2

ds2

ds2

ds2

ds2 ds2

ds2 ds2

ds2ds2

ds2ds2

Chapter 11: The Evolution of Populations 325

Untitled-284 325 5/5/2011 2:17:10 PM

A

B

BElOw lEvElHave students use sequence diagrams to summarize how behavioral, geographic, and temporal isolation can lead to reproductive isolation and speciation.

Teacher Toolkit, Section C, Sequence Diagram

PrE-aPTell students that when the Isthmus of 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, Quick-Write

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 temper-ate flies? geographic• What type of isolation kept lab flies

from mating? behavioral

Take It FurtherWhen 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 popula-tions of fruit flies were first isolated geographically. Then, because of how the allele mutation affected mating behaviors, the populations also became isolated reproductively.

vocabularyword 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.

answersA 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.

TEACH FROM VISUALS

Chapter 11: The Evolution of Populations 325

FIGURE 5.2 GEOGRAPHIC BARRIER

©Fr

ed B

aven

dam

/Min

den

Pict

ures

Formative AssessmentREVIEWING MAIN IDEAS

How can 1. reproductive isolation lead to speciation?

What are three types of barriers that 2. can lead to reproductive isolation?

CRITICAL THINKING

Apply 3. Why are the flash patterns of fireflies considered to be behavioral isolation?

Synthesize 4. How did geographic isolation affect the diversity Darwin observed in Galápagos finches?

CONNECT TO

SCIENTIFIC PROCESSWhat could have been used 5. as a control group in the fruit fly experiment described in Figure 5.1?

Geographic BarriersThe 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.

Temporal BarriersBarriers can also involve timing. Temporal isolation exists when timing 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.

Compare and Contrast What are the differences and similarities between behavioral isolation and temporal isolation?

Although snapping shrimp in the Atlantic and Pacific oceans look similar, they are distinct species that have evolved through geographic isolation.

R E A D I N G T O O L B OX

VOCABULARYTemporal comes from the Latin word tempus, meaning “time.”

SELF-CHECK OnlineHMDScience.com

PREMIUM CONTENT

10 °N

10 °S

20 °N

90 °W100 °W110 °W80 °W

Equator0°

PACIFICOCEAN

ATLANTICOCEAN

NORTHAMER I CA

SOUTHAMER I CA

Isthmus of Panama 0 500 1,000 kilometers

0 500 1,000 miles

N

S

EW

MapQuest

McDougal-Littel, Biology Program '05Isthmus of Panamabhspe-041105-004

2nd proof date: 9/27/05

326 Unit 4: Evolution

11.5

Untitled-284 326 5/5/2011 2:17:12 PM

A

1. If populations cannot mate successfully, genetic differences may accumulate in the populations. Over time, the populations may change so much that they become different species.

2. behavioral barriers, geographic barriers, and temporal barriers

3. The flash-pattern mating behaviors of fireflies are not related to the timing of mating, so this is considered behavioral isolation rather than temporal isolation.

4. The geographic isolation of the finches on different islands meant that each island’s 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.

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 com-pared against this group.

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.

answersA 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 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.

. Teach continuedTEACH FROM VISUALS

. Assess and Reteach

326 Unit 4: Evolution

11.5 FOrMaTIvE assEssMEnT