Download 17.3 The Process of Speciation

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17.3 The Process of Speciation
Lesson Objectives
Identify the types of isolation that can lead to the formation of new species.
Describe the current hypothesis about Galápagos finch speciation.
Lesson Summary
Isolating Mechanisms Speciation is the formation of new species. For one species to
evolve into two new species, the gene pools of two populations must become separated, or
reproductively isolated. Reproductive isolation occurs when members of two populations
do not interbreed and produce fertile offspring. Reproductive isolation can develop through
behavioral, geographic, or temporal isolation.
▶ Behavioral isolation occurs when populations have different courtship rituals or other
behaviors involved in reproduction.
▶ Geographic isolation occurs when populations are separated by geographic barriers, such
as mountains or rivers.
▶ Temporal isolation occurs when populations reproduce at different times.
Speciation in Darwin’s Finches Peter and Rosemary Grant’s work supports the hypothesis that speciation in the Galápagos finches was, and still continues to be, a result of the
founder effect and natural selection.
▶ Speciation in Galápagos finches may have occurred in a sequence of events that involved
the founding of a new population, geographic isolation, changes in the gene pool,
behavioral isolation, and ecological competition.
▶ For example, a few finches may have flown from mainland South America to one of the
islands. There, they survived and reproduced. Some birds may have crossed to a second
island, and the two populations became geographically isolated. Seed sizes on the second
island could have favored birds with larger beaks, so the population on the second island
evolved into a population with larger beaks. Eventually, these large-beaked birds became
reproductively isolated and evolved into a new species.
Isolating Mechanisms
1. What is speciation?
It is the formation of new species.
2. What does it mean for two species to be reproductively isolated from each other?
Members of the two species do not interbreed and produce fertile offspring.
3. What must happen in order for a new species to evolve?
Populations must be reproductively isolated from one another.
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4. List three ways that reproductive isolation occurs.
behavioral isolation, geographic isolation, temporal isolation
5. When does behavioral isolation occur?
It occurs when populations have differences in courtship rituals or other types of
behaviors that prevent them from interbreeding.
6. When does geographic isolation occur?
It occurs when populations are separated by geographic barriers such as rivers,
mountains, or bodies of water.
7. What is an example of temporal isolation?
SAMPLE ANSWER:
Each of three similar species of orchid in the same rain forest releases
pollen on different days.
8. Suppose a seamount forms from an underwater volcano. Birds on the mainland colonize
the island. How might this lead to speciation?
Answers may vary. The birds on the island may become reproductively isolated from
mainland birds through geographic isolation. As a result, the gene pool of the birds
that colonized the island would remain isolated, and the birds may evolve to become
a new species as they adapt to their island habitat.
Speciation in Darwin’s Finches
For Questions 9–13, complete each statement by writing the correct word or words.
9. Peter and Rosemary Grant spent years on the Galápagos Islands studying changes in
finch
populations.
10. Many finch characteristics appear in bell-shaped distributions typical of
traits.
polygenic
11. The ancestors of the Galápagos Island finches originally came from the continent of
South America
.
12. The populations of finches on separate islands are
from one another by large stretches of open water.
geographically
isolated
13. Big-beaked finches that prefer to mate with other big-beaked finches are behaviorally
isolated from small-beaked finches living on the same island.
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14. Write a paragraph that summarizes how speciation likely occurred in the Galápagos
finches. Use the following terms in your response: geographic isolation, gene pools,
behavioral isolation, and competition.
Many years ago, a few finches from South America arrived on a Galápagos island.
These finches were geographically isolated from the original South American population. They survived and reproduced. Over time, some finches from the first island
may have migrated to a new island, becoming a new geographically-isolated population. Differing conditions on the two islands, such as varied food sources, may have
caused the populations to evolve in separate ways. So, the allele frequencies in each
population’s gene pool would have also changed in different ways. Over time, migration, geographic isolation, and natural selection may have occurred multiple times on
different Galápagos islands. Additionally, populations on different islands may have
evolved separate breeding behaviors causing them to become behaviorally isolated.
Competition among individuals and responses to a changing environment could have
created further selective pressures on these isolated populations. In the end, the combination of all these processes likely resulted in the evolution of the 13 finch species
currently found in the Galápagos.
15. Explain why reproductive isolation must occur for separate populations of the same
species to evolve into different species.
Reproductive isolation must occur so that the gene pools are isolated. When the gene
pools are isolated, changes that occur in one gene pool will not be carried over to the
other gene pool. Thus, over time, accumulated gene pool changes can produce a new
species.
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17.4 Molecular Evolution
Lesson Objectives
Explain how molecular clocks are used.
Explain how new genes evolve.
Describe how Hox genes may be involved in evolutionary change.
Lesson Summary
Timing Lineage Splits: Molecular Clocks A molecular clock uses mutation rates in
DNA to estimate the length of time that two species have been evolving independently.
▶ Molecular clock models assume that neutral mutations, which do not affect phenotype,
accumulate in the DNA of different species at about the same rate.
▶ Two species evolving independently from each other will accumulate different neutral
mutations through time. The more differences between the DNA, the more time has
passed since the two species shared an ancestor.
Gene Duplication New genes evolve through the duplication, and then modification, of
existing genes.
▶ Organisms may carry multiple copies of the same gene. The extra copies of a gene may
undergo mutations.
▶ The mutated gene may have a new function that is different from the original gene. In this
way, new genes evolve.
▶ Multiple copies of a duplicated gene can turn into a gene family.
Developmental Genes and Body Plans Researchers study the relationship between
evolution and embryological development.
▶ Some genes, called Hox genes, control the forms of animals’ bodies.
▶ Small changes in Hox genes during embryological development can produce major
changes in adult organisms.
▶ Some scientists think that changes in Hox genes may contribute to major evolutionary
changes.
Timing Lineage Splits: Molecular Clocks
1. What is a molecular clock?
It is a model that uses DNA comparisons to estimate the length of time that two
species have been evolving independently.
2. Why are only neutral mutations useful for molecular clocks?
Neutral mutations accumulate in the DNA of different species at about the same rate
because they are unaffected by natural selection.
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3. Why are there many molecular clocks in a genome instead of just one?
Some genes accumulate mutations faster than others.
Use the diagram of an ancestral species to answer Questions 4–5. Each picture in the
diagram represents a gene. Each shaded portion of a gene represents a mutation.
4. Which species is most closely related
to Species B? Explain your answer.
Species C; Species B and Species C
A gene in an ancestral species
Nucleotide
have two mutations in common.
1
Species B and Species A have no
mutations in common. This shows
Two
mutations
Two
mutations
that Species B and C are more closely
related than Species B and A.
5. How can you tell that Species C is
probably not a descendant of the
organism with Gene 2?
Species C and the species with Gene
2
New
mutation
3
New
New
mutation mutation
2 do not share any mutations.
4
Species
A
5
Species
B
6
Species
C
Gene Duplication
For Questions 6–7, write the letter of the correct answer on the line at the left.
D
6. Multiple copies of a duplicated gene can turn into a group of related genes called
A. globins.
B. duplicates.
C. a Hox gene.
D. a gene family.
A
7. A chromosome may get several copies of the same gene during the process of
A. crossing-over.
B. gene mutation.
C. gene expression.
D. artificial selection.
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8. Complete the flowchart to show how a new gene can evolve from a duplicated gene.
Original gene
Duplication in ancestor
Mutation in one copy
Exact copy of original gene
New gene evolves new function.
Original gene keeps original function.
Developmental Genes and Body Plans
9. What genetic factors might be responsible for a change in an organism’s body plan?
A change in Hox gene activity during embryonic development can change an organism’s body plan.
10. How can Hox genes help reveal how evolution occurred?
Homologous Hox genes establish body plans in species that have not shared a
common ancestor in hundreds of millions of years. In addition, major evolutionary
changes may be based on Hox genes. A change in one Hox gene can result in major
body changes in an animal. Scientists may be able to trace Hox gene changes to
determine when and why certain species of organisms developed.
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Chapter Vocabulary Review
For Questions 1–8, complete each statement by writing the correct word or words.
1. In a small population, a random change in allele frequency is called
genetic drift
.
2. When birds cannot interbreed because they have different mating songs, they are
separated by
behavioral
isolation.
3. A situation in which allele frequencies change as a result of the migration of a small
subgroup of a population is known as the founder effect .
4. Two related species that live in the same area but mate during different seasons are
separated by
temporal
single-gene trait
5. A(n)
isolation.
is a trait controlled by only one gene.
bottleneck effect
6. The
is a change in allele frequency following a dramatic
reduction in the size of a population.
7.
Allele frequency
is the number of times an allele occurs in a gene pool,
compared to the total number of alleles in that pool for the same gene.
For Questions 8–16, write True if the statement is true. If the statement is false, change the
underlined word or words to make the statement true.
gene pool
True
True
geographic
True
Speciation
Genetic
equilibrium
stabilizing
True
8. All of the genes in a population make up the allele frequency of the
population.
9. Traits controlled by two or more genes are polygenic traits.
10. Reproductive isolation occurs when members of two populations do
not interbreed and produce fertile offspring.
11. The separation of two populations by barriers such as rivers or
mountains results in temporal isolation.
12. The Hardy-Weinberg principle states that allele frequencies in a
population should remain constant unless one or more factors cause
those frequencies to change.
13. Genetic drift is the formation of new species.
14. The founder effect occurs when the allele frequencies in a population
remain constant.
15. For polygenic traits, when individuals near the center of the bell curve
have higher fitness than individuals at either end, disruptive selection
takes place.
16. When researchers use a molecular clock, they compare stretches of
DNA to mark the passage of evolutionary time.
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The Chapter Mystery focused on the
emergence of the deadly strain of influenza
that led to the cataclysmic flu epidemic of
1918. Today, teams of scientists, doctors,
and other health specialists are working
hard to prevent contagious diseases from
spreading out of control.
EPIDEMIC
Learning
Preventing the Next Epidemic
The Global Outbreak Alert and Response Network, called “GOARN,” is a vital and littleknown group that serves as an international early warning system to prevent and contain
disease outbreaks. It is based at the World Health Organization headquarters in Geneva,
Switzerland. The World Health Organization is part of the United Nations system.
Read the following case study to learn more about GOARN’s work fighting a disease
outbreak known as SARS.
How GOARN Helped Prevent a SARS Epidemic
In late 2002, an outbreak of an unusual and deadly type of pneumonia occurred in southern
China. After additional cases appeared early in 2003 in Vietnam and Hong Kong, doctors named
the mysterious ailment “severe acute respiratory syndrome,” or SARS. It appeared to be spreading
fast and no drug seemed effective against it. SARS killed many of the people it infected. Within
months, cases of SARS had begun to occur in more than a dozen countries around the world.
From the start, the Global Outbreak Alert and Response Network (GOARN) quickly swung
into action. GOARN staff mobilized partners in many countries to support scores of professionals
in the field working to fight SARS. They lent support for tracking the disease and controlling
its spread. While GOARN staff worked to coordinate quarantine efforts and take blood samples
from infected patients, they also mobilized scientists in GOARN’s network of laboratories. These
laboratories, located in nine different countries, studied the blood samples to try to identify the
underlying cause for SARS.
GOARN set up a secure Web site so that its worldwide network of researchers could keep in
contact as they worked around the clock. The communications system was so well designed that
scientists could display patient samples and electron microscope images in real time to colleagues
continents away. Details of each lab’s analysis and testing of samples were immediately posted
online so researchers elsewhere could instantly act upon relevant information. Scientists at all
these laboratories participated in daily teleconferences to discuss progress and obstacles. With the
extraordinary global collaboration, scientists in GOARN’s global network were able to identify the
previously unknown mutant virus responsible for SARS in less than two weeks.
GOARN’s network of scientists succeeded in identifying the culprit and containing the spread
of SARS in record time. But researchers say that a virulent strain of flu could spread even faster
than SARS. So, night and day, the GOARN staff remains on alert, carefully monitoring outbreaks
of disease wherever they occur around the world.
Continued on next page ▶
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Science and Global
Awareness
1. What steps did GOARN take to fight SARS in the field?
GOARN staff worked to coordinate quarantine efforts and take blood samples from
infected patients. They lent support for tracking the disease and controlling its spread.
2. What steps did GOARN take to identify the underlying cause of SARS?
GOARN mobilized scientists in its network of laboratories. It set up a special Web site
to allow these researchers to share information about their work and collaborate
around the clock.
3. Was the worldwide collaboration between the laboratories successful in identifying the
virus responsible for SARS? If so, what were the key factors that led to its success?
The GOARN network was remarkably successful, finding the virus in just two weeks. The
key was that laboratories around the world all shared their work and results. The combined effort allowed them to quickly build on one anothers’ research.
4. How do you think GOARN could improve their response time to an emerging disease?
SAMPLE ANSWER:
They could recruit several doctors from every major population center
around the world and have the doctors flag any suspicious cases they see.
5. Today, emerging viruses can spread faster than ever before. What factors do you think
contribute to this fast pace of transmission?
SAMPLE ANSWER:
Today, much of the world’s population live in densely packed urban
areas, making transmission from one person to another very easy. In addition, modern transportation, such as airplanes, lets people in one part of the world reach
another part of the world in just a few hours.
Media Review
The skills used in this activity include information and media literacy, critical thinking and
systems thinking, and self-direction.
A number of books and movies are based on the potential for, or the occurrence of, a
disease outbreak. Two examples are Richard Preston’s 1995 nonfiction thriller The Hot Zone:
A Terrifying True Story and the 1995 Hollywood feature film “Outbreak.” At the library or
other local resource, find a book or a movie that dramatizes the threat of a disease epidemic.
Also, find out more about how GOARN helps to fight epidemics. You may want to start by
visiting the World Health Organization’s Web site.
After getting permission from your teacher, write a review discussing the work and how its
treatment of the subject compares with what you have learned about GOARN’s real-life
monitoring and prevention activities. Evaluate the students’ performance based on their
analysis of the accuracy of the film or book. They should discuss several ways the film or
book compares with what they learned about GOARN’s activities.
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