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Biology
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2. 16-3 The Process of Speciation
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
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3. 16-3 The Process of Speciation
Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation.
Speciation is the formation of new species.
A species is a group of organisms that breed with one another and produce fertile offspring.
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Isolating Mechanisms
What factors are involved in the formation of new species?
The gene pools of two populations must become separated for them to become new species.
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Isolating Mechanisms
Isolating Mechanisms
As new species evolve, populations become reproductively isolated from each other.
When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.
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Isolating Mechanisms
Reproductive isolation can develop in a variety of ways, including:
behavioral isolation
geographic isolation
temporal isolation
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Isolating Mechanisms
Behavioral Isolation 
Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.
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Isolating Mechanisms
Geographic Isolation 
Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains.
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Isolating Mechanisms
Geographic barriers do not guarantee the formation of new species.
If two formerly separated populations can still interbreed, they remain a single species.
Potential geographic barriers may separate certain types of organisms but not others.
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Isolating Mechanisms
Temporal Isolation  
Temporal isolation occurs when two or more species reproduce at different times.
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11. Testing Natural Selection in Nature
Studies showing natural selection in action involve descendants of the finches that Darwin observed in the Galápagos Islands.
The finches Darwin saw were different, but he hypothesized that they had descended from a common ancestor.
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12. Testing Natural Selection in Nature
Detailed genetic studies have shown that these finches evolved from a species with a more-or-less general-purpose beak. 
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13. Testing Natural Selection in Nature
Detailed genetic studies have shown that these finches evolved from a species with a more-or-less general-purpose beak. 
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14. Testing Natural Selection in Nature
Peter and Rosemary Grant tested Darwin’s hypothesis, which relied on two testable assumptions:
For beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection.
Differences in beak size and shape must produce differences in fitness, causing natural selection to occur.
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15. Testing Natural Selection in Nature
The Grants tested these hypotheses on the medium ground finch on Daphne Major, one of the Galápagos Islands.
During the rainy season, there is plenty of food.
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16. Testing Natural Selection in Nature
During droughts, food becomes scarce.
Individual birds with different-sized beaks had different chances of survival during a drought.
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17. Testing Natural Selection in Nature
When food was scarce, individuals with large beaks were more likely to survive.
This graph shows the survival rate of one species of ground-feeding finches, the medium ground finch. 
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18. Testing Natural Selection in Nature
The Grants provided evidence of the process of evolution.
Beak size can be changed by natural selection.
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19. Speciation in Darwin’s Finches
Describe the process of speciation in the Galápagos finches.
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20. Speciation in Darwin's Finches
Speciation in the Galápagos finches occurred by:
founding of a new population
geographic isolation
changes in new population's gene pool
reproductive isolation
ecological competition
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21. Speciation in Darwin's Finches
Founders Arrive 
A few finches—species A—travel from South America to one of the Galápagos Islands.
There, they survive and reproduce.
Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.
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22. Speciation in Darwin's Finches
Geographic Isolation
Some birds from species A cross to a second island.
The two populations no longer share a gene pool.
Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.
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23. Speciation in Darwin's Finches
Changes in the Gene Pool
Seed sizes on the second island favor birds with large beaks.
The population on the second island evolves into population B, with larger beaks.
Speciation in the Galápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.
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24. Speciation in Darwin's Finches
Reproductive Isolation
If population B birds cross back to the first island, they will not mate with birds from population A.
Populations A and B are separate species.
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25. Speciation in Darwin's Finches
Ecological Competition
As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them.
A new species—C—may evolve.
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26. Speciation in Darwin's Finches
Continued Evolution
This process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galápagos island chain.
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27. Studying Evolution Since Darwin
Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past.
Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved.
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16-3
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16-3
When two species do not reproduce because of differences in mating rituals, the situation is referred to as
temporal isolation.
geographic isolation.
behavioral isolation.
reproductive isolation.
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16-3
The most important factor involved in the evolution of the Kaibab and Abert squirrels of the American Southwest appears to be
temporal isolation.
geographic isolation.
behavioral isolation.
different food sources.
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16-3
One finding of the Grants' research on generations of Galápagos finches was that
natural selection did not occur in the finches
natural selection can take place often and very rapidly.
beak size had no effect on survival rate of the finches.
natural selection was slow and permanent.
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16-3
All of the following played a role in speciation of Galápagos finches EXCEPT
no changes in the gene pool.
separation of populations.
reproductive isolation.
natural selection.
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16-3
Beak size in the various groups of Galápagos finches changed primarily in response to
climate.
mating preference.
food source.
availability of water.
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