Name
Class
Date
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?
2. What does it mean for two species to be reproductively isolated from each other?
3. What must happen in order for a new species to evolve?
276
Name
Class
Date
4. List three ways that reproductive isolation occurs.
5. When does behavioral isolation occur?
6. When does geographic isolation occur?
7. What is an example of temporal isolation?
8. Suppose a seamount forms from an underwater volcano. Birds on the mainland colonize
the island. How might this lead to speciation?
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
populations.
10. Many finch characteristics appear in bell-shaped distributions typical of
traits.
11. The ancestors of the Galápagos Island finches originally came from the continent of
.
12. The populations of finches on separate islands are
one another by large stretches of open water.
13. Big-beaked finches that prefer to mate with other big-beaked finches are
isolated from small-beaked finches living on the same island.
277
isolated from
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.
Apply the Big idea
15. Explain why reproductive isolation must occur for separate populations of the same
species to evolve into different species.
278