1. The Process of Speciation
Isolation Notes

2. Vocabulary- Words to Know!
Species – interbreeding populations of organisms that can produce fertile offspring
Speciation – formation of a new species

3. Reproductive Isolation
Reproductive Isolation – when members of two populations cannot interbreed and produce fertile offspring
Horses and donkeys can mate and produce a mule. However, all mules are infertile females.

4. Isolating Mechanisms
If two populations become reproductively isolated, their gene pools can diverge, producing new species.
Isolated gene pools diverge into two separate species.
Reproductive isolation
Explain to students that a species is a population or group of populations whose members interbreed and produce fertile offspring. Interbreeding permits any genetic change that occurs to spread throughout populations of a species.
Tell students: If some members of a population stop breeding with other members, the species gene pool can split, as shown in the figure.
Click to reveal the point of reproductive isolation.
Point out that once members of two populations stop interbreeding, changes in one gene pool cannot spread to the other. When this happens, we say that reproductive isolation has occurred.
Ask: What happens when populations become reproductively isolated?
Answer: They may then evolve into two separate species through speciation.
Click to reveal this answer.
Common gene pool

5. How do organisms become isolated?
Behavioral isolation – when 2 populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior
Blue footed boobies

6. How do organisms become isolated? (cont.)
2. Geographic isolation – when 2 populations are
separated by geographic barriers such as rivers, mountains or bodies of water

7. How do organisms become isolated? (cont.)
3. Temporal Isolation- when 2 or more species reproduce at different times
Different seasons or different times of the day.
Example: 3 similar species of orchids live in the same rain forest. Each species has flowers that last only one day and must be pollinated (reproduce) on that day to produce seeds. Because the species bloom at different times of the year they cannot pollinate one another.

8. Speciation in Darwin’s Finches
Darwin found over a dozen different species of finches on the Galapagos Islands that all evolved from a common ancestor
How?
Speciation in Galapagos finches by founding of a new population
Geographical isolation
Changes in the new populations gene pool
Behavioral isolation
Ecological competition and evolution
Pg 496

9. 1. Speciation: Founders
Allele frequencies of the founding finches could have differed from those of the original population.
Ask: What is the current hypothesis about Galápagos finch speciation?
Answer: According to the current hypothesis, speciation in Galápagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, behavioral isolation, and ecological competition.
Tell students: Many years ago, a few finches from South America—species M in the figure—arrived on one of the Galápagos islands. These birds may have gotten lost or been blown off course by a storm. Once on the island, they survived and reproduced.
Explain that because of the founder effect, allele frequencies of this founding finch population could have differed from allele frequencies in the original South American population.
Ask: How might genetic drift, specifically the founder effect, have contributed to genetic variation in finch populations on different islands?
Answer: Only a few finches are likely to have crossed to other islands from the first island the finches inhabited. Their allele frequencies could have differed, by chance, from those of finches on the first island.

10. 2. Speciation: Geographic Isolation
Populations on the two islands no longer shared a common
gene pool
Tell students: The island’s environment was different from the South American environment. Some combination of the founder effect, geographic isolation, and natural selection resulted in differential reproductive success, adaptation, and evolution of this island population into a new species—species A, shown in the figure.
Explain that later, a few birds of species A crossed to another island.
Tell students: Because these birds do not usually fly over open water, they move from island to island very rarely. Thus, finch populations on the two islands were geographically isolated.
Ask for a volunteer to complete the sentence verbally.
Click to reveal the correct answers.
Tell students: The finch populations no longer shared a common gene pool.

11. 3. Speciation: Changes in Gene Pools
Populations on each island adapted to local environments.
On the second island,
selection favored individuals with larger, heavier beaks.
directional
Explain that over time, populations on each island further adapted to local environments. Plants on the first island may have produced small, thin-shelled seeds, whereas plants on the second island may have produced larger, thick-shelled seeds.
Click to reveal a statement about selection.
Ask for a volunteer to fill in the blank verbally.
Click to reveal the answer.
Tell students: On the second island, directional selection would have favored individuals with larger, heavier beaks. These birds could crack open and eat the large seeds more easily. Thus, birds with large beaks would be better able to survive on the second island.
Point out that over time, natural selection would have caused that population to evolve larger beaks, forming a new population, B, with a new phenotype.
Ask: Why did selection increase genetic variation among finches on the different islands?
Answer: Different traits were selected for because the islands had different environments.

12. 4. Speciation: Behavioral Isolation
Differences in beak size and mating behavior could lead to reproductive isolation.
The populations have become two distinct
species
Tell students: Imagine that a few birds from the second island cross back to the first island.
Ask: Will population A birds breed with population B birds?
Answer: Probably not.
Explain that during courtship, these finches closely inspect a potential partner’s beak. Finches prefer to mate with birds that have the same size beak as they do. Big-beaked birds prefer to mate with other big-beaked birds, and smaller-beaked birds prefer to mate with other smaller-beaked birds. Because the populations on the two islands have evolved differently sized beaks, they would probably not mate with each other.
Tell students: Differences in beak size, combined with mating behavior, could lead to reproductive isolation. The gene pools of the two bird populations remain isolated—even when individuals live in the same place.
Ask for a volunteer to verbally complete the statement shown.
Click to reveal the answer.
Tell students: The populations have now become two distinct species.

13. 5. Speciation: Competition and Evolution
Over many generations,
selection could have produced the 13 finch species found in Galápagos today.
natural
Explain to students that as these two new species live together on the first island, they compete for seeds. During the dry season, birds that are most different from each other have the highest fitness. That is because the more specialized birds experience less competition for certain kinds of seeds and other foods. Over time, the species evolve in a way that increases the differences between them.
Tell students: Birds on the first island that once belonged to species B may evolve into a new species, species C. These processes of geographic isolation, genetic change, and behavioral isolation could have repeated again and again across the Galápagos chain.
Ask for a volunteer to complete the statement shown.
Click to reveal the answer.
Tell students: Over many generations, natural selection could have produced the diversity among the 13 different finch species found there today.
Ask: How did selection lead to speciation?
Answer: Likely, speciation among the finches resulted from various combinations of geographic isolation and differences in phenotype, specifically in beak size, which correlated with different food sources. Even if two separate groups lived in the same area, their phenotypic differences would conceivably have been great enough to prevent them from mating with each other. Once populations become reproductively isolated, they are considered separate species.

14. Summary Isolated gene pools diverge into separate species.
Reproductive isolation occurs.
Ask for volunteers to add labels and captions to the figure so that it explains how speciation occurs.
Click to reveal the answers.
Members of a species share a gene pool.