1. Speciation
2019

2. 5/31 Objective: Explain the types of isolation that lead to the formation of new species.
DMA: The type of selection in which individuals of average size have greater fitness than small or large individuals is called _______________.

3. The Process of Speciation
Read the lesson title aloud to students.

4. Speciation – the rise of two more species from one existing species.

5. 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

6. Populations can become isolated in 3 ways:
Behavioral isolation – differences in courtship or mating.
Geographic isolation – physical barriers that divide a population into two or more groups.
Temporal isolation – some members of the population show signs of courtship at different times. Reproductive periods may change.

7. Reproductive Isolation
Reproductive isolation can develop from:
behavioral isolation
geographic isolation
temporal isolation
Tell students: Reproductive isolation can develop in several ways, including behavioral isolation, geographic isolation, and temporal isolation.
Click to reveal behavioral isolation.
Tell students: If two populations that were once able to interbreed evolve differences in courtship rituals or other behaviors, behavioral isolation can occur. For example, eastern and western meadowlarks are similar birds whose habitats overlap. But members of the two species will not mate with each other, partly because they use different songs to attract mates. Eastern meadowlarks don’t respond to western meadowlark songs, and vice versa.
Click to reveal geographic isolation.
Tell students: When two populations are separated by geographic barriers such as rivers, mountains, tepuis (shown), or bodies of water, geographic isolation occurs.
Point out that organisms that live on the top of tepuis are not found anywhere else in the world.
Point out that geographic barriers do not always guarantee isolation. Floods, for example, may link separate lakes, enabling their fish populations to mix. If those populations still interbreed, they remain a single species. Also, a geographic barrier may separate certain organisms but not others. A large river may keep squirrels and other small rodents apart but probably won’t isolate bird populations.
Click to reveal temporal isolation.
Tell students: A third isolating mechanism, known as temporal isolation, happens when two or more species reproduce at different times. For example, suppose three similar species of orchids live in the same rain forest. Each species has flowers that last only one day and must be pollinated on that day to produce seeds. Because the species bloom on different days, they cannot pollinate one another.

8. These barriers lead to reproductive isolation.
Occurs when members of different populations can no longer mate successfully with one another.
Not physically able
Offspring won’t survive and reproduce

9. Speciation: Founders Arrive
Allele frequencies of the founding finches could have differed from those of the original population.
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. 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. 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. The finch populations no longer shared a common gene pool.

11. 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. 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.
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. Speciation: Competition and Continued 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.