1. KEY CONCEPT Natural selection is not the only mechanism through which populations evolve.

2. Gene flow is the movement of alleles between populations.
bald eagle migration
Gene flow occurs when individuals join new populations and reproduce.
Gene flow keeps neighboring populations similar.
Low gene flow increases the chance that two populations will evolve into different species.
Predict: How does gene flow affect neighboring populations?
Fig. This map shows the locations where young banded eagles were found during the first summer after hatching.

3. Genetic drift is a change in allele frequencies due to chance.
Genetic drift causes a loss of genetic diversity.
It is most common in small populations.
A population bottleneck can lead to genetic drift.
It occurs when an event drastically reduces population size.
The bottleneck effect is genetic drift that occurs after a bottleneck event.
Fig. The bottleneck effect describes the effect of a destructive event that leaves only a few survivors in a population.
Initial population
Surviving population
Bottleneck effect

4. The founding of a small population can lead to genetic drift.
It occurs when a few individuals start a new population.
The founder effect is genetic drift that occurs after start of new population.
A bird carries a few seeds to a new location. These seeds “found” a new population.
A. The gene pool for a population of flowers has genetic diversity that results in red, yellow and blue phenotypes.
B. Alleles for yellow flower color increase in the new small population through genetic drift.
Apply: Why is genetic drift more likely to occur in smaller populations?

5. Genetic drift has negative effects on a population.
less likely to have some individuals that can adapt to a changing environment due to loss of genetic variation
harmful alleles can become more common due to chance
Example of Genetic Drift
Cheetahs exhibit much lower levels of variation than other mammals. In most species, related individuals share about 80 percent of the same genes. With cheetahs, this figure rises to approximately 99 percent. The genetic inbreeding in cheetahs has led to low survivorship (a large number of animals dying), poor sperm quality, and greater susceptibility to disease. Inbred animals suffer from a lack of genetic diversity.

6. Implications of the Founder Effect in National Parks
What are some population management strategies National Parks can use to prevent genetic drift from occurring in reintroduced species?
Yellowstone National Park wolf reintroduction.
Cherokee Nation American Bison reintroduction.

7. Sexual selection occurs when certain traits increase mating success.
Sexual selection occurs due to higher cost of reproduction for females.
males produce many sperm continuously
females are more limited in potential offspring each cycle
Fig. Widowbird
Apply: Male Irish elks, which are now extinct, had 12-foot wide antlers. Describe how sexual selection could have caused such an exaggerated trait to evolve.

8. There are two types of sexual selection.
intrasexual selection: competition among males
Ex: Head-butting of bighorn sheep
intersexual selection: males display certain traits to females
Ex: peacocks fanning out their tail feathers
Fig. Male frigate birds inflate an air sac in their chest to attract females. This trait has evolved through sexual selection. This trait is not always adaptive for the survival of the individual. The bright red air sac likely make the male frigate bird easy to spot by predators.

9. Identifying patterns
Graph 1: Tail feather length and reproductive success
Biologist were trying to determine the relationship between tail feather length and reproductive success in male widowbirds.
Interpret: According to the data, what gives a male widowbird greater reproductive success?

10. Graph 2: Sexual selection in guppies
Identifying patterns
Graph 2: Sexual selection in guppies
Three experiments were run to determine if female guppies prefer males with specific tail sizes.
Analyze: What is the relationship between tail size in male guppies and female preference for mates?
Infer: Why might the difference in preference be larger in Experiment 1 than in Experiment 2?