1. Interest Grabber Yes, No, or Maybe
Section 16-1
Yes, No, or Maybe
Some traits, such as a widow’s peak, fall into neat categories: You either have a widow’s peak or you don’t. Other traits, such as height, aren’t so easy to categorize.

2. Interest Grabber continued
Section 16-1
1. Make a list of physical traits that you think are influenced by genes. Then, write next to each trait whether you have the trait or not (e.g., a widow’s peak) or whether there are many variations of the trait (e.g., hair color).
2. Are most of the traits you listed clear-cut or are they mostly traits that have many variations? Which traits in your list are difficult to categorize?
3. Compare your list with that of another student. Did he or she think of any traits that you missed? Why do you think some traits are clear-cut, while others are not?

3. Section Outline 16–1 Genes and Variation
A. How Common is Genetic Variation?
B. Variation and Gene Pools
C. Sources of Genetic Variation
1. Mutations
2. Gene Shuffling
D. Single-Gene and Polygenic Traits

4. Generic Bell Curve for Polygenic Trait
Section 16-1
Frequency of Phenotype
Phenotype (height)

5. Figure 16–2 Relative Frequencies of Alleles
Section 16-1
Sample Population
Frequency of Alleles
allele for brown fur
allele for black fur
48% heterozygous black
16% homozygous black
36% homozygous brown

6. Figure 16–3 Phenotypes for Single-Gene Trait
Section 16-1
100 80 60 40 20
Frequency of Phenotype
(%)
Widow’s peak
No widow’s peak
Phenotype

7. Interest Grabber . . . All the Help I Can Get
Section 16-2
. . . All the Help I Can Get
Natural selection operates on traits in different ways. You might be able to predict which traits natural selection would favor if you think about the demands of an organism’s environment.
1. Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other.
2. Under Trait, write in several of the animal’s traits.
3. Under Advantage, write in how you think the trait would be helpful to the animal.

8. Section Outline 16–2 Evolution as Genetic Change
A. Natural Selection on Single-Gene Traits
B. Natural Selection on Polygenic Traits
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
C. Genetic Drift
D. Evolution Versus Genetic Equilibrium
1. Random Mating
2. Large Population
3. No Movement Into or Out of the Population
4. No Mutations
5. No Natural Selection

9. Genetic Drift Section 16-2 Sample of Original Population Descendants
Founding Population A
Founding Population B

10. Genetic Drift Section 16-2 Sample of Original Population Descendants
Founding Population A
Founding Population B

11. Genetic Drift Section 16-2 Sample of Original Population Descendants
Founding Population A
Founding Population B

12. Figure 16–6 Graph of Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.

13. Figure 16–7 Graph of Stabilizing Selection
Section 16-2
Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Selection against both extremes keep curve narrow and in same place.
Percentage of Population
Birth Weight

14. Figure 16–8 Graph of Disruptive Selection
Section 16-2
Disruptive Selection
Largest and smallest seeds become more common.
Key
Low mortality, high fitness
Population splits into two subgroups specializing in different seeds.
Number of Birds in Population
Number of Birds in Population
High mortality, low fitness
Beak Size
Beak Size

15. Interest Grabber Country Cousin/City Cousin
Section 16-3
Country Cousin/City Cousin
What happens when a population or group of living things is divided into two separate groups in two separate environments? To understand what goes on, think about someone who lives in another part of the United States or in another country.
1. Make a list of everyday things that this person encounters that you don’t. For example, does he or she eat different kinds of food? Does he or she live in a climate different from yours?
2. All humans are the same species. What might happen if groups of humans were separated for millions of years in very different environments, such as those you have just described?

16. Section Outline 16–3 The Process of Speciation A. Isolating Mechanisms
1. Behavioral Isolation
2. Geographic Isolation
3. Temporal Isolation
B. Testing Natural Selection in Nature
1. Variation
2. Natural Selection
3. Rapid Evolution
C. Speciation in Darwin’s Finches
1. Founders Arrive
2. Separation of Populations
3. Changes in the Gene Pool
4. Reproductive Isolation
5. Ecological Competition
6. Continued Evolution
D. Studying Evolution Since Darwin

17. Reproductive Isolation
Concept Map
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Temporal isolation
Geographic isolation
produced by
produced by
produced by
Behavioral differences
Different mating times
Physical separation
which result in
Independently evolving populations
which result in
Formation of new species

18. Go Online Links from the authors on restricting use of antibiotics
Interactive test
For links on population genetics, go to and enter the Web Code as follows: cbn-5161.
For links on speciation, go to and enter the Web Code as follows: cbn-5163.
Internet

19. Interest Grabber Answers
1. Make a list of physical traits that you think are influenced by genes. Then, write next to each trait whether you have the trait or not (e.g., a widow’s peak) or whether there are many variations of the trait (e.g., hair color).
Students’ answers will include dimples and detached earlobes.
2. Are most of the traits you listed clear-cut or are they mostly traits that have many variations? Which traits in your list are difficult to categorize?
Most traits listed likely have many variations.
3. Compare your list with that of another student. Did he or she think of any traits that you missed? Why do you think some traits are clear-cut, while others are not?
Some students may suggest that patterns of inheritance for traits with many variations (polygenic) are more complex than for clear-cut (single-gene) ones.
Section 1 Answers

20. Interest Grabber Answers
1. Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other.
Animal choices should be sufficiently familiar that students can describe several traits.
2. Under Trait, write in several of the animal’s traits.
Students should list traits such as size, color, and specialized behavior.
3. Under Advantage, write in how you think the trait would be helpful to the animal.
Students should indicate that adaptive value is clearer for some traits than for others. For example, white-tailed deer raise their tails upon sensing a predator. This may be an alarm signal for other deer, or it may induce the predator to chase the now-conspicuous deer.
Section 2 Answers

21. Interest Grabber Answers
1. Make a list of everyday things that this person encounters that you don’t. For example, does he or she eat different kinds of food? Does he or she live in a climate different from yours?
Students’ lists should include several social/environmental factors.
2. All humans are the same species. What might happen if groups of humans were separated for millions of years in very different environments, such as those you have just described?
Students may understand that humans would evolve separately in response to different environmental pressures.
Section 3 Answers

22. This slide is intentionally blank.
End of Custom Shows