1. 16-1 Genes and Variation
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Gene pool is the combined genetic information of all members of a particular population.
Relative frequency is the number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur.

2. Sources of Genetic Variation
A mutation is any change in a sequence of DNA. Mutations increase genetic variation, which is needed for natural selection to bring about evolutionary change.
Mutations do not always affect an organism’s phenotype or fitness.
Some can affect an organism’s fitness, or its ability to survive and reproduce in its environment.

3. Sources of Genetic Variation
Many heritable differences are due to gene shuffling that occurs during the production of gametes.
The separation of homologous chromosomes during meiosis is random with a possibility of 8.4 million different combinations of genes.
Crossing over during meiosis increases the number of different genotypes that can appear in offspring.
Sexual reproduction produces many different combination of genotypes.

4. Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait. - single-gene traits produce only two phenotypes. - polygenic traits have many possible genotypes and phenotypes.

5. Generic Bell Curve for Polygenic Trait
Section 16-1
The graph shows the distribution of phenotypes that would be expected for a polygenic trait. The symmetrical bell-like shape of this curve is typical of polygenic traits. This is called a normal distribution.
Frequency of Phenotype
Phenotype (height)

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. 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

8. Hardy Weinberg Equilibrium

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. 16-3 The Process of Speciation
A species is a group of similar organisms that can breed and produce fertile offspring. Individuals in the same species share a common gene pool.
A genetic change that occurs in one individual can spread through the population as that individual and its offspring reproduce.
If a genetic change increases fitness, that allele will eventually be found in many individuals of that population.

16. Speciation is the formation of new species.
Isolation Mechanisms
Speciation is the formation of new species.
As new species evolve, populations become reproductively isolated from each other.
In reproductive isolation, the members of two populations cannot interbreed and produce fertile offspring.
Can be due to behavioral isolation, geographic isolation, and temporal isolation.

17. Reproductive Isolation
Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive stratagies.
Geographic isolation – two populations are separated by geographic barriers (rivers, mountains, or bodies of water).
Temporal isolation – two or more species reproduce at different times.

18. 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

19. Speciation in Darwin’s Finches
In finches on the Galapagos Islands, a few finches traveled from S. America to one of the islands. They survived and reproduced.
Some birds crossed to other islands, and the different populations no longer shared a gene pool.
Differences in food sources on the islands favored birds with different types of beaks.
This led to the populations becoming reproductively isolated and evolving into separate species.
The reproductive isolation was reinforced by ecological competition between members of the different species whenever they did live in the same environment.

20. 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

21. 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

22. 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

23. 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

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