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Interest Grabber 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. Section 16-1

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Page 1: Interest Grabber 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,

Interest Grabber

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.

Section 16-1

Page 2: Interest Grabber 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,

Interest Grabber continued

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 thetrait (e.g., hair color).

2. Are most of the traits you listed clear-cut or are they mostly traitsthat have many variations? Which traits in your list are difficult tocategorize?

3. Compare your list with that of another student. Did he or she think ofany traits that you missed? Why do you think some traits are clear-cut,while others are not?

Section 16-1

Page 3: Interest Grabber 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,

Answers

1.Students’ answers will include dimples

and detached earlobes.

2. Most traits listed likely have many variations.

3.Some students may suggest that patterns of inheritance for traits with many variations (polygenic) are more complex than for clear-cut (single-gene) ones.

Page 4: Interest Grabber 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,

Section Outline

16–1 Genes and Variation

A. Darwin’s Ideas Revisited

B. Gene Pools

C.Sources of Genetic Variation

1.Mutations

2.Gene Shuffling

D.Single-Gene and Polygenic Traits

Section 16-1

Page 5: Interest Grabber 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,

Darwin’s Ideas

Darwin did not know how heredity worked:

1. He did not know the source of the variation that was so central to his theory.

2. He could not explain how inheritable traits were passed from one generation to the next.

Page 6: Interest Grabber 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,

Words to Know

Gene pool – combined genetic information of all the members of a particular population

Relative frequency – the number of times an allele occurs in a gene pool compared with the number of times other alleles occur

The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

Mutation – any change in the sequence of DNA

Page 7: Interest Grabber 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,

Mutations

•Can occur because of mistakes in the replication of DNA•Can be a result of radiation or chemicals in the environment•Can be limited to one or a few bases of DNA •Can affect lengthy segments of a chromosome•Do not always affect an organism’s phenotype – its physical, behavioral, and biochemical characteristics (Example: A DNA codon altered by a point mutation from GGA to GGU will still code for the same amino acid, glycine.)•Many mutations do change the phenotype•Some mutations affect fitness; others do not affect the organism’s ability to survive and reproduce

Page 8: Interest Grabber 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,

Words to Know

The number of phenotypes produced for a given trait depends on how many genes control the trait.

Single-gene trait – a trait controlled by a single gene that has two alleles (Widow’s peak)

Polygenic traits – traits controlled by two or more genes; each gene of a polygenic trait often has two or more alleles (Height)

Page 9: Interest Grabber 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,

Fre

qu

enc

y o

f P

hen

oty

pe

Phenotype (height)

Generic Bell Curve for Polygenic Trait

Section 16-1

Page 10: Interest Grabber 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,

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

Figure 16–2 Relative Frequencies of Alleles

Section 16-1

Page 11: Interest Grabber 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,

Fre

qu

ency

of

Ph

eno

typ

e(%

)100

80

60

40

20

0 Widow’s peak No widow’s peak

Phenotype

Figure 16–3 Phenotypes for Single-gene Trait

Section 16-1

Page 12: Interest Grabber 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,

Interest Grabber

. . . All the Help I Can Get

Natural selection operates on traits in different ways. You might be ableto predict which traits natural selection would favor if you think about the demands of an organism’s environment.

Section 16-2

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.

Page 13: Interest Grabber 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,

1.Animal choices should be sufficiently familiar that students can describe several traits.

2. Students should list traits such as size, color, and specialized behavior.

3. 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.

Page 14: Interest Grabber 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,

Section Outline

16–2 Evolution as Genetic ChangeA. 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

Section 16-2

Page 15: Interest Grabber 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,

Natural Selection on Single-Gene Traits

1. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.

Example: • If a population of lizards lives in dark soil, those with

red skin coloring would be easier prey. Eventually, more lizards with dark coloring would survive and change the gene pool frequencies.

Page 16: Interest Grabber 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,

Natural Selection on Polygenic Traits

Natural selection can affect the distribution of phenotypes in any of three ways:

1. Directional selection

2. Stabilizing selection

3. Disruptive selection

Page 17: Interest Grabber 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,

Directional Selection

Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end; entire curve shifts

Example: An increase in the average size of the beaks in a particular species of Galapagos finches; better fitness as they competed for food

Page 18: Interest Grabber 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,

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Figure 16–6 Graph of Directional Selection

Section 16-2

Page 19: Interest Grabber 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,

Stabilizing Selection

Individuals near the center of the curve have higher fitness than individuals at either end of the curve; keeps the center of the curve at its current position, but it narrow the overall graph

Example: Human infants at birth- low birth weight babies are less likely to survive and large birth weight babies are more likely to have difficulty being born

Page 20: Interest Grabber 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,

Key

Per

cen

tag

e o

f P

op

ula

tio

n

Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Figure 16–7 Graph of Stabilizing Selection

Section 16-2

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Page 21: Interest Grabber 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,

Disruptive Selection

Individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle; the single curve splits into two curves

Example: A population of birds lives in an area where medium-sized seeds become less common. Birds with unusually small or large beaks would have higher fitness.

Page 22: Interest Grabber 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,

Disruptive Selection

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Figure 16–8 Graph of Disruptive Selection

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

Section 16-2

Page 23: Interest Grabber 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,

Genetic Drift

•A random change in allele frequency•In small populations, individuals that carry a particular allele may leave more descendents than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.•Can occur when a small group of individuals colonizes a new habitat

Page 24: Interest Grabber 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,

Founder Effect

A situation in which the allele frequencies change as a result of the migration of a small subgroup of a population

Example: Evolution of several hundred species of fruit flies on the Hawaiian Islands (All descended from the same mainland)

Page 25: Interest Grabber 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,

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Section 16-2

Page 26: Interest Grabber 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,

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Section 16-2

Page 27: Interest Grabber 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,

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Section 16-2

Page 28: Interest Grabber 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,

Interest Grabber

Country Cousin/City Cousin

What happens when a population or group of living things is dividedinto two separate groups in two separate environments? To understand what goes on, think about someone who lives in another part of theUnited States or in another country.

Section 16-3

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?

Page 29: Interest Grabber 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,

Answers

Country Cousin/City Cousin

1.Students’ lists should include several

social/environmental factors.

2.Students may understand that humans would evolve separately in response to different environmental pressures.

Page 30: Interest Grabber 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,

Section Outline

16–3 The Process of SpeciationA. 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 of 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

Section 16-3

Page 31: Interest Grabber 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,

Concept Map

Section 16-3

results from

which include

produced by produced byproduced by

which result in

which result in

Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species


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