Transcript Section 16-2 - Xavier High School

Interest Grabber
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.
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?
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 clearcut (single-gene) ones.
Section Outline
Section 16-1
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
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.
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
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
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)
Generic Bell Curve for
Polygenic Trait
Frequency of Phenotype
Section 16-1
Phenotype (height)
Figure 16–2 Relative
Frequencies of Alleles
Section 16-1
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
Figure 16–3 Phenotypes for
Single-gene Trait
Section 16-1
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s peak
Interest Grabber
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.
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 nowconspicuous deer.
Section Outline
Section 16-2
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
Natural Selection on SingleGene 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.
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
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
Figure 16–6 Graph of
Directional Selection
Section 16-2
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
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
Figure 16–7 Graph of
Stabilizing Selection
Section 16-2
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
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.
Figure 16–8 Graph of
Disruptive Selection
Section 16-2
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
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
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)
Genetic Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Interest Grabber
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?
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.
Section Outline
Section 16-3
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 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
Concept Map
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species