Biology Chapter 16

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Biology Chapter 16

• Evolution Unit
• Evolution of Populations

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16-1 Genes and Variation

1. As Darwin developed his theory of evolution, he
   was not aware of how _____________ passed from
   one generation to the next

heritable traits
and how variation appeared in organisms.
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• B. Evolutionary biologists connected
  Darwins work and Mendels work during the
  1930s.
• 1. Changes in ________ produce heritable
  variation on which _______________ can operate.
• 2.

genes
natural selection
  Discovery of DNA demonstrated the molecular
nature of mutation and genetic variation.
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II. How Common is Genetic Variation?

• A. Individual fishes, reptiles, and mammals
  are typically heterozygous for between 4-8 of
  their genes.
• B. Variation and Gene Pools
• 1. Genetic variation is ______________
• ________________
• 2. Population ____________________
  _______________________________

studied in populations.
a group of individuals of the same
species that interbreed.
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• 3. Gene pool

all genes, including all the different
alleles, that are present in a population.
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• 4. 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.
• Example

Fur color in a population of mice 40 B
(black fur) 60 b (brown fur)
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Relative Frequencies of AllelesFigure 162 
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
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Evolution is any change in the
relative frequency of alleles

• MICROEVOLUTION ______________
  __________________________________________ in a
  population.
• Microevolution
• refers to
• ___________
• change in allele
• frequency over
• time.

small scale
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III. Sources of Genetic Variation

• A. Two sources of genetic variation
• 1. Mutation
• a. Ultimate source of variation.
• b. Any change in a sequence of DNA
• c. Most mutations
• are bad.
• Example UV,
• radiation, toxins

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• d. Mutations that produce changes in an
  organisms phenotype and increase an organisms
  fitness, or its ability to reproduce in its
  environment, will be passed on.

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• 2. Genetic shuffling that results from sexual
  reproduction.
• a. Independent assortment during meiosis
  produces 8.4 million possible combinations.
• b. Crossing-over.

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IV. Single-Gene and Polygenic Traits

• A. The number of phenotypes produced for a
  given trait depends on how many genes control
  the trait.
• 1. Single-gene trait Single gene that has two
  alleles. Example Free earlobes (FF, Ff) or
  attached earlobes (ff).

Free
Attached
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Phenotypes for Single-Gene Trait
100 80 60 40 20 0
Frequency of Phenotype ()
Free Earlobes (FF, Ff)
Attached Earlobes (ff)
Phenotype
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• Polygenic traits Traits that are controlled by
  two or more genes.
• One polygenic trait can have many possible
  genotypes or phenotypes.
• Example Height, eye color, skin color.

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16-2 Evolution as Genetic Change

• I. Natural Selection on Single-Gene Traits
• A. Reminder Evolution is any change over
  time in the relative frequencies of alleles in
  a population. Populations, not individual
  organisms, evolve over time.
• B. Natural selection on single-gene traits can
  lead to changes in allele frequencies and thus
  to evolution.

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• Effect of Color Mutations on Lizard Survival
  (Figure 16-5)
• 1. Organisms of one color may produce fewer
  offspring than organisms of other colors.
• Example Red lizards are more visible to
  predators and therefore, may be more likely to be
  eaten and not pass on that red gene.

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II. 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.

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A. Directional Selection

• 1. One of the two possible extremes is favored.
  
• Example Dark-colored peppered moths in regions
  of England with industrial pollution.

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Directional Selection Figure 166
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
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B. Stabilizing Selection

• 1. Intermediate characteristics are favored.
• Examples Human babies with very high or very
  low birth weights have lower survival than babies
  with intermediate weights.

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Stabilizing Selection Figure 167
Section 16-2
Stabilizing Selection
Key
Low mortality, high fitness
Selection against both extremes keep curve narrow
and in same place.
High mortality, low fitness
Percentage of Population
Birth Weight
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C. Disruptive Selection

• 1. Natural selection moves characteristics
  toward both extremes, and intermediate phenotypes
  become rarest.
• Example Populations of West African birds with
  either large or small, but not intermediate size
  beaks.

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 Disruptive Selection Figure 168
Section 16-2
Disruptive Selection
Largest and smallest seeds become more common.
Key
Population splits into two subgroups specializing
in different seeds.
Low mortality, high fitness
Number of Birdsin Population
Number of Birdsin Population
High mortality, low fitness
Beak Size
Beak Size
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III. Genetic Drift

• In small populations, an allele can become more
  or less common simply by chance.
• B. Genetic drift is a random change in allele
  frequency.

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Genetic Drift
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C. Two types of genetic drift

• Genetic bottleneck
• If a population crashes, then there will be a
  loss of alleles from the population.
• Example Northern Elephant Seals, Cheetahs.

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Genetic Bottleneck
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• 2. Founder effect A population can become
  limited in genetic variability if its founded
  by a small number of individuals.
• Example Polydactyly in Amish.

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Figure 16-9 Founder Effect
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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IV. Hardy-Weinberg and Genetic Equilibrium

• A. What would be necessary for no change to
  take place?
• 1. Hardy-Weinberg principle states that allele
  frequencies in a population will remain constant
  unless one or more factors cause those
  frequencies to change.
• 2. If allele frequencies remained constant
  then it there would be genetic equilibrium.

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• 3. If allele frequencies do not change, THEN the
  population will not evolve.
• Hardy-Weinberg Equation
• p2 2pq q2 1
• a. The population is made of homozygous
  dominant genotypes (p2) heterozygous genotypes
  (2pq) homozygous recessive genotypes (q2).
• b. The sum of the frequencies must always equal
  the entire population (100).

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• c. Example If 10 of the population exhibits
  attached earlobes (homozygous recessive
  phenotype ff), then ___ of the population is FF
  or Ff and exhibits the ________ phenotype (____
  earlobes).

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dominant
free
Solution Dominant Recessive p
q 1 (free) (attached
earlobes) 1 p 10
100 p 100 - 10 p
90
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• 5. Five conditions necessary for Hardy-Weinberg
  Equilibrium
• NOTE Hardy-Weinberg equilibrium rarely exists
  in natural populations but understanding the
  assumptions behind it gives us a basis for
  understanding how populations evolve.

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Conditions necessary for Hardy-Weinberg
Equilibrium

• a. The population is very large.
• b. The population is isolated (no migration of
  individuals, or alleles, into or out of the
  population).
• c. Mutations do not alter the gene pool.
• d. Mating is random.
• e. All individuals are equal in reproductive
  success (no natural selection).

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IV. Agents of Change
Agent Example
1. Mutation Alteration in an organisms DNA. Ultimate source of variation.
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Sickle Cell Mutation
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IV. Agents of Change
Agent Example
2. Gene Flow The movement of alleles from one population to another. Occurs when individuals move between populations.
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IV. Agents of Change
Agent Example
3. Genetic Drift The CHANCE alteration of gene frequencies in a small population. Can occur when populations are reduced in size (genetic bottleneck) or when a few individuals start a new population (founder effect).
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IV. Agents of Change
Agent Example
4. Nonrandom Mating Occurs when one member of a population is not equally likely to mate with any other member.
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Queen Victoria Hemophilia
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IV. Agents of Change
Evidence Example
Natural Selection Some individuals will be more successful than others in surviving and reproducing. Certain traits give them a better fit with the environment.
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Small Tree Finch
Large Ground Finch
Woodpecker Finch
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16-3 The Process of Speciation

• I. How do we get new species?
• A. What is a Species?
• 1. Species
• This means that the individuals of the same
  species share a common gene pool.

a group of interbreeding organisms that
breed with one another and produce fertile
offspring.
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Diversity in Humans
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• 2. If a beneficial genetic change occurs in one
  individual, then that gene can be spread through
  the population as that individual and its
  offspring reproduce.

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• B. Isolating Mechanisms (Leads to a new
  species!)
• Reproductive Isolation members of two
  populations cannot interbreed and produce fertile
  offspring.

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Reproductive Barriers
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• PRE-Mating Reproductive Isolation involves
  mechanisms which do not allow mating to occur in
  the first place.
• 1. Behavioral Isolation Members of two
  populations are capable of interbreeding but have
  differences in mating displays or courtship
  rituals.
• a.
• b.
• c.

specific scents (pheromones of insects).
color patterns/strutting.
specific sounds or calls.
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Courtship Dance
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Different Mating Songs
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• 2. Geographic/Ecological Isolation Two
  populations are separated by geographic barriers
  such as rivers, mountains, or bodies of water.

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When has speciation occurred?
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• 3. Temporal Isolation Two or more species live
  in the same habitat but have different
  mating/reproductive seasons.
• a. Brown trout and Rainbow trout are found in
  the same streams but Rainbow trout spawn in the
  Spring and Brown trout spawn in the Fall.
• b. Three similar species of orchid living in
  the same tropical habitat each release pollen
  on different days therefore, they cannot
  pollinate one another.

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Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
produced by
produced by
produced by
which result in
Independentlyevolving populations
which result in
Formation ofnew species
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• NOTE Several isolating mechanisms can compound
  one another to insure mating doesnt occur. This
  permits two species to occupy the same valuable
  habitat and prevents wastage of valuable gametes.

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• POST-Mating Reproductive Isolation
  (fertilization occurred and zygote formed)
• 1. Hybrid inviability hybrid zygotes fail to
  develop or fail to reach sexual maturity.
• 2. Hybrid sterility Hybrids fail to produce
  functional gametes.
• Example horse x donkey gt mule (sterile).

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Hybrid Sterility

Donkey
Horse

Mule (sterile)
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II. Testing Natural Selection in Nature

• A. Peter and Rosemary Grant from Princeton
  University worked to band and measure finches on
  the Galapagos Islands for over twenty years. By
  documenting natural selection in the wild, the
  Grants provided evidence of the process of
  evolution.

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• B. Grants realized Darwins hypothesis relied on
  two TESTABLE assumptions
• 1. Variation must occur in beak size and shape
• 2. Natural Selection takes place. a.
  Different finches compete and eat different
  food.
• b. During the rainy season, there is plenty
  of food.

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• c. During dry-season drought, some foods become
  scarce forcing birds to become feeding
  specialists.
• d. Each species selects the type of food its
  beak handles best. Example Birds with big,
  heavy beaks can crack open big, thick seeds that
  no other birds can open.
• e. Grants observed that average beak size in
  that population increased dramatically over time.
  This is an example of directional selection.

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III. Speciation in Darwins Finches

• Speciation in the Galapagos finches occurred by
  the following events
• A. Founders Arrive A few finches from the South
  American mainland (species A) flew to the
  Galapagos Islands.
• B. Geographic Isolation Some birds from species
  A crossed to another island in the Galapagos
  group.

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• C. Changes in the Gene Pool Over time,
  populations on each island became adapted to
  their local environments causing a separate
  species to form. On the second island, the
  larger seeds would favor individuals with larger,
  heavier beaks thus forming species B.
• D. Reproductive Isolation If birds from the
  second island cross back to the first island and
  mating does not occur between the two
  populations, then reproductive isolation has
  occurred. The two populations have become
  separate species.

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• E. Ecological Competition As these two species
  compete on the first island, the more specialized
  birds have less competition. During a dry
  season, individuals that are most different from
  each other have the highest fitness. Over time,
  species evolve in a way that increases the
  differences between them. The species-B birds on
  the first island may evolve into a new species,
  C.
• F. Continued Evolution Isolation on different
  islands, genetic change, and reproductive
  isolation led to 13 different finch species found
  there today.

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Finch Species
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IV. Studying Evolution Since Darwin

• A. Data from genetics, physics, biochemistry,
  geology and biology supports the theory that
  living species descended with modification from
  common ancestors that lived in the ancient past.

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B. Unanswered Questions

• 1. No scientist suggests that all evolutionary
  processes are fully understood.
• 2. Why is understanding evolution important?
• a. Drug resistance in bacteria and viruses.
• b. Pesticide resistance in insects.
• c. Evolutionary theory helps us respond to
• these changes to improve human life.