Evolution of Populations:

1
Evolution of Populations

• Georgia Performance Standards
• SB5b Explain the history of life in terms of
  biodiversity, ancestry, and the rates of
  evolution.
• SB5d Relate natural selection to changes in
  organisms.

• Essential Questions
• Why is important to understand evolutionary
  theory?
• What is the role of natural selection in
  speciation?
• Why are there species alive now that were not
  found in the past fossil record?
• How does fossil and biochemical evidence support
  the evolutionary theory?

2
Process of Speciation

• Factors such as natural selection and chance
  events can change the relative frequencies of
  alleles in a population.
• But how do these changes lead to the formation of
  new species, or speciation?

3
Process of Speciation

• A species as a group of organisms that breed with
  one another and produce fertile offspring.
• They 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.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Mechanisms of Evolution

• Hardy-Weinberg principle states that when allelic
  frequencies remain constant, a population is in
  genetic equilibrium.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• This equation allows us to determine the
  equilibrium frequency of each genotype in the
  population.

• Homozygous dominant (p2)

• Heterozygous (2pq)

• Homozygous recessive (q2)

6
Hardy-Weinberg Equation

• Used to calculate the frequency of alleles
• p2 2pq q2 1
• Frequency of WW Frequency of Ww Frequency of
  ww 1
• The combined frequencies of all alleles must be
  100

7
Five conditions are required for Hardy-Weinberg
equilibrium

• Evolution v/s Equilibrium
• Five conditions are required to maintain genetic
  equilibrium from generation to generation

1. The population is very large
2. The population is isolated
3. Mutations do not alter the gene pool
4. Mating is random
5. All individuals are equal in reproductive success

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
9
Hardy-Weinberg principle

• The Hardy-Weinberg principle states that allele
  frequencies in a population will remain constant
  unless one or more factors cause those
  frequencies to change.
• The situation in which allele frequencies remain
  constant is called genetic equilibrium
  (juh-net-ik ee-kwih-lib-ree-um).
• If the allele frequencies do not change, the
  population will not evolve.

10
Checkpoint Questions

• Describe how natural selection can affect traits
  controlled by single genes.
• Describe three patterns of natural selection on
  polygenic traits. Which one leads to two distinct
  phenotypes?
•  How does genetic drift lead to a change in a
  populations gene pool?
•  What is the Hardy-Weinberg principle?
• How are directional selection and disruptive
  selection similar? How are they different?

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Microevolution

• When the relative frequencies of alleles in a
  population change over a number of generations,
  evolution is occurring on its smallest scale
  (microevolution)

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Genes and Variation

• Genetics, molecular biology, and evolutionary
  theory work together to explain how inheritable
  variation appears and how natural selection
  operates on that variation

13
There are several potential causes of
microevolution

• Genetic drift is a change in a gene pool due to
  chance
• Genetic drift can cause the bottleneck effect
• Gene flow can change a gene pool due to the
  movement of genes into or out of a population
• Mutation changes alleles
• Nonrandom mating
• Natural selection leads to differential
  reproductive success

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Genetic Drift

• A change in the allelic frequencies in a
  population that is due to chance

• In smaller populations, the effects of genetic
  drift become more pronounced, and the chance of
  losing an allele becomes greater.

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Genetic Drift

• Natural selection is not the only source of
  evolutionary change.
• The smaller a population is, the farther the
  results may be from what the laws of probability
  predict. This kind of random change in allele
  frequency is called genetic drift.
• How does genetic drift take place?
• In small populations, individuals that carry a
  particular allele may leave more descendants than
  other individuals do, just by chance.
• Over time, a series of chance occurrences of this
  type can cause an allele to become common in a
  population.

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Genetic Drift
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Founder Effect

• Occurs when a small sample of a population
  settles in a location separated from the rest of
  the population

• Alleles that were uncommon in the original
  population might be common in the new population.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Bottleneck

• Occurs when a population declines to a very low
  number and then rebounds

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Gene Flow

• Increases genetic variation within a population
  and reduces differences between populations

Nonrandom Mating

• Promotes inbreeding and could lead to a change in
  allelic proportions favoring individuals that are
  homozygous for particular traits

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Gene Pools

• All members of a population can interbreed, they
  share a common group of genes, called a gene
  pool.
• A gene pool is the combined genetic information
  of all the members of a particular population.
• Typically contains two or more allelesor forms
  of a certain genefor each inheritable trait.
• The relative frequency of an allele is the number
  of times that allele occurs in a gene pool
  compared with the number of times other alleles
  occur.

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Relative Frequencies of Alleles
Section 16-1
Frequency of Alleles
Sample Population
allele for brown fur
allele for black fur
48 heterozygous black
16 homozygous black
36 homozygous brown
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Sources of Genetic Variation

• The two main sources of genetic variation are
  mutations and the genetic shuffling that results
  from sexual reproduction.
• Sexual reproduction can thus produce many
  different phenotypes, but this does not change
  the relative frequency of alleles in a
  population. (Card deck analogy)

23
Evolution as Genetic Change 

• Natural selection on single-gene traits can lead
  to changes in allele frequencies and, thus, to
  evolution.
• Ex Color Mutations

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Single-Gene and Polygenic Traits

• Inheritable variation can be expressed in a
  variety of ways.    
• The number of phenotypes produced for a given
  trait depends on how many genes control the trait

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Single-gene trait

• Trait controlled by a single gene
• Variation in this gene leads to only two distinct
  phenotypes
• The number of phenotypes a given trait has is
  determined by how many genes control the trait.

In humans, having a widows peak or not having a
widows peak is controlled by a single gene with
two alleles. As a result, only two phenotypes are
possible.  
26
Polygenic Traits

• Most traits are controlled by two or more genes
  and are, therefore, called polygenic traits.
• Each gene of a polygenic trait often has two or
  more alleles.
• As a result, one polygenic trait can have many
  possible genotypes and even more possible
  phenotypes.

EX height (A bell-shaped curve is also called a
normal distribution)
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Checkpoint Questions

•  What two processes can lead to inherited
  variation in populations?
• How does the range of phenotypes differ between
  single-gene traits and polygenic traits?
• What is a gene pool? How are allele frequencies
  related to gene pools?
• How could you distinguish between a species in
  which there is a lot of variation and two
  separate species?

28
Evolution as Genetic Change 

• determines which alleles are passed from one
  generation to the next.
• can change the relative frequencies of alleles in
  a population over time.

• Natural selection
• does not act directly on genes, but on
  phenotypes.
• affects which individuals having different
  phenotypes survive and reproduce and which do
  not.

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Exactly what factors change the relative
frequencies of alleles in a population?

• In genetic terms, any factor that causes alleles
  to be added to or removed from a population will
  change the relative frequencies of alleles.
• Evolution is any change in the relative
  frequencies of alleles in a populations gene
  pool.
• Evolution acts on populations, not on
  individuals.

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Natural Selection on Single-Gene Traits 

•  Natural selection on single-gene traits can lead
  to changes in allele frequencies and, thus, to
  evolution.
• EX Color Mutations (organisms of one color may
  produce fewer offspring than organisms of another
  color.

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Evolution as Genetic Change

• Natural Selection on Polygenic Traits
• Fitness varies in polygenic traits.
• Where fitness varies, natural selection can act.
     

• Natural selection can affect the distributions of
  phenotypes in any of three ways
• directional selection
• stabilizing selection
• disruptive selection.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Natural Selection

• Acts to select the individuals that are best
  adapted for survival and reproduction

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Stabilizing selection operates to eliminate
  extreme expressions of a trait when the average
  expression leads to higher fitness.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Directional selection makes an organism more fit.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Disruptive selection is a process that splits a
  population into two groups.

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Sexual selection operates in populations where
  males and females differ significantly in
  appearance.

• Qualities of sexual attractiveness appear to be
  the opposite of qualities that might enhance
  survival.

Natural Selection
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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Prezygotic isolation prevents reproduction by
  making fertilization unlikely.

• Prevents genotypes from entering a populations
  gene pool through geographic, ecological,
  behavioral, or other differences

Eastern meadowlark and Western meadowlark
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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Prevents offspring survival or reproduction

Liger
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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Allopatric Speciation

• A physical barrier divides one population into
  two or more populations.

Abert squirrel
Kaibab squirrel
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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Sympatric Speciation

• A species evolves into a new species without a
  physical barrier.

• The ancestor species and the new species live
  side by side during the speciation process.

41
Isolating Mechanisms

• What happens to a gene pool as one species
  evolves into one or more species?    
• As new species evolve, populations become
  reproductively isolated from each other.
• When the members of two populations cannot
  interbreed and produce fertile offspring,
  reproductive isolation has occurred.
• At that point, the populations have separate gene
  pools.

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Reproductive Isolation

• Develops in a variety of ways
• behavioral isolation occurs when two
  populations are capable of interbreeding but have
  differences in courtship rituals or other types
  of behavior.
• geographic isolation two populations are
  separated by geographic barriers such as rivers,
  mountains, or bodies of water.
• do not guarantee the formation of new species
• temporal isolation two or more species
  reproduce at different times.

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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
44
Section 17-4
Species
that are
in
under
under
form
in
in
can undergo
can undergo
can undergo
can undergo
can undergo
Go to Section
45
Checkpoint Questions

• How is reproductive isolation related to the
  formation of new species?
• What type of isolating mechanism was important in
  the formation of different Galápagos finch
  species?
• 3. Explain how behavior can play a role in the
  evolution of species.
• 4. Leopard frogs and tree frogs share the same
  habitat. Leopard frogs mate in April tree frogs
  mate in June. How are these species isolated from
  each other?