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Evolution of Populations:

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Evolution of Populations: Georgia Performance Standards: SB5b: Explain the history of life in terms of biodiversity, ancestry, and the rates of evolution. – PowerPoint PPT presentation

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Title: 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.

4
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.

5
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

8
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?

11
Microevolution
  • When the relative frequencies of alleles in a
    population change over a number of generations,
    evolution is occurring on its smallest scale
    (microevolution)

12
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

14
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.

15
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.

16
Genetic Drift
Sample of Original Population
Descendants
Founding Population A
Founding Population B
17
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.

18
Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Bottleneck
  • Occurs when a population declines to a very low
    number and then rebounds

19
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

20
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.

21
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
22
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

24
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

25
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)
27
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.

29
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.

30
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.

31
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.

32
Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Natural Selection
  • Acts to select the individuals that are best
    adapted for survival and reproduction

33
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.

34
Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
  • Directional selection makes an organism more fit.

35
Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
  • Disruptive selection is a process that splits a
    population into two groups.

36
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
37
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
38
Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
  • Prevents offspring survival or reproduction

Liger
39
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
40
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.

42
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.

43
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?
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