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The Evolution of Populations

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Title: The Evolution of Populations Author: Sally Memminger Last modified by: Sally Memminger Created Date: 9/26/2010 1:39:37 PM Document presentation format – PowerPoint PPT presentation

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Title: The Evolution of Populations


1
The Evolution of Populations
2
Review of Genetic Terminology
  • In Genetic Crosses of two organisms
  • P refers to the parental generation.
  • F1 are the offspring of the parents or the
    first generation.
  • F2 are the offspring of the F1 or the second
    generation.

3
Genetic Terminology continued
  • Alternative forms of genes cause variations in
    inherited characteristics in offspring.
  • The alternative forms are called alleles.
  • Offspring get two alleles for each trait, one
    from each parent.

4
Genetic Terminology Continued
  • If two alleles are different, the dominant allele
    will be fully expressed meaning that is the trait
    that is seen in the offspring.
  • The recessive allele is not seen in the offspring
    if the dominant allele is present.

5
More Genetic terminology
  • Homozygous means organisms have two of the same
    alleles for a particular trait.
  • Example If the dominant allele is
  • T for tall and t for short, then TT
  • is homozygous dominant and tt is
  • homozygous recessive.

6
Genetic terminology continued
  • Heterozygous means organisms have two different
    alleles for a trait.
  • Example using previous alleles
  • Tt
  • This organism would express the dominant trait.
    It is heterozygous dominant.

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8
Finallyfor genetic terminology
  • Phenotype is the physical expression of an
    organisms traits, or what does it look like
    (appearance, chemistry, behavior, etc.).
  • Evolution can only work on phenotype.
  • Genotype is an organisms genetic makeup.

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10
Mutation and Sexual Reproduction produce genetic
variation that makes evolution possible
11
Microelvolutionis evolution on the smallest
scale.
  • Change in allelic frequencies of a population
    over generations.
  • Mutations are the only source of new genes and
    new alleles.

12
Mutations and Genetic Variation
  • Only mutations in gametes can be passed to
    offspring.
  • Point mutations are changes in one DNA base in a
    gene.
  • Chromosomal mutations delete, disrupt, duplicate,
    or rearrange parts of chromosomes and are almost
    always harmful.

13
Sexual Recombination most important source of
genetic variation.
  • Recombination of new alleles that already exist
    in a population.
  • Due to
  • Crossing over during meiosis.
  • Independent Assortment during meiosis.
  • Fertilization

14
Mutations
15
Sources of Genetic Variation
16
Population Genetics
  • Study of how populations change genetically over
    time.
  • Remember Population is a group of the same
    species that live in the same area, interbreed,
    and produce fertile offspring.

17
Gene Pool
  • All of the alleles present for all genes in the
    population.
  • In a diploid organism, each individual has two
    alleles for each gene (trait), and may be either
    homozygous or heterozygous.
  • If all members are homozygous for the same
    allele, the allele is said to be fixed
  • there will be no change.
  • The greater the number of fixed alleles, the
    lower the species diversity.

18
Factors That Can Alter Allelic Frequencies
  • Mutations can alter gene frequency, but they are
    rare.
  • Major Factors that alter allelic frequency are
  • Natural Selection
  • Genetic Drift
  • Gene Flow

19
Natural Selection Affects Allelic Frequencies.
  • Results in alleles being passed to the next
    generation in proportions different from the
    frequencies in the parents generation.
  • Individuals with variations better suited to the
    environment tend to produce more offspring than
    those with variations that are less suited.

20
Genetic Drift Affects Allelic Frequency
  • The unpredictable flucuation in allelic frequency
    from one generation to the next.
  • Affects smaller populations more.
  • Random, non-adaptive change in allelic frequency.
  • Examples of Genetic Drift
  • Founder Effect
  • Bottleneck Effect

21
Founder Effect
  • A few individuals become isolated from a larger
    population.
  • Establish a new population whose gene pool is not
    the same as the original population.

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23
Bottleneck Effect
  • Sudden change in the environment (for example, an
    earthquake, flood, or fire) drastically reduces
    the size of the population.
  • The few survivors that pass through the
    restrictive bottleneck may have a different
    gene pool than the original population.

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25
Gene Flow
  • Occurs when a population gains or loses alleles
    by additions or subtractions from the population.
  • Immigration and Emmigration
  • Tends to reduce differences between populations

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27
Hardy-Weinberg Theorem
  • Describes a population that is not evolving.
  • States that the frequencies of alleles and genes
    will remain the same over generations, unless
    acted on by forces other than segregation and
    recombination.
  • Population is at Hardy-Weinberg Equilibrium.

28
Five Conditions for Hardy-Weinberg Equilibrium
  • No Mutations
  • Random Mating
  • No Natural Selection
  • The population must be extremely large. (No
    Genetic Drift)
  • No Gene Flow

29
Hardy-Weinberg Equation
  • p q 1

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31
Natural Selection only mechanism that
consistantly causes adaptive evolution.
  • Relative Fitness is the contribution an organism
    makes the gene pool of the next generation
    relative to the contribution of other members.
  • Measured by reproductive success.
  • Natural Selection works on phenotype directly and
    genotype indirectly

32
Frequency Distribution of Heritable Traits can
be altered in three ways.
  • Directional Selection
  • Disruptive Selection
  • Stabilizing Selection

33
Directional Selection
  • Individuals with one extreme of a phenotypic
    range are favored, shifting the curve toward that
    extreme.
  • Example Large black bears survived extreme cold
    better than smaller ones.

34
Disruptive Selection
  • Favors individuals on both extremes of a
    phenotypic range rather than individuals with
    intermediate phenotypes.
  • Example A population has individuals with
    either large beaks or small beaks, but few with
    intermediate sizes.

35
Stabilizing Selection
  • Acts against both extreme phenotypes and favors
    intermediate varieties.
  • Birth weights of most humans lie in a narrow
    range, as those babies who are very large or very
    small have higher mortality.

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37
How is genetic variation preserved in a
population?
  • Diploidy Recessive trait may be hidden from
    selection.
  • Heterozygous Advantage Individuals who are
    heterozygous sometimes have a selective advantage
    for survival.

38
Heterozygous Advantage
39
Why doesnt natural selection produce perfect
organisms?
  • Selection can only act on existing variations.
  • Evolution is limited by historical constraints.
  • Adaptations are often compromises.
  • Chance, natural selection and the environment
    interact.
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