Chapter 23 How populations change

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Chapter 23 How populations change
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• Darwin proved that species are products of
  evolution
• But, without a mechanism to explain natural
  selection, there were issues
• Like begets like, but not exactly
• What was missing? Way to explain how chance
  variations can show up in a population, while
  also accounting for precise transmission from
  parent to offspring

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Population Genetics

• Emphasizes the variation within pops and
  recognizes the importance of quantitative
  characters (those characteristics that vary along
  a continuum)
• Modern synthesis ties in ideas from paleontology,
  taxonomy, biogeography, and population genetics
• Scientists contributing Dobzhansky, Wright,
  Mayr, Simpson, Stebbins (page 446)

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• Population localized group of individuals
  belonging to the same species
• Species group of pops whose individuals have
  the potential to interbreed and produce fertile
  offspring
• Gene Pool total aggregate of genes in a pop at
  any one time

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Allele Frequencies

• Allele frequencies the abundance of each kind
  of allele in a population
• To compare, use Hardy-Weinberg equilibrium (equati
  on p2 2pq q2 1)
• p2 frequency of homo dom
• 2pq frequency of hetero
• q2 frequency of homo recess

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Hardy-Weinberg Theorem describes a
non-evolving population

• Must meet 5 conditions for no change in
  frequencies of alleles at a given gene locus
• 1. No gene mutations have occurred.
• 2. Population is very large.
• 3. Population is isolated from other pops of
  the species (no immigration or
  emigration.)
• 4. No natural selection
• 5. Mating is random.

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Hardy-Weinberg example

• The allele for the ability to roll ones tongue
  is dominant (R) over the allele for the lack of
  this ability (r).
• In a population of 500 individuals, 25 show the
  recessive phenotype. How many individuals would
  you expect to be homo dom and heterozy?

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• The equation p2 2pq q2 1
• Therefore, p q 1
• 500 organisms, 25 are rr thus q2 .25
• so 125 organisms are rr
• If q2 .25, the q.5
• Thus, p .5 1, leaves p .5
• So, p2 .25, so 125 organisms are RR
• 2pq leaves the heterozygotes, so
• 2(.5)(.5) .5 or 50, so 250 organisms are Rr

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Hardy-Weinberg equilibrium

• Describes how the Mendelian system has no
  tendency to alter allele frequencies.
• Like a deck of cards no matter how many times
  deck is reshuffled to deal out new hands, the
  deck itself stays the same.

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Causes of Microevolution

• Microevolution generation to generation change
  in a pops allele frequencies
• Two main causes genetic drift and natural
  selection

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Genetic Drift

• A random change in allele frequencies over the
  generations, brought about by chance alone.
• Impact is minor in large pops, but is significant
  in small pops.
• In the absence of other forces, random change in
  allele frequencies leads to the homozygous
  condition and a loss of genetic diversity over
  the generations.(This happens in all pops it
  just happens faster in small ones.)

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Situations that can shrink pops down to small
size.

• 1. BOTTLENECK effect
• When disasters dramatically reduce pop size
  surviving pop is not representative of original
  gene pool, alleles are not equally represented
• Reduces genetically variability b/c at least some
  alleles are likely to be lost from gene pool

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Figure 23.5 The bottleneck effect an analogy
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• 2. FOUNDER effect
• Genetic drift in a new colony
• Occurs when a few individuals from a larger pop
  colonize an isolated area (island, lake, pond,
  etc.)
• The smaller the sample size, the less the genetic
  makeup of the colonists will represent the gene
  pool of the larger pop that they left

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Section 16-2
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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Section 16-2
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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Section 16-2
Sample of Original Population
Descendants
Founding Population A
Founding Population B
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Natural Selection

• Differential success in reproduction
• Results in alleles being passed along to next
  generation in numbers disproportionate to their
  relative frequencies in the present generation
• Only natural selection is likely to ADAPT a
  population to its environment
• Accumulates and maintains favorable genotypes in
  a pop

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Mutation

• Change in DNA
• If can be transmitted in gametes, can change gene
  pool
• Individual loci changes are not going to be seen
  easily, but cumulative impact of mutations on ALL
  loci can be significant if pop is large and orgs
  reproduce a great deal.

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Gene Flow can also change allele frequencies

• Gene flow is the physical flow of alleles into or
  out of a population.
• Immigration alleles coming in (added)
• Emigration alleles moving out (lost)
• Gene flow counteracts differences that arise
  through mutation, natural selection, and genetic
  drift.
• Gene flow helps keep separated pops genetically
  similar reduces differences between pops

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Genetic Variation raw material for Natural
Selection

• Genetic variation occurs within and between
  populations
• many are at molecular level and cannot be
  seen
• not all are heritable some are
  environmentally induced
• (Ex. Map butterflies)

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Figure 23.7 A nonheritable difference within a
population
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Quantitative characters and Discrete characters

• Quantitative characters
• traits that vary along a continuum in a pop
  (like plant height)
• Discrete characters
• traits that can be classified on an either-or
  basis (like flower color)

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Polymorphism

• Morphs when two or more forms of a discrete
  character are represented in a pop, this is what
  the different forms are called
• If two or more distinct morphs are each
  represented in high frequencies, population is
  polymorphic

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Figure 23.0 Shells
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Figure 23x2 Polymorphism
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Measuring Genetic Variation

• Pop Geneticists use whole gene measurements and
  molecular measurements gene diversity and
  nucleotide diversity
• Ex. Fruit flies gene diversity using loci,
• nucleotide diversity using DNA
  fingerprinting
• Note humans have little genetic variation
  compared to other species same nucleotide
  sequence at 999 out of every 1000 nucleotide
  sites in your DNA

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Geographic variation

• Differences in gene pools between pops or
  subgroups of pops
• Due to fact that at least SOME environmental
  factors are likely to differ from one place to
  another thus, natural selection can contribute
  to this.
• Ex. In pop, one type of geographic variation
• Cline -- graded change in trait along a
  geographic axis

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Figure 23.8 Clinal variation in a plant
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Variation in the gene pool.pages 455-457

• Mutation and sexual recombination generate
  genetic variation
• Mutations
• these are random events
• inheritance depends on if mutation occurs in
  an exon or not
• many chromo mutations are lethal
• asexual reproduction produces clones

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• Sexual Recombination
• meiosis shuffles alleles and sex deals them
  out at random
• gametes vary extensively even within one
  organism

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• Diploidy and balanced polymorphism preserve
  variation
• Diploidy
• hides much of genetic variation in form of
  recessives
• Balanced Polymorphism
• ability of natural selection to maintain
  stable frequencies of two or more
  phenotypic forms in a pop
• promoted by heterzygote advantage and
  frequency-dependent selection (456-7)

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Neutral variation

• Confers no selective advantage of some
  individuals over others
• Not affected by natural selection some neutral
  alleles will increase in the gene pool and other
  will decrease by genetic drift
• Hard to say what is neutral detriment can be
  pinpointed, but no effect is harder to pinpoint

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Three effects or modes of selection on a
characteristic

• 1. Directional Selection
• 2. Stabilizing Selection
• 3. Disruptive Selection

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Figure 23.12 Modes of selection
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Directional Selection

• When individuals at one end of the population
  curve have higher fitness than individuals in the
  middle or at either end of the population curve.
• Causes entire curve to move as character trait
  changes
• Ex ?beak size of Galapagos finches
• ?peppered moths
• ? antibiotic resistance

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Figure 23.13 Directional selection for beak size
in a Galápagos population of the medium ground
finch
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Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
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Stabilizing Selection

• When individuals near the center of the curve
  have higher fitness than individuals at either
  end of the curve
• Intermediate forms of a trait are favored and
  alleles that specify extreme forms are eliminated
  from a pop.
• Counteracts the effects of mutation, gene flow,
  and genetic drift preserves the most common
  phenotypes.
• Ex. Weight of human babies at birth

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Stabilizing Selection
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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Disruptive or Diversifying Selection

• When individuals at the upper and lower ends of
  the curve have higher fitness than individuals
  near the middle.
• Forms at both ends of the range of variation are
  favored and intermediate forms are selected
  against selection creates two, distinct
  phenotypes
• Ex. Bird beak size no middle sized seeds,
  only large seeds and small seeds thus, small and
  large beaks are favored

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Figure 23.14 Diversifying selection in a finch
population
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Disruptive Selection
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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Sexual Selection

• Maintained by natural selection
• May lead to pronounced differences between sexes
  sexual dimorphism

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Figure 23.16x1 Sexual selection and the
evolution of male appearance
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Natural Selection cannot fashion perfect orgs

• Limited by historical constraints
• Ex. Body structure for erect posture
• Adaptations are often compromises
• Ex. Seal on land vs. water
• Not all evolution is adaptive
• Ex. Storm blows ALL orgs to new place, not
  just best suited
• Selection can only edit existing variations
• See page 461