The hierarchical nature and processes of different levels of ecological systems: now we focus on the population

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The hierarchical nature and processes of
different levels of ecological systems now we
focus on the population
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This lecture parts of Ch 16/26 Population
Evolutionary Unit
Population Ecology
Genetics
Evolution

• Darwins finches

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Objectives

• Understand (micro)evolution and its relation to
  genetics
• Sources of genetic variation
• Forces causing change in gene frequency in pop
• Natural selection
• Small population size
• Assortative mating
• Gene flow

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A pre-test on phenotypic variation, genetic
variation, natural selection, fitness, evolution,
adaptation (pre-adapt)
Cyanide experienced by an insec tpopulation
Non-Cyan- resistant
Cyan-resistant individuals
Use these words to explain evolution of cyanide
resistance.
TIME
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• What is definition of (micro)evolution?
• Change in allele frequency in a
• population through time
• What must be present for natural selection to
  cause evolutionary change?
• Genetic (allele) variation
• What is the ultimate source of genetic variation?
• Mutation, a change in nucleotide in DNA
• ---gt change in amino acid it specifies
• ---gt change in phenotype of organism

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Genetic variation is also produced by chromosome
recombination during 1)meiosis and by 2)
fertilization.
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Does this slide show phenotypic or genotypic
variation? What underlies different patterns of
coloration (different phenotypes) in the
population?
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How much genetic variation exists? Why is
genetic variation important?

• In changing environments, the reservoir of
  genetic variation may take on positive survival
  value.
• Rapid environmental change by humans may exceed
  the capacity of a population to respond by
  evolution --gt extinction

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Are most mutations beneficial? Are most
mutations dominant? What happens to harmful
mutations?

• Most mutations are harmful and recessive natural
  selection weeds out most deleterious alleles,
  leaving those that best suit organisms to their
  environments.
• Mutations are likely to be beneficial when the
  relationship of the organism to its environment
  changes organism is pre-adapted to change.
• Selection for beneficial mutations is the basis
  for evolutionary change, enabling organisms to
  exploit new environmental conditions.

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What forces can cause change in genotype
frequency?

• 1) Natural selection --gt differentiates
  subpopulations
• 2) Effects of small population size
• a) Genetic drift
• b) Founder effect
• c) Population bottlenecks
• 3) Assortative (non-random) mating
• 4) Gene flow ( dispersal/migration) --gt
  homogenizes subpopulations

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Evolution by natural selectionFinches beak size
changes in response to change in seed hardness in
drought years.
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Natural selection

• change in the frequency of traits in a population
  because of differential survival and reproduction
  of individuals with those traits.
• Individuals with the most offspring are selected
  and the proportion of their genes increases over
  time.
• Fitness the genetic contribution by an
  individual to future generations.
• Relative fitness Maximum 1 most fit

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Types of natural selection

• Intermediates Extremes
• most fit most fit
• Two extremes
• favored at same time

• Most common Removes
  Uncommon
• in unchanging genetic Retains
  variation
• environments variation
• Removes variation

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Genetic drift
5 of 10 plants 2 of 10 plants leave
offspring leave offspring

• Generation 1 Generation 2 Generation 3p
  p .7, q .3 p .5, q
  .5 p 1.0, q 0
• Change in allele frequency due to random
  variation in births and deaths.

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Population Bottleneck period of small pop. size.
subject to genetic drift
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Population bottlenecks often result in reduced or
no genetic variation (e.g. cheetahs).
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Small populations experience genetic drift,
founder events, and population bottlenecks. Each
causes a loss in genetic variation.
genetic drift

• Allele
• becomes
• fixed no
• variation.

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Summarize the results. What is the potential
consequence for small populations?
polymorphic genes
Population size
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• Assortative mating when individuals choose mates
  non-randomly with respect to their own genotypes.
• Negative mates differ genetically --gt increases
  proportion of heterozygotes
• Most individuals do this to avoid inbreeding.
• Positive like mating with like (includes
  inbreeding) --gt increases proportion of
  homozygotes

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Positive assortment

• increases the proportion of homozygotes
• unmasks deleterious recessive alleles --gt
  inbreeding depression (decrease in fitness)

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homozygosity depends on level of inbreeding.
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Gene flow (migration)--gt mixes alleles between
subpopulations and homogenizes them.

• Maintains genetic variation
• What represents gene flow in
  animals?
• plants?
• Animals dispersal of the adult animal or gametes
• Plants dispersal of pollen and seeds

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Summary of forces

• Remove genetic variation
• Natural selection
• Small population size
• Maintain genetic variation
• Mutation
• Gene flow
• Varying selective pressures in
• time and space

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Genetic structure (differentiation) of
populations is determined by ecological factors,
e.g. heavy metals from mines causing natural
selection (foreground).
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Sample exam question

• In the previous picture the plants in the
  foreground are tolerant of heavy metals those in
  the background are not. Use the 7 words to
  define on the lecture outline to write a
  scenario whereby the original plant population
  got subdivided into two - as the one in the
  foreground evolved tolerance to heavy metals.

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Sample exam question.A species of scale
insects extracts fluids from branches of pine
trees. They have very limited movement. In an
experiment, these insects were transplanted 1)
between branches of the same pine tree, and 2)
from one pine tree to another pine tree of the
same size.

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• State the hypothesis/prediction that was being
• tested as an Ifthen
• Summarize the results in one concise sentence.
• Do the results support the hypothesis?
• Predict whether gene flow or natural selection
  would be a more powerful force affecting the
  genetic structure of this insect. Explain your
  choice.
• 5. Predict whether the genetic makeup of
  populations of the insect on adjacent trees would
  be homogeneous or differentiated. Explain your
  choice.

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Sample exam questionacclimation3 species
grown in both hot moderate tempthen PS rate
of both groups of plants was measured at a range
of temperatures.
Red raised in hot T Blue raised in moderate T
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• What is the major question being addressed in
  this experiment?
• 2. Describe how Larrea and Tiderstromia
  responded relative to the temperatures at which
  it was grown.
• 3. What is the likely mean temperature (high or
  moderate) of Larrea and Tiderstromia?
• 4. What is the likely temperature range during
  the year (high or low) of Larrea and
  Tiderstromia? Explain.
• 5. What is the major conclusion of the
  experiment?