Chapter 8 Natural Selection: Empirical studies in the wild

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Chapter 8 Natural Selection Empirical studies in
the wild

• Assigned reading chapter 8.

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Evolution by Natural Selection

• Recall Darwin proposed evolution was the
  inevitable outcome of 4 postulates
• 1. There is variation in populations.
  Individuals within populations differ.
• 2. Variation is heritable.

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Evolution by Natural Selection

• 3. In every generation some organisms are more
  successful at surviving and reproducing than
  other. Differential reproductive success.
• 4. Survival and reproduction are not random, but
  are related to variation among individuals.
  Organisms with best characteristics are
  naturally selected.

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Evolution by Natural Selection

• If 4 postulates are true then the population will
  change from one generation to the next.
• Evolution will occur.

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Evolution by Natural Selection

• Recall -- Darwinian fitness ability of an
  organism to survive and reproduce in its
  environment.
• Fitness measured relative to others of its species

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Evolution by Natural Selection

• Adaptation is a characteristic or trait of an
  organism that increases its fitness relative to
  individuals that do not possess it.

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Natural Selection and coat color in the oldfield
mouse is there variation?

• The oldfield mouse is widely distributed in the
  southeastern U.S. It is preyed upon by a variety
  of visually hunting predators such as hawks and
  owls.
• The mouse displays considerable variation in coat
  color both within and between populations across
  its range.

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Natural Selection and coat color in the oldfield
mouse

• Most populations of the mouse are dark colored,
  but populations on beaches and barrier islands
  have lighter colored coats.
• Hoekstra et al. carried out a series of
  experiments to evaluate the hypothesis that
  natural selection favors a match between coat
  color and background color.

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Is variation in coat color heritable?

• There is lots of phenotypic variation in coat
  color in oldfield mice.
• For natural selection to occur the variation must
  be heritable. Several genes affect coat color in
  these mice.

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Genetics of coat color

• Melanocortin-1 receptor gene (Mc1R). This gene
  produces either a dark pigment (Eumelanin) or a
  light pigment (Phaeomelanin) depending on signals
  it receives from other genes.

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Genetics of coat color

• If a protein called alpha-MSH binds to the Mc1R
  gene then the dark pigment eumelanin is produced.
• If alpha-MSH cannot bind then a light-colored
  pigment phaeomelanin is produced.

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Genetics of coat color

• In populations with many light-colored mice two
  mutations are common
• (1) mutant that prevents alpha-MSH binding to
  Mc1R
• (2) mutant allele that produces an excess of a
  protein called ASP that competes with alpha-MSH
  to bind to the MC1R.
• Both mutant alleles result in light-colored mice.
  Thus there is a clear genetic basis for the
  observed variation in coat color.

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Does variation affect fitness?

• Does coat color affect the survival and
  ultimately reproduction (i.e. fitness) of
  oldfield mice?
• Two experiments suggest it does.

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Does variation affect fitness?

• Kaufman (1974) experiment
• Pairs of mice (one dark-coated, one light coated)
  and an owl were placed in large cages located in
  habitats with different backgrounds (light or
  dark and with different vegetation densities).

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Does variation affect fitness?

• In all cases mice that better matched the
  background survived better than mice that matched
  less well.

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Does variation affect fitness?

• Kaufman et al. made silicone mouse models painted
  light or dark.
• Placed the models in different habitats and
  measured how often the models were attacked.
• Clear differences in attack rates. Models that
  matched their background were attacked much less.

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Natural Selection and coat color in the oldfield
mouse

• Thus for oldfield mice all 4 postulates are
  satisfied. There is (i) variation in coat color
  and it is (ii) heritable.
• There is (iii) differential reproductive success
  (or in this case differential survival which is a
  necessary precursor to reproduction).
• That differential reproductive success is (iv)
  related to the variation (different coat colors
  survive better in different habitats).

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Another example of natural selection Darwins
finches

• Evolution of beak shape in Darwins Finches.
• Peter and Rosemary Grants (and colleagues) work
  on Medium Ground Finches Geospiza fortis
• On Daphne Major since 1973.

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Evolution of beak shape in Darwins Finches.

• Postulate 1. Is the population variable?
• Finches vary in beak length, beak depth, beak
  width, wing length and tail length.

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Evolution of beak shape in Darwins Finches.

• Postulate 2 Is variation among individuals
  heritable?
• Variation can be a result of environmental
  effects.
• Heritability proportion of the variation in a
  trait in a population that is due to variation in
  genes.

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Evolution of beak shape in Darwins Finches.

• Peter Boag compared average beak depth of parents
  with that of their adult offspring.
• Strong relationship between offspring and parent
  beak depths.

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FIG 3.7
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Evolution of beak shape in Darwins Finches.

• Postulate 3 Do individuals differ in their
  success at survival and reproduction?
• 1977 drought 84 of G. fortis individuals died,
  most from starvation. In two other droughts 19
  and 25 of the population died.

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Evolution of beak shape in Darwins Finches.

• Seed densities declined rapidly during drought
  and the small soft seeds were consumed first.
• Average size and hardness of remaining seeds
  increased over the course of the drought.

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FIG 3.8b
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FIG 3.8A
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Fig 3.8c
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Evolution of beak shape in Darwins Finches.

• Postulate 4 Are survival and reproduction
  nonrandom?
• Do those who survive and reproduce have different
  characteristics than those that dont?

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Evolution of beak shape in Darwins Finches.

• As drought progressed small soft seeds
  disappeared and large, hard Tribulus seeds became
  a key food item.
• Only birds with deep, narrow beaks could open
  them.

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Evolution of beak shape in Darwins Finches.

• At end of the 1977 drought the average survivor
  had a deeper beak than the average non-survivor
  and also a larger body size.

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FIG 3.9
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Did the population evolve?

• Chicks hatched in 1978 had deeper beaks on
  average than those hatched in 1976.
• Population evolved.

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Strong association between parent and offspring
beak sizes. Hence narrow-sense heritability is
high.
There is a difference in beak dimensions (selectio
n differential) between breeders and original
population.
Response to selection in that beak dimensions
increased in the offspring.
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Fig 3.10
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Evolution of beak shape in Darwins Finches.

• Variation in weather from year to year on Daphne
  Major over 30 years has led to variation in the
  traits that are favored by selection.
• Population has evolved over time.

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Fig 3.11 A
Over the course of 30 years (1970 to 2000) beak
size evolved. Rose sharply during drought (red
line) then declined to pre-drought dimensions.
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Agents of selection operating in opposite
directions gall flies. Gall flies induce
plants to produce galls in which the larva
develops in a protected environment.
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Gall diameter is variable. Some individuals
produce large galls and others small ones.
Relatives produce similar size galls and there
is heritable variation in gall size.
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Stabilizing selection on gall size

• There are two major predators of larvae in galls
  birds and parasitic wasps.
• Parasitic wasps cannot reach larvae enclosed in
  very large galls, but birds spot large galls more
  easily and consume the larvae. There is thus
  stabilizing selection on gall size with
  intermediate sized galls favored.

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Milk drinking evidence for natural selection

• Milk contains the sugar lactose and young mammals
  produce an enzyme, lactase, to break it down.
  Most humans (about 70) stop producing lactase
  after weaning, but many western Europeans retain
  the ability to digest lactose into adulthood.

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Milk drinking evidence for natural selection

• Humans began to domesticate cattle in NW Europe
  about 10,000 years ago and this new food source
  favored individuals able to digest milk into
  adulthood.
• The frequency of alleles for lactose tolerance
  are highest in NW European populations and lowest
  in SE Europe the in populations furthest from the
  origin of cattle domestication.

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Milk drinking evidence for natural selection

• A similar pattern is found when comparing animal
  herding societies with nearby non-herding
  populations.
• The herders have much higher tolerance for
  lactose than their non-herding neighbors.

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Humans as agents of selection

• Humans act as strong agents of selection.
• This has occurred through deliberate choice
  (artificial selection for desired traits in crops
  and domesticated animals) and inadvertently
  through environmental change.

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Artificial Selection

• Artificial Selection. Humans have selectively
  bred for desirable traits in domestic animals and
  plants for millenia.
• Process has produced our crop plants, garden
  plants, pets, and domestic animals.
• Recall Darwin closely studied pigeon breeding as
  a process analogous to natural selection.

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Artificial Selection

• Cauliflower, broccoli, kale, brussels sprouts all
  descended from wild cabbage.
• All these crops can be crossed and produce
  fertile offspring.
• Cauliflower edible bit is the inflorescence or
  flower stalk.

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Artificial Selection

• Cauliflower has large dense infloresence. This
  results from mutant loss of function alleles of
  two genes that affect flower structure and
  infloresence density.

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Artificial Selection

• Early farmers choosing among their crops selected
  those with largest infloresences. Process has
  resulted in cauliflowers that are homozygous for
  both loss of function alleles.

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Pesticides and herbicides act as agents of
selection
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Resistance to pesticides

• Insects and plants treated with chemicals
  designed to kill them have rapidly developed
  resistance.
• Heavy spraying creates an environment in which
  any mutations that offer resistance are strongly
  selected for and spread rapidly.

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Resistance to pesticides in houseflies
Inverted triangle indicates first occurrence of
resistance and R indicates when most Populations
were resistant. Bar width indicates extent of the
pesticides use.
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Rapid evolution of herbicide resistance
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Resistance to pesticides

• Farmers are now using evolutionary biology to
  reduce rate of evolution of resistance.
• Resistance frequently comes with a cost and in
  pesticide-free environments non-resistant pests
  may have an advantage and outcompete resistant
  forms.

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Resistance to pesticides

• To maintain non-resistant genes in pest
  populations farmers are now setting aside
  pesticide free refuges that are not sprayed.
• For example farmers using BT-corn (corn
  containing a gene that produces a natural
  pesticide) must set aside 20 of their plantings
  as non-BT corn

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Resistance to pesticides

• States in which large areas of refuges were used
  have shown much slower rates of BT-resistance in
  pests than states where smaller areas of refuges
  were set aside.

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Hunting and fishing as agents of selection

• Humans have intensively fished all the worlds
  oceans and that fishing pressure has resulted in
  fish populations evolving in response.
• For example, because under fishing pressure few
  individuals survive to breed late in life, fish
  such as cod today mature much younger and at
  smaller sizes than they did 20 years ago.

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Hunting and fishing as agents of selection

• In a similar fashion selective shooting by trophy
  hunters of males with larger horns has led to the
  evolution of smaller horns in hunted populations.

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Evolution of shorter male horns due to hunting