How organisms evolve: Part 2

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How organisms evolve Part 2
Chapter 15 Sections 15.2 (finish) 15.3
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What causes evolution?3. Small population size

• Genetic drift
• Chance events change allele frequencies
• Much more likely to impact small populations
• Example What if only half the individuals in a
  population, chosen randomly, reproduce?
• What happens to the frequency of a particular
  allele, just by chance?
• Key Examine difference between large vs. small
  population

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Changes in allele A (started at 0.5)

• Remember, for each generation, half the
  population is chosen at random to reproduce!

Based on our definition of evolution, did
evolution occur for either population? If so,
which one?
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What causes evolution?3. Small population size
(cont.)

• Population bottlenecks
• By chance, very few individuals survive the
  particular bottleneck event.

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Population bottlenecks real example

• Northern elephant seal

What was the bottleneck event for this
species? From how many elephant seals is the
current population descended?
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What causes evolution?3. Small population size
(cont.)

• Founder effect
• Isolated colonies founded by a small number of
  organisms
• A subset of the alleles in the original
  population make it to the new population
• Example Ellis-van Creveld syndrome
• Short arms and legs, multiple fingers, heart
  defects.
• Common among the Amish of Lancaster County, PA
• One of the few founders of this colony had this
  syndrome

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What causes evolution?4. Non-random mating

• Assortative mating
• Example snow geese prefer mates with the same
  color plumage

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What causes evolution?4. Non-random mating

• NOTE The examples of male-male competition
  (bighorn sheep) and female choice (peahens and
  peacocks) belong in the section on natural
  selection, not in the section on non-random
  mating!
• These are examples of sexual selection, a type of
  natural selection

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What causes evolution?5. Natural selection!

• Natural selection occurs because not all
  phenotypes are equally beneficial in a particular
  environment
• Individuals with the best-adapted phenotypes
  will, on average, leave the most offspring.
• To the extent that the phenotype was determined
  by genotype, the most beneficial alleles will
  increase in the population.
• Thus, natural selection acts on individuals, but
  causes changes in populations over generations.

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• Traits that are selected often present a
  compromise of various selective pressures.
• Example Length of male giraffes neck

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What causes evolution?5. Natural selection!

• Types of selective pressures
• Abiotic factors
• Example Animals in the intertidal have
  adaptations to keep from drying out.
• This burrowing anemone can withdraw into the sand
  and close up during low tide.

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What causes evolution?5. Natural selection!

• Types of selective pressures
• Competition between organisms
• Example Some coral reef fish compete for algae
  growing on the reef. How are they adapted to do
  so?

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Reef Herbivores Damselfish

• Damselfish farm algae and defend their
  territories.

Dusky damselfish (EcoDives, Key West)
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Reef Herbivores Tang gangs!

• Blue tangs raid damselfish territories in gangs

Photo Karen Haberman
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What causes evolution?5. Natural selection!

• Types of selective pressures
• Predation!
• Example Animals have various methods of dealing
  with their predators.
• Do all animals do this in the same way?

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Chemical defense/toxicity/ warning coloration
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Giant petrel chick gacking behavior

• Photo Karen Haberman

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Cryptic coloration

• Poorwill and frog

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What causes evolution?5. Natural selection!

• Types of selective pressures
• Sexual selection

Female choice
Male-male competition
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What causes evolution?5. Natural selection!

• Selection can influence populations in three ways

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Directional selection

• Selects for one extreme of the characteristic
• Selection of antibiotic resistance in bacteria

Range of particular characteristic
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Stabilizing selection

• Selects for average values of the characteristic
• Often a balance between two selective pressures
• Bright coloration attract mate, but also
  predators
• Too dull ? no predation, but no mate either
• So coloration may be intermediate

Range of particular characteristic
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Disruptive selection

• Selects for extremes, and against individuals
  with intermediate values
• Example Beak size of black-bellied seedcrackers
• Large beaked Feed on hard seeds
• Smaller, pointed beak Feed on soft seeds.
• Intermediate beaks not very good at eitherone.

Range of particular characteristic