Chapter 13 Evolution and Natural Selection

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Chapter 13Evolution and Natural Selection
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Evolution

• Evolution is a change in the frequency of
  genetically determined characteristics within a
  population over time
• Microevolution occurs when there are minor
  differences in allele frequency between
  populations of the same species

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Evolution

• Macroevolution occurs when there are major
  differences that have occurred over long periods
  that have resulted in so much genetic change that
  new kinds of species are produced

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Development of Evolutionary Thought

• For centuries, people believed that the various
  species of plants and animals were unchanged from
  the time of their creation
• Today we know this is not true, but they knew
  nothing of DNA, meiosis, and genetics

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Development of Evolutionary Thought

• In the mid-1700s,Georges-Louis Buffon, a French
  naturalist wondered if animals underwent change
  (evolved) over time.
• Buffon had a student, Jean-Baptiste de Lamark who
  suggested how these changes might occurred. His
  ideas were known as acquired characteristics.

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Development of Evolutionary Thought

• Lamarks theory of acquired characteristics
  states that traits gained during an organisms
  life and transmitted to the offspring.
• For example, giraffes had short necks, but in
  order to reach the leaves, their necks
  stretching. When these giraffes reproduced their
  offspring acquired their parents longer necks.

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Theory of Natural Selection

• Charles Darwin (1809-1882) was the son of a
  physician and member of a family with a history
  scientific curiosity.
• After briefly studying medicine, he enrolled at
  Cambridge University, receiving his B.A. degree
  in 1831.
• After graduating, Darwin left on a multi-year
  expedition (1831-1836) on the HMS Beagle,
  primarily as a social companion to the captain.

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Natural Selection

• The purpose of the trip was to chart poorly known
  coastal waters off South America.
• While the crew were surveying, Darwin went on
  many expeditions to collect plants and animals on
  mainland South America and the islands which the
  ship visited.
• Darwin noted that the temperate species in South
  America looked much more like tropical South
  American species than they did temperate species
  from Europe.

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Natural Selection and Charles Darwin

• When the Beagle arrived at the Galapagos Islands,
  900km west of South America, he observed many new
  species, yet similar to those on the mainland.
• This included 13 species of finches, with
  dramatically different beak morphologies.
• Some were very unfinchlike with bills like
  insect-eating warblers.
• Some Galapagos finch species were found only on
  one island, others had more than one.

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Natural Selection

• After his return to England, Darwin developed his
  ideas on evolution and natural selection by 1844,
  but delayed publishing.
• He spent 20 more years collecting more
  observations to support his theory.
• In 1858, Alfred Wallace, a naturalist working in
  Indonesia, sent Darwin a manuscript with
  essentially the same theory as Darwin
• This prompted Darwin to finish his book, On the
  Origin of Species by Means of Natural Selection
  in 1859.

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Origin of Species

• The first point of the Origin of Species is that
  all organisms are related through descent from
  some unknown ancestor that lived in the past.
• Over time, species changed via adaptations which
  fit them to specific ways of life.
• These ideas, he called descent with modification.

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Origin of Species

• The second point in the Origin of Species is that
  the dominant mechanism for evolution of species
  is natural selection.
• Darwin argued that evolution of species in the
  natural world was analogous to artificial
  selection of domesticated animals and plants.
• Individuals with desired characteristics are
  breed with other individuals with desired
  characteristics.
• This can produce tremendous changes in the
  phenotype of a species in a relatively short
  period of time.

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Origin of Species

• The theory of natural selection is based on the
  following assumptions about the nature of living
  things
• 1. All organisms produce more offspring than
  survive.
• 2. No two organisms are exactly alike.
• 3. Among organisms, there is a constant struggle
  for survival.
• 4. Individuals that possess favorable
  characteristics for their environment have a
  higher rate of survival and produce more
  offspring.
• 5. Favorable characteristics become more common
  in the species, and unfavorable characteristics
  are lost.

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Natural Selection - Giraffes

• Using Darwin-Wallace theory with giraffes
• 1. In each generation more giraffes would be
  born than the food supply could support.
• In each generation, some giraffes would inherit
  longer necks, and some would inherit shorter
  necks.
• All giraffes would compete for the same food
  source.
• Giraffes with longer necks would obtain more
  food, have a higher survival rate, and produce
  some offspring.
• As a result, succeeding generations would show an
  increase in the number of individuals with longer
  necks.

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Natural Selection and Evolution

• The gene pool of a species is modified by
• chance events in small populations (founder
  effect, genetic drift, genetic bottlenecks)
• immigration and emigration
• Mutations
• Natural selection
• As a result, the gene pool in the next generation
  differs from that of the previous generation -
  microevolution.

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Natural Selection Darwins Finches

• Darwin hypothesized that differences in bill-size
  lead to differences in the success of Galapagos
  finches when feeding.
• Detailed research over the last 30 years have
  shown that bill size varies among finches and is
  inheritable (genetically controlled).
• During drought years in the Galapagos, the local
  plants produce fewer seeds than during normal,
  wet years.
• All the finches prefer to eat the smaller, easier
  to handle seeds first but soon they are all gone.
• All that remain are the larger seeds, which are
  harder to open.
• Larger billed birds can open these larger seeds,
  but smaller billed birds cannot.

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Natural Selection Darwins Finches

• Larger billed birds survive drought better and
  leave more offspring - more fit.
• The alleles that favor large bills become more
  common among the next generation.
• The average size of birds and their bills
  increases in the population.
• Larger billed birds are less fit during wet years
  when small seeds are abundant.
• Larger billed birds are less
• efficient when feeding on
• small seeds.

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Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium allows us to
  predict the frequencies of alleles in the
  offspring if we know the frequency of alleles in
  the parental population.
• If a population demonstrates genotype frequencies
  from generation to generation that are consistent
  with Hardy-Weinberg, then it is NOT evolving.

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Hardy-Weinberg Equation

• The Hardy-Weinberg Equation is p2 2pq q2 1
• p2 stands for the frequency of homozygous
  dominant alleles
• 2pq stands for the frequency of heterozygous
  alleles
• q2 stands for the frequency of homozygous
  recessive alleles

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Hardy-Weinberg Equilibrium

• Conditions necessary for gene frequencies to
  remain constant are the following
• 1. Mating must be completely random
• 2. Mutations must not occur
• 3. The migration of individual organisms into
  and out of the population must not occur.
• 4. The population must be very large
• 5. All genes must have an equal chance of being
  passed onto the next generation

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Effects of Natural Selection

• In natural selection, particular phenotypes match
  (or dont) the local environmental conditions.
• Some phenotypes do well, live long, and produce
  abundant offspring (and other dont).
• If these phenotypic characters have a genetic
  basis, individuals that have favored alleles will
  contribute more alleles to the next generation.

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Effects of Natural Selection

• In stabilizing selection, the environment favors
  individuals at the middle of the range of
  phenotypes, reducing phenotypic variation.
• With the range of color possibilities, the ones
  that do not blend into the environment, will be
  more likely to be killed by predators.

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Effects of Natural Selection

• In directional selection, the environment favors
  individuals at the one end of the range of
  phenotypes, shifting phenotypic variation.
• For example, periods of drought favor larger
  individuals among Darwins finches in the
  Galapagos.
• Bigger birds compete better for scarce food,
  survive better, and leave more offspring.
• During wetter periods, the opposite is true.

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Effects of Natural Selection

• In diversifying selection, the environment
  selects against average individuals and favors
  the extremes, increasing phenotypic variation.
• Melanistic moths are either light colored or dark
  colored to match light or dark tree bark.
• Intermediate colors match neither background and
  increase predation by birds.