The Origin of Species

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The Origin of Species

• Chapter 23

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Outline

• The Nature of Species
• Pre and Postzygotic Isolating Mechanisms
• Geography of Speciation
• Hawaiian Drosophila
• Darwins Finches
• Lake Victorias Cichlid Fishes
• New Zealand Buttercups
• Diversity of Life Through Time
• Pace of Evolution
• Speciation and Extinction

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The Nature of Species

• Any concept of species must account for
• distinctiveness of species occurring together in
  a single locality
• connection among populations of the same species
  that are geographically separated

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The Nature of Species

• Distinctiveness of sympatric species
• Sympatric species are distinctive entities that
  are phenotypically different, utilize different
  parts of the habitat, and behave separately.
• Two species that occur together and appear to be
  nearly identical are termed sibling species.

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The Nature of Species

• Geographic variation within species
• Populations within a species that occur in
  different areas and are distinctive may be
  classified as subspecies or varieties.
• Biological species concept
• If sympatric species commonly exchange genes,
  their gene pools should become homogenized.

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Geographic Variation - Milk Snake
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Biological Species Concept

• Ernst Mayr
• groups of actually or potentially interbreeding
  natural populations which are reproductively
  isolated from other such groups.
• Individuals that cannot produce fertile offspring
  are termed reproductively isolated, and thus
  members of different species.

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Biological Species Concept

• Application problems
• difficult to apply the concept to populations
  that do not occur together in nature (allopatric
  populations)
• no meaning for asexual species

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Prezygotic Isolating Mechanisms

• Prezygotic isolating mechanisms prevent the
  formation of zygotes.
• Ecological isolation
• Even if two species occur in the same area, they
  may utilize different portions of the environment
  and thus do not hybridize because they do not
  encounter each other.
• lions and tigers in India

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Tiglon
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Prezygotic Isolating Mechanisms

• Behavioral isolation
• Many birds differ in courtship rituals which
  tends to keep species distinct.
• Temporal isolation
• varying breeding seasons
• Mechanical isolation
• reproductive structural differences
• Prevention of gamete fusion
• broadcasting of gametes

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Prezygotic Isolation
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Postzygotic Isolating Mechanisms

• Postzygotic isolating mechanisms prevent the
  proper functioning of zygotes after they form.
• Hybridization often produces embryos that die
  during early development.
• hybrid sterility
• abnormal sex organ development
• chromosome alterations

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Postzygotic Isolation in Leopard Frogs
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Problems with the Biological Species Concept

• Extent of reproductive isolation
• high levels of hybridization
• Difficult to apply to species that do not occur
  together in nature
• Cannot be applied to asexual reproduction
• No single definition may be universally
  applicable.

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Reproductive Isolation and Evolutionary Change

• Most reproductive isolating mechanisms initially
  arise for some reason other than to prevent
  reproduction.
• adaptations for changing environments
• selection may reinforce isolating mechanisms
• Initially incomplete isolating mechanisms are
  reinforced by natural selection until they are
  completely effective.

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Reproductive Isolation and Evolutionary Change

• Role of natural selection in speciation
• Reinforcement is driven by natural selection
  favoring the perfection of reproductive
  isolation.
• Random changes may cause reproductive isolation
• Given long enough periods of time, any two
  isolated populations will diverge due to genetic
  drift.

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Reproductive Isolation and Evolutionary Change

• Adaptation and speciation
• Adaptation and speciation are probably related in
  many cases.
• As species adapt, they accumulate many
  differences that may lead to reproductive
  isolation.

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Geography of Speciation

• Speciation is a two-part process
• identical populations must diverge
• reproductive isolation must evolve to maintain
  the differences
• Allopatric speciation
• Geographically separated populations appear much
  more likely to have evolved substantial
  differences leading to speciation.

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Geographic Isolation
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Geography of Speciation

• Sympatric speciation
• In recent years, a number of cases have appeared
  that appear difficult to interpret in any way
  other than sympatric speciation.

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Sympatric Speciation

• Instantaneous speciation through polyploidy
• individual is born that is reproductively
  isolated from other members of species
• polyploidy - more than 2 sets of chromosomes
• autopolyploidy - all chromosomes arise from a
  single species
• allopolyploidy - two species hybridize
• Disruptive selection

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Rapid Evolution

• Adaptive radiation - process producing a cluster
  of species, occupying a series of similar
  habitats, all evolving from a recent ancestor
• requires both speciation and adaptation to
  different habitats
• Adaptation may be driven by the need to minimize
  competition for available resources with other
  species (character displacement).

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Hawaiian Drosophila

• More than one-third of the worlds species of
  Drosophila occur on the Hawaiian Islands.
• Native Hawaiian flies are closely associated with
  native plants.
• When ancestors first reached the islands, they
  encountered empty niches, facilitating
  diversification and speciation.
• New species evolved as new islands arose.

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Hawaiian Drosophila
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Darwins Finches

• The 14 species of finches Darwin encountered
  comprise four groups, all derived from one
  similar mainland species, and radiated in the
  absence of competition from other birds.
• ground finches
• tree finches
• warbler finches
• vegetarian finches

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Darwins Finches
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Lake Victoria Cichlid Fishes

• Lake Victoria was home to a cluster of over 300
  cichlid species.
• Researches estimate the first cichlid entered the
  lake 200,000 years ago from the Nile.
• Many species probably originated as the lake
  dried down 14,000 years ago, isolating
  populations.
• Diversity was reduced due to competition from
  exotic species.

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Cichlid Fishes of Lake Victoria
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New Zealand Alpine Buttercups

• More species of alpine buttercups (Ranunculus)
  grow on two islands of New Zealand than all of
  North and South America combined.
• research found evolutionary mechanisms behind
  high diversity is recurrent isolation associated
  with recession of glaciers
• repeated invasion with glaciation followed by
  isolation

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New Zealand Alpine Buttercups
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Pace of Evolution

• Punctuated equilibrium
• proposed by Stephen Jay Gould and Niles Eldredge
• Evolution normally proceeds in spurts, with long
  periods of little movement in between.
• contrasted to the theory of gradual evolutionary
  change (gradualism)
• The proposed stasis would be expected in large
  populations experiencing stabilizing selection
  over long periods of time.

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Macroevolution
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Speciation and Extinction Through Time

• There have been 5 major mass extinctions
  interspersed within relatively consistent
  extinction patterns.
• most famous - end of Cretaceous period (65 mya)-
  dinosaurs went extinct
• Mammals quickly experienced evolutionary
  radiation.
• Biological diversity tends to rebound after mass
  extinctions.

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Diversity Through Time
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The Future of Evolution

• Human impacts on the environment will affect the
  evolutionary process in many ways.
• environment alteration
• climate change
• decreased population sizes
• increased genetic drift
• increased extinction

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Summary

• The Nature of Species
• Pre and Postzygotic Isolating Mechanisms
• Geography of Speciation
• Hawaiian Drosophila
• Darwins Finches
• Lake Victorias Cichlid Fishes
• New Zealand Buttercups
• Diversity of Life Through Time
• Pace of Evolution
• Speciation and Extinction

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