The Origin of Species

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The Origin of Species
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• Two basic patterns of evolutionary change can be
  distinguished
• Anagenesis
• Cladogenesis

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Origin of Species

• How do we define species?
• A population of organisms that produces viable
  fertile offspring in nature
• When does this definition fall apart?
• Asexual, extinct and blurred organisms
• What definition is used in these cases?
• Morphospecies concept

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• What is the driving factor behind speciation
• (what event must happen to produce a new
• species)?
• Reproductive isolation
• Pre-zygotic
• Temporal isolation
• Behavioral/sexual isolation
• Mechanical isolation
• Gametic isolation
• Post-zygotic
• Hybrid inviability
• Hybrids may not be able to reproduce due to
  differences in courtship behaviors, sterility
  (chromosomes may be incompatible with meiosis)
• Hybrid breakdown

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Pre-Zygotic Barriers
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Post-Zygotic Barriers
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• Which type of reproductive isolation mechanism is
  at work?
• Firefly signals with specific flashes to attract
  mates
• Male dragonfly has appendages that clasps female
  during mating
• Brown trout breed in the fall and rainbow trout
  in the spring
• 1 type of garter snakes lives in the water and
  the other lives on land

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What sort of reproductive barrier is this?
Prezygotic
Has reproductive isolation occurred?
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• Horse and donkey produce a sterile mule

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Biogeography of Speciation

• What is the difference between
• allopatric and sympatric speciation?
• Allopatric - involves a geographical barrier
  between 2 groups
• Sympatric - the result of a genetic isolation
  without a geographical barrier

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Conditions for Allopatry

• Long physical isolation and different selective
  pressures
• Populations of a species become separated
  geographically and undergo genetic drift
  natural selection
• Genetic drift continues to change gene pool
• The fittest traits among the new group survive

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• Allopatric speciation may occur relatively
  rapidly
• Kaibab squirrel is an example of allopatric
  speciation in progress
• Separated by the Grand Canyon
• Show marked phenotypic differences

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Ring Species

• Examples of various stages in the gradual
  divergence of new species from common ancestors
• Pops are distributed around some geographic
  barrier
• Pops that have diverged the most meet where the
  ring closes
• Some populations can interbreed, others cant

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Ring Species
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Adaptive Radiation

• Islands are laboratories of speciation
• Adaptive radiation is the evolution of diversely
  adapted species from a common ancestor upon
  introduction to new environmental opportunities
• Archipelagos are the often the home of adaptive
  radiations

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Hawaiian Archipelago
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Sympatric Speciation

• Genetic alterations result in a reproductive
  barrier
• Can occur in a single generation
• More frequently seen in plants
• Nondisjunction and selfing leads to polyploidy
• Autopolyploid
• Alloployploid
• Evolution of wheat

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Sympatric Speciation in Animals
normal light
monochromatic light

• Cichlid fish - resulted from sexual selection
• Genetic divergence between species is likely to
  be small, suggesting that speciation in nature
  has occurred relatively recently

200 species of cichlid fishes in Lake Victoria,
Africa!
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Genetic Mechanisms of Speciation

• Adaptive Divergence
• Reproductive barriers evolve as secondary result
  of divergence
• Barriers evolve to enhance reproduction within a
  group - not to eliminate reproduction b/t groups
• Barriers occur as a side effect of the
  accumulated adaptive divergences

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Tempo of Speciation

• Gradualism
• Punctuated Equilibrium

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From Speciation to Macroevolution

• Speciation the boundary between micro and
  macroevolution
• Cumulative change over vast amounts of time
  accounts for macroevoulution
• How do evolutionary novelties evolve?

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Eye Evolution
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Origin of Novelties

• How do large scale novelties arise?
• Exaptation - modifications of older structures
• Pandas thumb, stinger of bees, etc
• Gen. changes that lead to dev. changes
• Allometric growth rel. rates of growth differ
• Change one stage - big change
• Alter timing of developmental events, (age of
  sexual activity)
• Alteration in homeotic genes
• Evolution, however, is not goal oriented

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An individual that has more than two chromosome
sets, all derived from a single species
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A species with multiple sets of chromosomes
derived from different species
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• Tetrapod evolution
• Fish Hox gene leads to fin development
• Chicken same Hox gene leads to leg development

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