The Origin of Species

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The Origin of Species

• Chapter 24

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Objectives

• You will define the biological species concept.
• You will define reproductive isolation.
• You will describe two modes of speciation.
• You will define macroevolution.

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Biological species concept

• This defines a species as a population or group
  of populations whose members have the potential
  to interbreed with one another and produce
  viable, fertile offspring, but who cannot produce
  viable, fertile offspring with members of other
  species.
• Simply
• They can interbreed
• They can produce fertile offspring

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Prezygotic barriers- isolating gene pools of
species

• Function before the fertilization of eggs
• Impede mating between species
• Examples
• Habitat Isolation- may live in the same area but
  in different habitats
• Behavioral Isolation- response to mating signals
• Temporal Isolation- breeding during different
  times of day or different seasons
• Mechanical Isolation- attempt to mate but
  anatomically incompatible.
• Gametic Isolation- gametes may meet but dont
  fuse to form zygote

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Postzygotic barriers

• Prevent hybrid zygote from developing into
  fertile adult.
• Examples
• Reduced Hybrid Viability embryo may be aborted
• Reduced Hybrid Fertility organism is sterile
  i.e. the mule (cross between horse and donkey)
• Hybrid Breakdown first generation may be viable
  and fertile but offspring are feeble or sterile.

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Speciation

• The origin of a new species
• Two modes
• Allopatric speciation
• Speciation that takes place in populations with
  geographically separate ranges.
• Gene flow is interrupted or reduced between two
  populations
• Sympatric speciation
• Speciation takes place in geographically
  overlapping populations.
• Chromosomal changes and nonrandom mating reduce
  gene flow.

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How Sympatric Speciation Occurs

• Requires some type of reproductive barrier to
  isolate the gene pool without geographic
  separation.
• Ex. Switch to a different habitat, food source,
  or other resource.
• Ex. Rigid mating preference by females.

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Macroevolution

• This is the origin of new taxonomic groups
• Species, families, even kingdoms
• Level of change evident over the time scale of
  the fossil record.

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