Speciation

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Speciation

• The How and Why of Species

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What is a Species?
A species is one or more populations of organisms
with the potential to interbreed with one another
but NOT with members of other such groups
naturally.
But What about the liger? Well get there. I
promise!
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Determining Separate Species
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Biological Species Concept

• Proposed by Ernst Mayr who said
• Species are groups of interbreeding natural
  populations that are reproductively isolated from
  other such groups."
• Explains why the members of a species resemble
  one another and differ from other species.
• Breeding organisms pass genes to offspring
• By contrast, genes are not transferred to other
  species, and different species therefore look
  different
• Has some fallacies asexual organisms, hybrids,
  ring species, chronospecies

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What Leads to New Species?
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What leads to New Species?

• Diversifying or Directional Selection can lead to
  new species.
• More on this later!
• When differences between subpopulations become
  large enough that gene flow between them may stop.

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Reproductive Isolating Mechanisms
Prezygotic Barriers
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Reproductive Isolating Mechanisms

• Prezygotic Barriers
• Geographic Isolation
• Ecological Isolation.
• Temporal(Time) Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gametic Isolation

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Reproductive Isolating Mechanisms

• Postzygotic Barriers
• Reduced Hybrid Viability
• Reduced Hybrid Fertility
• Hybrid Breakdown

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Modes of Speciation
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Allopatric speciation of squirrels in the Grand
Canyon
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Allopatric Speciation

• New species arise as a result of geographic
  isolation
• Rivers change course, mountains rise, continents
  drift, organisms migrate, and what was once a
  continuous population is divided into two or more
  smaller populations
• Allopatric means different homelands

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Sympatric Speciation
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Sympatric Speciation
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Sympatric Speciation

• Two subpopulations become reproductively isolated
  within the same geographic area.
• First proposed by Darwin in the 1850s.

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Parapatric Speciation
Images from Evolution Berkeley
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Parapatric Speciation

• No specific extrinsic barrier to gene flow
• Continuous population exists but the population
  does not mate randomly
• Individuals are more likely to mate with their
  geographic neighbors than with individuals in a
  different part of the populations range
• Divergence may happen because of reduced gene
  flow within the population and varying selection
  pressures across the populations range

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Competitive Exclusion
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Competitive Exclusion

• Also known as Gauses Law
• Two species that compete for the exact same
  resources cannot stably coexist.
• As a result, competing related species often
  evolve distinguishing characteristics in areas
  where they both coexist

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Rate of Speciation
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Rate of Speciation

• Often can take millions of years, but can
  occasionally occur faster.
• Banana trees ? moth species
• Gradualism
• Punctuated equilibrium

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And Now, Those Hybrids
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And Now, Those Hybrids

• Ligers hybrid between lions and tigers
• Zebroids hybrid between horses and zebras
• Cama hybrid between a camel and a llama
  (artificial insemination)
• Wolphin - bottlenose dolphin and a false killer
  whale
• Remember, most of these hybrids are sterile and
  cannot reproduce with each other.