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How Species Form: Speciation

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How do new species form?

  • At what point can we say a new species has been made?

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What are “species”?

  • Recall that it can be difficult to define species
  • Grey area between species due to evolution
  • Biological Species:
    • A reproductively compatible population
      • ie. They can interbreed under natural conditions

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This definition has limitations!

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Eg. asexual organisms

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Remember hybrids?�

Horses and donkeys have different chromosome numbers… �this ultimately makes mules infertile

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Just ONE Ingredient needed for Speciation

REPRODUCTIVE ISOLATION

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The Hawaiian Archipelago

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Reproductive Isolation

  • Once organisms are sufficiently different or separated that they can no longer breed, then they no longer share their gene pool

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  • This means their allele frequencies will change independently of each other

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Two Paths of Speciation�

(1) Transformation / Anagenesis

      • One species gradually takes on new characteristics over time�

(2) Divergence / Cladogenesis

      • Promotes biological diversity
      • One species branches off into two or more different species

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Modes of Speciation

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Transformation�caused by directional selection

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Divergence�Caused by disruptive selection or�other means of reproductive isolation

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There are approximately 6500 languages spoken on Earth today

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Reproductive Barriers

  • Geographic Barriers
    • Egs. rivers, oceans, mountains�
  • Biological Barriers
    • Prezygotic and Postzygotic

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Prezygotic

Postzygotic

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Pre-zygotic Barriers: prevent fertilization

  1. Behavioural Isolation
    • Courtship displays
    • Other mating cues
      • egs. �specific song by bird species�pheromones: chemical signals / scents��

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1. Behavioural Isolation

Female “Greater Bird of Paradise”

Male “Blue Bird of Paradise”

???

Male “Red Plumed Bird of Paradise”

Male “Greater Bird of Paradise”

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Pre-zygotic Barriers: prevent fertilization

2. Temporal Isolation

    • Populations breed at different times of the day, seasons, or year

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Temporal Isolation

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Pre-zygotic Barriers: prevent fertilization

3. Ecological Isolation

    • Similar species, living in different habitats within an ecosystem

eg. garter snakes: �T. siratalis lives near water�T. ordinoides lives in open fields and meadows

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Ecological Isolation

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Pre-zygotic Barriers: prevent fertilization

4. Mechanical Isolation

    • Anatomical incompatibility prevents mating

�eg. �flowers are adapted to specific pollinators

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Mechanical Isolation

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Pre-zygotic Barriers: prevent fertilization

5. Gametic Isolation

    • Gametes of different species cannot recognize eggs from other species

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Gametic Isolation

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Post-Zygotic Barriers : after fertilization

  • prevent offspring from developing into a viable, fertile hybrid
    1. Zygotic mortality
      • Genetic incompatibility stops development of the embryonic hybrid�
    2. Hybrid infertility
      • Results in sterile offspring e.g. mule�
    3. Hybrid inviability
      • 1st generation of hybrids are viable and fertile, but if they mate with another hybrid or parent species the offspring are sterile or weak

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Modes of Speciation

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Sympatric Speciation?

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Parapatric Speciation

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Parapatric Speciation

  • distinct species with a contact zone between populations where some hybridization may occur.
  • They remain distinct due to sexual selection and different selection pressures

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Wildlife Corridors

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