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Can Different Species Hybridize?

    Reproductive Isolating Mechanisms

    A biological species is defined as a group of similar organisms
    able to interbreed (under natural conditions) to produce fertile, viable offspring.
    Biological species are reproductively isolated from one another.

    Evolution of reproductive isolating mechanisms
    prevents nascent species from interbreeding.

    Isolating mechanisms can operate at two basic levels.

    • Prezygotic Mechanisms prevent formation of viable zygotes.

    • Postzygotic Mechanisms prevent hybrids from passing on their genes.

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I. Pre-Zygotic Reproductive Isolating Mechanisms

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    Ecological Isolation

    Closely related species may inhabit different ecosystems within a region.
    Different habitat preferences lower their probability of mating.

    Example 1: Central California populations of Rana spp.

    • The Red-legged Frog (Rana draytonii)
        Tends to breed in large, permanent ponds.

      • The Yellow-legged Frog (Rana boylii)
          Breeds almost exclusively in fast-moving streams.

      Conservation note:
      • Male R. draytonii prefer large females to small females.
      • Male R. draytonii living in ponds occupied by (introduced, exotic)
        x Bullfrogs (R. catesbeiana) will often amplex with females
        x of the larger exotic species.
      • This results in wasted mating effort.
      • Introduction of bullfrogs to Red-legged Frog territory has become a conservation problem.

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    Ecological Isolation: Turdus spp.

    Example 2. European populations of Turdus spp.

    • The Common Blackbird (Turdus merula)
      lives and breeds in forest.

    • The closely related Ring Ouzel (Turdus torquatus)
      lives and breeds on moors.

    • Even where forest and moor abut, the two species do not interbreed.

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    Ecological Isolation:
    Anopheles Mosquitos

    Example 3: Anopheles maculipennis group
    • Once believed to comprise one species,
      xxx Anopheles actually contains six species.

    • Each occupies a different estuarine niche
      xxx (freshwater, brackish water, marine).

    • Their ecological preferences make matings
      xxx among them rare.

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    Temporal Isolation: Rana spp.

    Two related species occupying the same geographical range
    may have different periods of sexual activity or breeding seasons.

    Example 1: Closely related Rana species in California Coastal Ecosystems

    • The Red-legged Frog (Rana draytonii) breeding season
      x lasts from ~ November - late April.

    • The Yellow-legged Frog (Rana boylii) breeding season
      x lasts from ~ late April - June.

    • The breeding seasons may overlap in some areas.

    • The combination of ecological and temporal isolation prevents hybridization.

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    Temporal Isolation: Fruit Flies

    Example 2: Closely related Fruit Flies in Hawaii

      • Drosophila persimilis breeds in early morning.

      • Closely related Drosophila pseudoobscura
        breeds in the afternoon.

      • Never the twain shall mate.

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    Behavioral Isolation

    Animals with complex courtship behaviors
    usually perform species-specific "call and response" signals between male and female before
    actual mating takes place.

    These rituals prevent wasted mating effort that would halt gene transmission due to sterile or inviable hybrids.

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    Mechanical Isolation: Shell Coiling

    Morphological differences between species prevent hybridization.

      Example 1: Shell Coiling in Euhadra

      • In snails, the direction of shell coiling
        is controlled by a single (maternal effect) gene.

      • Left-coiling snails cannot mate with right-coiling snails.

      • Gene flow between them will cease.

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    Mechanical Isolation: Pollination

    Example 2: The Bucket Orchid and the Orchid Bee

    • Male Orchid Bees obtain a wax from the orchid
      x that they use to make a substance to attract female bees.
    • The anatomy of the Bucket Orchid allows pollination
      x only by this species of bee.
    • This partnership is so precise that if either species
      x became scarce or extinct, the other would follow.

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    Gametic Isolation

    If the sperm and ova of two species have incompatible
    plasma membrane receptors, they cannot form a zygote.

    Example: Sympatric Sea Urchin Species

    • Sea urchins synchronously broadcast gametes into the ocean.

    • Sperm and eggs from the same species fuse to form zygotes.

    • Planktonic larvae eventually settle to metamorphose into adults.

    • The Giant Red Urchin (Strongylocentrotus franciscanus) and the Purple Urchin (Strongylocentrotus purpuratus) cohabit the rocky intertidal along the western U.S., but do not interbreed.

    • Their gametes do not recognize one another,
      maintaining species integrity.

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II. Postzygotic Isolating Mechanisms

    Hybrid Inviability: Drosophila

    Sperm and egg from the two species may combine, but the genetic information
    is insufficient to carry the organism through normal development.

    The embryo dies after a few cleavages, or some time before birth/hatching.

    Example 1: Drosophila spp.

    • Despite their superficially similar appearance, D. melanogaster and D. simulans have incompatible alleles for nuclear pore proteins.

    • Dysfunction of this vital gene results in inviable hybrids.

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    Hybrid Inviability:
    Panthera spp.

    Example 2: Tigers (Panthera tigris) and Leopards (Panthera pardus)

    • Lions and Leopards are sister taxa.
      • Hybrid offspring are viable, but sterile.

    • Tigers and Lions are more genetically distant.
      • Hybrid offspring are viable and robust,
        but sterile.

    • Tigers and Leopards are genetically distant.
      • Zygotes divide, but embryo miscarries
        or is stillborn.

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    Hybrid Inviability: Rana spp.

    Example 3: Rana draytonii and Rana catesbeiana

    • These two species have been separate for tens of thousands of years.

    • If zygotes form, they are inviable.

    • This has created a conservation problem in areas where Bullfrogshave been introduced in Red-legged Frog habitat.

    • Adding insult to injury, Bullfrogs will voraciously eat juvenile frogs.

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    Hybrid Sterility

    Some species are closely related enough
    to produce viable hybrids, but the hybrids are sterile.

    Example 1: Tigers (Panthera tigris) and Lions (Panthera leo)

    • Tigers and Lions have been separate for millions of years.
    • Their hybrid offspring are viable and robust, but sterile.
      • male tiger x lioness --> tigon
      • male lion x tigress --> liger
    • Non-homologous chromosomes lead to meiotic dysfunction.
    • maternally and paternally imprinted genes differ

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    Hybrid Sterility

    Example 2: Horse (Equus caballus) and Donkey (Equus asinus)

    • Horses and donkeys have been separate species for millions of years.
    • Their hybrid offspring are viable and robust, but sterile.
    • male horse x female donkey --> mule
    • male donkey x female horse --> hinny
    • As above, reciprocal cross offspring are somewhat different
      • maternal mitochondrial input
      • maternally and paternally imprinted genes differ

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    Hybrid Breakdown

    Two related species can hybridize, and their F1 offspring are fertile.
    But successive generations (F2 and beyond) suffer lower viability or fecundity.
    Thus, they cannot become an established population.

    Example: Rice cultivars

    • Cultivars of domestic rice have been artificially selected for centuries.
    • Some are closely related enough to hybridize.
    • F1 hybrids are fertile and viable.
    • F2 generation is stunted and sterile.

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    Revisiting the Question:
    Can Different Species Hybridize?

    Reproductive isolating mechanisms
    can prevent different species from interbreeding.

    But when interspecies mating does occur,
    there are several possible outcomes.

    1. Species reinforcement

    • Hybrids have lower fitness than either parent species.

    • Reproductive isolation is maintained
      due to lack of hybrid survival/reproduction.

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(click on pic for source)

    2. Species Fusion

    Species fusion

    • Two related species in a hybrid zone may have
      weak reproductive isolating barriers.

    • Over time, the two species gene pools essesntially become one.

    • Individuals may no longer be recognizable
      as members of two separate biological species.

    This has been seen in some species of Darwin's Finches.

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    3. Hybrid Equilibrium

    Species stability/Hybrid equilibrium

    • Hybrids are continually produced
      by the two parent populations in a hybrid zone.

    • A narrow hybrid zone can foster
      constant hybridization with reduced hybrid survival.

    • Example: Bombina) hybrid zone in Eastern Europe

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    4. Hybrid Speciatioun

    Hybrid speciation
    • Hybrids are reproductively isolated from parent species.

    • Hybrids may sometimes be reproductively superior
      to parent populations.

    • If hybrid x hybrid matings confer Darwinian fitness,
      hybrid speciation can occur.


      The Red Wolf (Canis rufus) is a product of
      ancient hybridization between the Gray Wolf (Canis lupus)
      and the Coyote (Canis latrans)

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    Hybrid Speciation: Tragopogon

    In the early 1900s, three species of goatsbeard plants were accidentally introduced into the northwestern US.

    • T. pratensis (Meadow Salsify)
    • Tragopogon dubius (Western Salsify)
    • T. porrifolius (Oyster Plant)

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    Tragopogon Hybrids

    In the 1950s, in areas of Idaho and Washington where
    T. dubius, T. pratensis, and T. porrifolius had established sympatry, two new species of Tragopogon were found.

    • Tragopogon miscellus is a 4n hybrid
        of T. dubius x T. pratensis.

    • Tragopogon mirus is a 2n hybrid
        of T. dubius and T. porrifolius.

    Neither hybrid can interbreed with its parent species.

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    Tephritid Fruit Fly Hybrids

    Rhagoletis is a genus of common agricultural pests
    that lay eggs in many crops, from apples to blueberries.

    • Rhagoletis zephyria feeds exclusively on snowberry plants.

    • Rhagoletis mendax feeds exclusively on blueberry plants.

    The two species of Rhagoletis occasionally hybridize..

    • Hybrid R. "lonicera" has genetic markers from both parent species.
    • It feeds mainly on invasive exotic honeysuckle (Lonicera).

    Because flies almost always breed on the plant species
    they ate as larvae, relatively rapid reproductive isolation can occur.

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