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    Reproductive Isolating Mechanisms

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

    The definition is sometimes extended to require that such reproduction must occur under natural, not artificial (e.g., captive) conditions.

    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.

Pre-Zygotic Reproductive Isolating Mechanisms

    Ecological Isolation

    Two closely related species may occupy different ecosystems within a geographical region.
    They are ecologically isolated if their habitat preferences lower their probability of mating.

    Example 1: Central California populations of Rana spp.

    • The Red-legged Frog (Rana draytonii) tends to breed
      x in large ponds.
    • The Yellow-legged Frog (Rana boylii) breeds almost exclusively
      x in fast-moving streams.
    • Their habitat preferences contribute to their reproductive isolation.

      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.

    Example 2: European populations of Turdus spp.

    • The Common Blackbird (Turdus merula) lives and breeds in forest.
    • The Ring Ouzel (Turdus torquatus), a close cousin, lives and breeds on moors.
    • Even when forest and moor abut, the two species do not interbreed.

    Example 3: Anopheles maculipennis group
    • Once believed to be a single species,
      xxx there are actually six species in this taxon.
    • Each occupies a different estuarine niche
      xxx (freshwater, brackish water, marine).
    • Their ecological preferences make matings
      xxx among them rare.

    Temporal Isolation

    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.

    Example 1: Closely related Fruit Flies in Hawaii

      • Drosophila persimilis breeds in early morning.
      • Closely related Drosophila pseudoobscura breeds in the afternoon.

        + =

    Behavioral Isolation

    Species with complex courtship behaviors usually exhibit
    stereotyped "call and response" signals between male and female
    before actual mating takes place.

    These rituals prevent wasted mating effort that would halt
    gene transmission by inviable or infertile hybrids.

    Bird of Paradise, Teminick Tragopan:

    Superb Lyrebird, Fireflies:

    Mechanical Isolation

    Morphological differences between species prevent hybridization.

      Example 1: Snail Shell Coiling

      • In some snail species, the direction of shell coiling
        x is controlled by a single (maternal effect) gene.
      • Left-coiling snails cannot mate with right-coiling snails.
      • Such mutations could quickly lead to further differentiation
        x and, possibly, speciation.

      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.

    Gametic Isolation

    In this case, sperm and ova of the two species are chemically incompatible,
    and will not join to 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.
    • These develop into planktonic larvae that eventually settle
      x to metamorphose into adults.
    • The Giant Red Urchin (Strongylocentrotus franciscanus) and
      x the Purple Urchin (Strongylocentrotus purpuratus) cohabit the rocky intertidal
      x along the western U.S., but do not interbreed.
    • Their gametes do not recognize one another, maintaining species integrity.

Postzygotic Isolating Mechanisms

    Hybrid Inviability

    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
      x and D. simulans have incompatible alleles
      x for nuclear pore proteins.
    • Dysfunction of this vital gene results in inviable hybrids.

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

    • Tigers and Lions are sister taxa.
    • Their hybrid offspring are viable and robust, but sterile.
    • A mating between a lion and leopard will produce sterile hybrids.
    • A mating between a tiger and leopard will produce inviable hybrids.
    • Zygotes divide, but embryo miscarries or is stillborn.

    Example 2: Rana draytonii and Rana catesbeiana

    • Although members of the same genus, they have been geographically
      x separated for tens of thousands of years.
    • If zygotes form, they are inviable.
    • This has created a conservation problem in areas where Bullfrogs
      x have been introduced in Red-legged Frog habitat.
    • Adding insult to injury, Bullfrogs will voraciously eat juvenile frogs.
    x =

    Hybrid Sterility

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

    • Tigers and Lions are sister taxa, but separate for millions of years.
    • Their hybrid offspring are viable and robust, but sterile.
    • Chromosomes are not homologous, so do not migrate normally at meiosis.
    • male tiger x lioness --> tigon
    • male lion x tigress --> liger
    • reciprocal cross offspring are somewhat different

    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

    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.

    Interspecies Hybridization

    A hybrid zone is a region where
    • the geographic ranges of two closely related species overlap
    • hybrids between the two species are known to occur.

    Hybridization between closely related species can have one of several possible outcomes.

    • Species Reinforcement
      • Hybrids have lower fitness than either parent species.
      • Reproductive isolation is maintained due to lack of hybrid survival/reproduction.

    • Species Fusion
      • Two species in a hybrid zone may have weak reproductive isolating barriers.
      • The two species may, over time, eventually share a common gene pool.

    • 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.

    • Hybrid Speciation
      • Hybrids may be reproductively superior to parent populations.
      • Positive assortative mating among hybrids can lead to hybrid speciation.