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    Evolutionary Force #3:
    Migration

    Random evolutionary change is caused by

    • mutation
    • non-infinite population size
    • migration
    • non-random mating
    • natural selection

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    Migration vs. Dispersal

    Although the term "migration" is often used in this context,
    it is not quite accurate.

    Migration is the movement of groups of individuals

    • from one area to another within their normal geographic range
    • on a seasonal basis (summer/winter or wet season/dry season)

    Dispersal is the permanent movement of individuals (and their genes)

    • into (immigration)
    • out of (emigration)

    ...a source population or deme.

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(click on pic for source)

    Effective Population Size

    Effective population size (Ne) an important parameter in
    evolutionary biology, population genetics and conservation biology.

    • census size - number of all individuals in the population.
    • effective size - number of individuals contributing offspring
      to the next generation

    Effective population size is the translation of census population size
    into population size that would lose genetic diversity at the same rate
    as the census population size. (Husemann, et al., 2016)

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    Effective Population Size and Conservation

    Effective population size is based on the loss of genetic diversity via
    • inbreeding
    • genetic drift

    In a very small population, effective population size can differ from census size.

    In a very large population

    • individuals are generally not genetically related
    • sex ratio usually approaches 50:50

    In a very small population

    • individuals are likely to be genetically related
    • sex ratio is more likely to be skewed
    • Both of these can reduce effective population size.
    • The smaller the population, the faster the loss of genetic diversity.

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    Skewed Sex Ratio Reduces
    Effective Population Size

    Effective population size (Ne) is equal to

    Ne = 4 x [(Nf x Nm)/(Nf + Nm)]

    Where

    • Nf = number of breeding females
    • Nm = number of breeding males

    • Males contribute 50% of their genes to each generation.
    • Females contribute 50% of their genes to each generation.

    If sex ratio is not 50:50, individuals of the scarcer sex
    will pass a disproportionate number of genes to the next generation.

    In animals with complex behaviors, skewed sex ratio can cause other problems.

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    Gene Flow

    Recall that gene flow is the movement of genes
    between populations or demes.

    Migration

    • spreads novel, mutant alleles
    • homogenizes both donor and recipient demes
    • increases the effective size of each deme

    Lack of gene flow may contribute to speciation.
    But this is unpredictable, and depends on many other factors.

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    Measuring the Effects of Migration
    on Allele Frequencies

    If two demes have different allele frequencies at a particular locus,
    then migration between them can change allelic frequencies at that locus.

    Example:

      In Deme X
      • allele A frequency is P
      • allele a frequency is Q

      In Deme Y
      • allele A frequency is p
      • allele a frequency is q

    1. Individuals from Deme Y migrate into the territory of Deme X.
    2. The proportion of Deme Y individuals in the post-migration Deme X is m


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    Agh! More Math!

    Let the Matings Commence.

    The frequency of a alleles (q) in Deme X (recipient)
    will be changed by the addition of A alleles from Deme Y (donor).

    After one generation...
    The new frequency of allele a in Deme X (recipient) (qt+1) can be expressed as:

    qt+1 = (1-m) x qt + mP

    Where:

    • qt = original frequency of allele a in Deme X (recipient)
    • P = original frequency of allele A in Deme Y (donor)
    • m = post-migration % of Deme Y (donor) individuals in Deme X (recipient)


    The change in q from one generation to the next is equal to:

    qt + 1 - qt

    Where:
    • qt = original frequency of allele a in Deme X (recipient)
    • qt+1 = first post-migration frequency of allele a in Deme X (recipient)

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    Solve This for Bonus Points

    Two somewhat isolated ponds are home to two demes of Rana aurora frogs.

    Gene locus affects the color of the frogs' legs.

    • Allele R (dominant) encodes red legs.
    • Allele r (recessive) encodes yellow legs.

  • In Deme X, the allele relative frequencies are 0.2 : 0.8 (dominant: recessive)
  • In Deme Y, the allele relative frequencies are 0.6 : 0.4 (dominant: recessive)

    If 10 frogs from Deme Y migrate into Deme X's pond and stay to breed,
    what will the frequency of allele r be in Deme X after one generation?

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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


    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.

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    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.

        + =

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    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:

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    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.

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    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.

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II. 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 =

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    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

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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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Hybrid Speciation

When closely related species hybridize, several outcomes are possible.

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    Tragopogon Hybrids

    In the early 1900s, three species of Goatsbeard were introduced into the northwestern U.S.
    • Western Salsify (Tragopogon dubius)
    • Meadow Salsify (T. pratensis
    • Oyster Plant (T. porrifolius)

    In the 1950s, botanists found two new species in Idaho and Washington.
    These were found in areas where the three species were sympatric.

    • Tragopogon miscellus is a tetraploid hybrid of T. dubius x T. pratensis.
    • Tragopogon mirus is an allopolyploid hybrid of T. dubius and T. porrifolius.
    Neither new species can breed successfully 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 important crops, from apples to blueberries.

    Two species of tephritid fruit flies are known to occasionally hybridize..

    • Rhagoletis zephyria feeds exclusively on snowberry plants
    • Rhagoletis mendax feeds exclusively on blueberry plants
    • Rhagoletis zephyria x Rhagoletis mendax = Rhagoletis "lonicera" hybrid
      • The hybrid feeds on introduced invasive exotic honeysuckle (Lonicera) vines
      • It has genetic markers from both parent species.

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