Plant Speciation

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Plant Speciation Evolution (PBIO 475/575)

• Isolation Mechanisms

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Isolation Mechanisms

• General definition--limit or prohibit gene flow
  between divergent sets of populations, or between
  species
• Most extreme among the most closely related (
  sister) species
• Doesn't have to be absolute to be effective
• More than one isolation mechanism commonly
  operates in a given species pair or group of
  closely related species

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Isolation Mechanisms

• Flurry of activity (with "biosystematics")
  earlier in the 20th century but not much in
  recent decades
• One of the most fascinating aspects of plant
  evolution, desperately in need further work
• Need investigation on molecular basis of
  particular isolation mechanisms

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Types of Isolation Mechanism

• Geographic (spatial)
• Ecological (environmental)
• Reproductive
• Pre-mating
• Temporal
• Ethological
• Mechanical
• Gametic

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Types of Isolation Mechanism

• Reproductive (cont.)
• Post-mating
• Incompatibility
• Hybrid inviability
• Hybrid sterility
• Hybrid breakdown

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Geographic Isolation

• Generally believed to be a trivial case--arguably
  found in "races" or subspecies and not species
  per se
• Initial event in some speciation models
• "Temporary" situation dependent on lack of
  secondary contact (or long delay)
• Absence of genetic basis--an "externally
  influenced" mechanism, until populations diverge
  to point of genomic incompatibility

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Ecological Isolation

• Different sets of populations gradually adapt to
  divergent ecological conditions ? shifts in
  physiological tolerances or preferences
• Related species in nearly all plant groups
  exhibit this, regardless of other operating
  isolation mechanisms
• As with spatial isolation, often referred to as
  an "external" isolation mechanism (influenced by
  outside environment), but it is ultimately
  genetically based

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Ecological Isolation
Habitat preferences in Michigan violets

• e.g., oaks (Quercus), irises (Iris), violets
  (Viola)

Ballard (1994)
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Pre-mating Isolation Mechanisms

• Pre-mating--inhibits movement of pollen to
  stigmatic surface
• Temporal--flowering time or time of receptivity
  or gamete shedding of reproductive structures
• Diurnal--shifts in time of day e.g., evening
  primrose (Oenothera)

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Pre-mating Isolation Mechanisms

• Temporal isolation (cont.)
• Seasonal--shifts in calendar days e.g.,
  witch-hazels (Hamamelis), violets (Viola)

Ballard (1990)
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Pre-mating Isolation Mechanisms

• Ethological
• Attracts divergent types of pollinators in the
  two sets of populations

Beattie (1974)
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Pre-mating Isolation Mechanisms

• Ethological isolation (cont.)
• Potential divergence in
• floral structure (e.g., narrow tube vs. broad
  sac)
• overall petal color
• flower outline
• nectar-guides or guiding hairs
• ? reduction of interspecific pollinator
  attraction
• Results in different pollinator guilds visiting
  related species
• e.g., touch-me-nots (Impatiens), irises (Iris),
  violets (Viola and several other genera)

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Pre-mating Isolation Mechanisms
Three sympatric N. American violet species and
common hybrids

• Ethological isolation (cont.)

Ballard (1990)
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Pre-mating Isolation Mechanisms

• Ethological isolation (cont.)
• In some plant groups, this and mechanical
  isolation (next slides) work synergistically to
  partition a wide range of pollination agents
  (bugs, birds, wind, water) among species
• Effective traits probably form a continuum with
  those accomplishing mechanical isolation
• Differential attraction of species may be at
  level of single species e.g., single yucca moth
  species and yucca species, fig wasp species and
  fig species

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Pre-mating Isolation Mechanisms

• Mechanical isolation
• Divergent floral architecture (e.g., orientation
  of reproductive structures) among related species
• ? Limits or prohibits pollen transfer
• Attracts same type of pollinator, possibly even
  same individuals, but prevents interspecific
  pollen deposition

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Pre-mating Isolation Mechanisms
Nototribic (head up) pollination

• Mechanical isolation (cont.)
• e.g., milkworts (Polygala), orchids (Platanthera
  et al.), violets (Viola)

Sternotribic (head down) pollination
Beattie (1974)
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Pre-mating Isolation Mechanisms

• Gametic
• in externally fertilized organisms (via water or
  wind)
• analogous to incompatibility in plants
• thus far mostly identified in animals (e.g.,
  fish), not well characterized in plants

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Post-mating Isolation Mechanisms

• Four main categories artificial, lie on a
  continuum of interspecific genetic divergence
• Not mutually exclusive with other mechanisms
• Incompatibility
• Pre-fertilization (gametic mortality)
• Pollen grains fail to germinate on style or
• Pollen tube aborts before it reaches ovule
• Post-fertilization (zygotic mortality)
• Embryo dies before seed maturity or
• Embryo matures but seed fails to germinate

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Post-mating Isolation Mechanisms

• Hybrid inviability--lack of vigor prevents
  maturation to reproductive state
• Hybrid sterility--vigorous enough to reach
  reproductive maturity, but sex organs or gametes
  are nonfunctional
• Genic--widespread condition nuclear
  incompatibility of parental genotypes
• Chromosomal--structural differences between
  parental chromosomes e.g., Clarkia, grasshopper
  in stasipatric model

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Post-mating Isolation Mechanisms

• Cytoplasmic--incompatibility between cytoplasmic
  and nuclear genes of parents e.g., wild potatoes
  (Solanum)
• Hybrid breakdown--F2 and later-generation
  individuals weak, inviable or very susceptible to
  diseases common in plant hybrids

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Theoretical Sequence

• Geographic isolation
• Manifested earliest (in many speciation models)
• Weakest (not appreciable upon early secondary
  contact)
• Common among species in many plant groups
• Environmental isolation follows
• Not as weak as geographic isolation alone
• Hybrids may still be common
• Also regarded as a last-stage reinforcing
  mechanism against gene flow

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Theoretical Sequence

• Pre-mating reproductive mechanisms develop
• Strongest mechanisms
• Widely documented or inferred
• Post-mating mechanisms develop last
• Invoked only where earlier mechanisms do not
  effectively halt gene flow
• Weaker and more reproductively costly than others
• Serve as reinforcing mechanisms
• Poorly understood because more difficult to
  observe

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Additional Observations

• Strength of each mechanism is dependent on
• length of time in place
• level of sophistication or extreme of development
• number of "overlaid" mechanisms operating
  together to limit gene flow
• Basis of isolation (genetic vs. environmental)
  not always clear for certain mechanisms
• Action of one isolation mechanism may "induce"
  another e.g., flowering time--characteristically
  later among populations within species and among
  species growing in wet vs. dry sites or in
  higher-altitude vs. lower-altitude sites

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Multiple Isolation Mechanisms

• One mechanism may predominate, but is often
  accompanied by "secondary" ones
• Evidently quite commonprobably multiple weak
  mechanisms more common than single strong one to
  limit gene flow
• Composite strength of 2 or more weak mechanisms
  at least as effective as 1, may be less "costly
  in terms of phenotypic differentiation

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Multiple Isolation Mechanisms

• Interrelationship of geographic and ecological
  isolation
• In allopatric speciation (believed to be most
  common model), geographic isolation is a
  prerequisite
• Niche Exclusion principle of ecology--closely
  related species cannot occupy precisely the same
  ecological niche

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Multiple Isolation Mechanisms

• Geographic and ecological isolation (cont.)
• Closely related species rarely completely overlap
  in geographic distribution
• But allopatric ( geographically isolated) close
  relatives may occupy similar or identical niches
  and lack strong isolation
• Potential evidence that ecological isolation can
  play roles in both initial differentiation and
  final reinforcement
• Temporal isolation generally precludes mechanical
  or ethological isolation

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End Stage of Speciation

• After gene flow has been limited or halted for
  sufficiently long time, closely related species
  have diverged genetically
• In secondary contact, progeny from hybridization
  are less vigorous or sterile
• Additional reinforcing mechanisms often evolve
  to further prohibit gene flow
• Pre-mating isolation mechanisms often devolve
  or become weaker once genetic differentiation is
  complete and hybridization ceases

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Bibliography

• Ballard, H. E., Jr. 1990. Hybrids among three
  caulescent violets, with special reference to
  Michigan. Michigan Botanist 2943-54.
• Ballard, H. E., Jr. 1994. Violets of Michigan.
  Michigan Botanist 33131-199.
• Beattie, A. J. 1974. Floral evolution in Viola.
  Annals of the Missouri Botanical Garden
  61781-793.
• Briggs, D. and S. M. Walters. 1997. Plant
  variation and evolution, 3rd ed. Cambridge
  University Press, Cambridge, United Kingdom. 512
  pp.

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Bibliography

• Futuyma, D. J. 1979. Evolutionary biology.
  Sinauer Associates, Inc., Sunderland,
  Massachusetts. 565 pp.
• Grant, V. 1971. Plant speciation. Columbia
  University Press, New York, New York. 435 pp.
• Grant, V. 1991. The evolutionary process A
  critical study of evolutionary theory, 2nd ed.
  Columbia University Press, New York, New York.
  487 pp.