Plant Speciation

1
Plant Speciation Evolution (PBIO 475/575)

• Gene Flow and Population Subdivision Genetics of
  Selection

2
Gene Flow

• Gene flow movement of alleles within and
  between populations (between species
  hybridization)
• Dependent on physical movement of pollen
  (pollination), seeds, spores or vegetative
  propagules (dispersal)
• Movement of pollen and seeds independent of each
  other
• Total gene flow is composite of
  pollen/microgametophyte movement and seed/spore
  dispersal

3
Gene Flow

• Ultimate source of allelic variation is mutation,
  but mutations ( alleles) get transferred among
  populations by gene flow
• May or may not result in gene frequency changes
• Gene flow partly determines population genetic
  structure ( population subdivision)
• Affected by ecological factors, too
• Distribution (aggregation) of individuals in
  space
• Intraspecific and interspecific competition
• Habitat factors (light, moisture, etc. and extent
  of site heterogeneity)

4
Gene Flow

• Life-form, breeding system, pollination type,
  seed dispersal mechanism, species geography--all
  play role in influencing gene flow
• Generalizations possible across species groups
• Endemic or narrowly distributed species usually
  have lower within-population differentiation than
  widespread species
• Annuals usually have lower within-population
  differentiation than perennials or woody plants

5
Gene Flow

• Seed dispersal mechanism is more complex in
  relation to within-population and
  among-population differentiation
• Mech. Gene div. WP diff. AP diff.
• gravity 0.14 0.10 0.28
• attachment 0.20 0.14 0.26
• explosive 0.09 0.06 0.24
• ingested 0.18 0.13 0.22
• wind 0.14 0.12 0.14

6
Gene Flow

• Even rare immigration from another population may
  offset isolation or inbreeding
• "Tail" (extreme distance) of gene flow in medium
  to large seeds as important as the average
  distance--rare events may have profound impacts
• Historically thought to be extensive and nearly
  "panmictic" numerous recent studies suggest gene
  flow is much more restricted than this ideal
  (especially in plants and low-vagility animals

7
Gene Flow

• Three major models
• Stepping-stone--gene flow only among adjacent
  evenly spaced populations (linear)
• Wright's island--gene flow through large central
  population to satellites, equal among all
  populations (spokes of wheel)
• Isolation by distance--similar spatial
  arrangement to WI, but gene flow directly between
  any pair and rate determined by distance between
  populations

8
Pollination

• Effective transfer of pollen onto stigma
• Pollen transfer ranges from within-flower
  (autogamy) all the way to between-species
  (hybridization)

Stuessy (1990)
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Pollination

• Pollen vectors diverse across land plants,
  important to test
• Wind (anemophily)--common in gymnosperms,
  Hamamelids (e.g., birch)
• Water (hydrophily)--common in aquatics,
  pteridophytes and allies

Insect pollination unexpectedly confirmed in
anemophilous Plantago
Briggs Walters (1997)
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Pollination

• Pollen vectors (cont.)
• Insects (entomophily)--ubiquitous in angiosperms,
  worldwide
• Birds (ornithophily)--widespread, in warmer
  (subtropical tropical) regions
• Mammals, marsupials--ditto for ornithophily, not
  as frequent except in tropics

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Seed Dispersal

• Movement of seed/asexual propagule through spaces
  (hopefully to suitable germination site)
• "Active" but without an intermediary (e.g.,
  explosive dispersal), or "active" by one of the
  same vectors as for pollination, or "passive" by
  gravity
• Successful dispersal does not mean successful
  germination and establishment
• Immigration--special case involving movement into
  a particular population from outside

12
Seed Dispersal

• Studies of patterns in nature
• Most studies utilize "traps" for fruits, seeds
  for only 1-2 years duration recently some have
  used genetic markers
• Generalized results
• Average distances of gravity-dispersed seeds
  short (no surprise)
• ADs unexpectedly often shorter in wind-borne
  seeds than expectations
• Bird- and animal-dispersed seeds travel longer
  distances

13
Seed Dispersal

• Some species have "mixed" strategies
• Amphicarpum purshii (Fabaceae) produces small
  aerial fruits on some spikelets, subterranean
  cleistogamous ( self-fertilizing) fruits
• Many Viola species produce open-pollinated
  flowers--gtexplosive capsules, and cleistogamous
  flowers--gtcapsules that "rot away", spilling out
  the seeds

14
Case Studies of Gene Flow

• Texas bluebonnet (Lupinus texensis)--distribution
  of allozyme frequencies in populations was more
  extensive than movement of pollen--gt"extra" must
  be due to more extensive seed dispersal or
  another factor
• "Wild" populations of horseradish (Raphanus
  sp.)--paternity analysis of population showed
  that 1-4 pollen parents were responsible for
  fertilizing the different ovules on each plant--gt
  cautions against "single father" presumption in
  all plants

15
Case Studies of Gene Flow

• Bladder campion (Silene latifolia)--chloroplast
  DNA (maternally inherited) demonstrates movement
  of seeds (dispersal), isozymes reflect movement
  of seeds and pollen both both types of gene flow
  contributed about equally
• Long-distance dispersal in Hawaiian fern
  Asplenium adiantum-nigrum--HI plants are
  tetraploids, but diploids only known from western
  Europe different HI populations have unique
  alleles corresponding to different regions of
  Europe--gt3-17 long-distance dispersal events!

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Principles of Selection

• Heritable variation in replication rate causes
  evolution through selection
• This variation originates as random alterations
  of DNA, etc.
• Rate of replication is selected directly through
  phenotypes
• Characters affecting the rate of replication are
  selected indirectly and may also evolve in the
  process

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Principles of Selection

• Adaptation by selection under one set of
  conditions may be associated with loss of
  adaptations under another set of conditions
• Evolution proceeds through sequential
  substitution of superior (more fit) variants
• A trait evolves from a prior trait only if they
  are connected by a series of modifications, each
  of which is individually advantageous

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Principles of Selection

• Selection tends to improve performance under a
  particular set of conditions, does not
  necessarily optimize performance across all
  conditions
• Selection is caused by differences in replication
  rates among individuals outcome of selection
  often depends on competition, not so much on
  growth conditions

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Models of Selection

• Stabilizing--yields phenotypic conformity by
  eliminating extremes
• Directional--unimodal shift in phenotypic mean
  for advantageous trait
• Disruptive--selection for more than one phenotype

Briggs Walters (1997)
20
Key Points on Selection

• Selection acts directly on the phenotype in the
  broadest sense (may be morphological or life
  history trait, physiological/biochemical trait,
  etc.)
• Selective advantage can be viewed in terms of
  "fitness (total number of descendants over
  individuals life span) left by a more "fit"
  individual compared with other individuals

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Case Studies of Selection

• Cyanogenic variants in European Trifolium
• Typical acyanogenic form common in northeastern
  Europe
• Cyanogenic form (poisonous to livestock) in SW
  Europe
• Cline of intermediates

Briggs Walters (1997)
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Case Studies of Selection

• Cyanogenic variants in Trifolium (cont.)
• Cyanogenic form more frequent at lower altitudes
  in the Alps
• Correlation beween cyanogenic form and January
  mean temperatureincreased temp? more
  cyanogenesis
• Cyanogenesis controlled by simple allele system

Briggs Walters (1997)
23
Case Studies of Selection

• Copper tolerance in Agrostis capillaris
• Plants collected along transects and different
  sites around a copper mine
• Copper ppm established for plots
• Seeds grown in range of copper concentrations to
  adulthood

Briggs Walters (1997)
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Case Studies of Selection

• Copper tolerance in Agrostis (cont.)
• Index of copper tolerance used
• Seedlings showed greater variation than adults ?
  selection operating between seedling and adult
  stage

Briggs Walters (1997)
25
Case Studies of Selection

• Copper tolerance in Agrostis (cont.)
• Seedlings on contaminated soils more likely to
  survive to maturity (? selected for high copper
  tolerance)
• Seedlings on non-contaminated soils better
  competitors in non-contaminated pasture
• Evidence for gene flow of copper tolerance
  allele(s) from contaminated into non-contaminated
  populations

26
Case Studies of Selection

• Geographic variants in barley
• 11 barley varieties mixed in equal proportions,
  shipped to 10 experimental stations across U.S.
• Seed harvest from first year planted in next
  season at each station

27
Case Studies of Selection

• Geographic variants in barley (cont.)
• Representative sample of seed harvest from each
  station at end of study planted out in
  Washington variety proportions compared
• ? Different varieties with locally adaptive
  traits outcompeted other varieties within a few
  years!

Briggs Walters (1997)
28
Case Studies of Selection

• Adaptive and non-adaptive traits in Spergula
  arvensis
• Seed coat types show geographic and clinal
  variation
• Seed coat based on single allele system

Briggs Walters (1997)
29
Case Studies of Selection

• Adaptive and non-adaptive traits in Spergula
  (cont.)
• Cultivation of seed types showed smooth form was
  less tolerant of high temperatures and low
  humidity than papillate form
• Seed coat differences correlated with genotypic
  differences in physiological tolerance
• Papillate seed form germinated better under dry
  conditions ? "non-adaptive" trait is linked
  somehow with other traits that have direct
  adaptive significance

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Bibliography

• Bell, G. 1997. Selection The mechanism of
  evolution. Chapman and Hall, New York.
• Briggs, D. and S. M. Walters. 1997. Plant
  variation and evolution, 3rd ed. Cambridge
  University Press, Cambridge, United Kingdom. 512
  pp.
• Grant, V. 1991. The evolutionary process A
  critical study of evolutionary theory. Columbia
  University Press, New York, New York. 487 pp.
• Hamrick, J. L. and J. D. Nason. 1996.
  Consequences of dispersal in plants. In Rhodes,
  O. E., R. K. Chesser, and M. H. Smith (eds.).
  Population dynamics in ecological space and time.
  University of Chicago Press, Chicago, Illinois.
  Pp. 203-236.
• Stuessy, T. 1990. Plant taxonomy The systematic
  evaluation of comparative data. Columbia
  University Press, New York, New York. 514 pp.