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AD. Plant Speciation – Part 2. A. B. C. D. E. F. Spring 2011. Major topics. Variation in plant populations and species (1) Gene flow and reproductive isolating barriers (1) Speciation mechanisms (modes) (2) Species concepts (2). Speciation.

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major topics
Major topics
  • Variation in plant populations and species (1)
  • Gene flow and reproductive isolating barriers (1)
  • Speciation mechanisms (modes) (2)
  • Species concepts (2)
speciation
Speciation
  • Speciation = permanent severing of two or more sets of populations so that migrants from one population system would be at a disadvantage when entering the other
  • Disadvantages: lack of mates (reproductive isolation); less competitive (adaptation)
  • Speciation may result from adaptive changes or chance events (or both)
modes of speciation
Modes of Speciation
  • Traditionally viewed as the gradual accumulation of differences via selection to produce two isolated sets of populations that cannot interbreed
  • Allopatric speciation or local speciation (different ends of the same spectrum)
slide5

Disruptive selection

is the beginning

of gradualistic

speciation.

  • Selection is against the
  • mean values of the
  • phenotypic range.
  • ● Geographic isolation
  • is expected.
  • Reproductive barriers
  • complete the process
  • in most organisms.
allopatric or local speciation
Allopatric or Local Speciation

Population A

Species A

A

D.S.

geographic

barrier

arises

time

reproductive

isolation

gene

flow

D.S.

B

Species B

Population B

allopatric or local speciation7
Allopatric or Local Speciation

allopatric speciation =

geographic isolation

followed by

reproductive isolation;

splitting of one population or

long distance dispersal

Species A

gene

flow

local speciation =

peripheral populations

become neospecies; these

may become fully reproductively

isolated species

Species B

sympatric speciation
Sympatric Speciation
  • The other main mode of speciation
  • But first we need to review chromosomes and chromosomal changes
chromosome number
Chromosome Number

Haploid number – lowest chromosome number in the spores or gametes (egg or sperm) (written as n)

Diploid number – lowest chromosome number in the somatic (non-sex) cells (written as 2n); for a given species, should be 2x the gametes

In angiosperms, chromosome numbers range from 2n = 4 to 2n = 250; average is about 2n = 26. Ferns can have much higher numbers.

For most species, chromosome numbers are fixed, and all individuals have the same number.

In some cases, chromosome numbers show instability, and vary widely within certain groups. Can be chromosome number variation within a single species or even a single individual (not counting gametes).

chromosome sets ploidy
Chromosome Sets - Ploidy
  • POLYPLOIDY – Presence of the three or more sets of chromosomes in somatic cells (often written as X—2X, 3X, 4X, etc.).
  • ANEUPLOIDY – Loss or gain of whole chromosomes.
slide12

Aneuploidy in

Claytonia virginica

(Portulacaceae)

Spring Beauty

2n = 12, 14, 16, 17-37, 40, 42, 44,

46, 48, 50, 72, 81, 85, 86, 87, 91,

93, 94, 96, 98, 102, 103, 104, 105,

110, 121, 173, 177, 191

polyploidy an example from rumex polygonaceae
Polyploidy:An example from Rumex(Polygonaceae)

2n n X Ploidy level

R. sanguineus 20 10 2X diploid

R. obtusifolius 40 20 4X tetraploid

R. hydrolapathum 120 60 12X dodecaploid

slide14

Mitosis vs. Meiosis

http://www.pbs.org/wgbh/nova/miracle/divide.html

modes of speciation15
Modes of Speciation
  • Speciation may occur without geographic isolation, but reproductive isolation is still necessary
  • Sympatric speciation (through polyploidy) occurs frequently in plants
    • autopolyploidy (without hybridization)
    • allopolyploidy (with hybridization)
slide16

Autopolyploid Speciation

Increase in ploidy due to unreduced

gametes and selfing.

autopolyploidy
Autopolyploidy

Meiotic error &

selfing occur.

Species A?

(2n = 16; 4X)

Species A

(2n = 8; 2X)

Normal

meiosis &

selfing or

out-crossing

occurs.

Species A

(2n = 8; 2X)

autopolyploidy18
Autopolyploidy
  • Autopolyploidy results in a form of reproductive isolation, but often there is no accompanying morphological or ecological differentiation
  • Many species with autopolyploidy are considered to include autopolyploid races; in other cases, distinct species are recognized based on ploidy levels if morphological/ecological differentiation has occurred
allopolyploidy
Allopolyploidy

Species A

(AA)

X

Species D

(DD)

Hybrid AD

(often infertile

& cannot cross

with either

parent)

gamete A

gamete D

allopolyploidy21
Allopolyploidy

Chromosome

doubling

AADD(now sex

cells-AA,AD,DD-

can be produced!)

Hybrid AD

Parent AA

X

AADD

gamete A

AAD(infertile

but could persist

through vegetative

reproduction)

gameteAD

allopolyploidy in tragopogon
Allopolyploidy in Tragopogon

3) T. porrifolius

3) RR

See Box 6E

in the text

for more

information.

4) T. mirus

4) DDRR

PR

DR

PR

2) T. pratensis

PD

2) PP

1) T. dubius

1) DD

5) T. miscellus

5) PPDD

many plant hybrids are possible
Many plant hybrids are possible

American and Oriental plane trees (Platanus occidentalis and P. orientalis, respectively) have been geographically isolated for at least 20 MY, but when artificially hybridized are still able to form a fully fertile hybrid, the London plane, which is used as a smog-resistant planter species in major North American cities.

slide26

The World Famous...

“Brassica

Triangle”

slide27

An example of hybrid, polyploid speciation

in wheat,Triticum aestivum (Poaceae)

polyploid crops
Polyploid Crops
  • Wheat – 6X
  • Potatoes – 4X
  • Maize – 4X
  • Sugarcane – 10X or 12X
  • Sweet Potato – 6X
  • Cotton – 4X
  • Banana – 3X
polyploidy in tracheophyte evolution
Polyploidy in Tracheophyte Evolution
  • Recent studies have shown that it is likely that at least 95% of all ferns have evidence of polyploidy in their lineages.
  • Estimated that as much as 75% of all angiosperms have at least one episode of polyploidy at some point of their evolutionary history!
modes of speciation30
Modes of Speciation

sympatric speciation =

reproductive isolation of

parent species from

their derivatives through

hybridization and

chromosome doubling (or just

chromosome doubling)

without geographic isolation

modes of speciation summary
Modes of Speciation: Summary
  • Allopatric speciation
  • Local speciation
  • Sympatric speciation
    • Autopolyploidy (without hybridization)
    • Allopolyploidy (with hybridization)

Geographic or habitat

isolation accompanies

reproductive isolation

Reproductive isolation

occurs without

geographic isolation

extensive variation in species concepts among biologists
Extensive variation in species concepts among biologists:
  • Biological – Gap in interfertility – “reproductive isolation”
  • Recognition – Common fertilization system
  • Phenetic – Gap in variation between species (discontinuity)
  • Evolutionary – Common evolutionary fate through time
  • Cladistic – Defined by an apomorphy as being monophyletic
  • Taxonomic/Diagnostic – Unique combination of characters
  • Genealogical – Being more closely related to each other than to

members of any other group

biological species concept
Biological Species Concept
  • Relies upon the concept of post-divergence reproductive isolation (lack of gene flow through allopatric speciation)
  • Assumes that species cannot interbreed
  • In general, works well for vertebrates
  • Well………This doesn’t work for plants!!
morphological species concept
Morphological Species Concept
  • the idea that one can observe either special, uniquely derived characters or identify morphological discontinuities between putatively related organisms
  • Need to take into account the range of morphological variation
  • Since diversity is present in all populations, it is implicit to somehow describe the diversity, and then evaluate any discontinuities
  • Ecological differentiation is usually also included as a criterion
  • A ‘practical’ concept, especially where identification is the primary goal
which is the best species concept to use for plants
Which is the best species concept to use for plants?
  • How do you evaluate what is “best” for your situation?
  • Do you have enough information to use any of the species concepts, or do some require more information than you have available?
  • In practice, some form of the morphological species concept is generally used but may be complemented by molecular/genetic data when available
slide37

But it is important to know your organisms—what

biological processes might have an impact on variation?

Figure 6.18 from the text

Morphological intermediacy may well be an indicator of

hybridization, but could be due to clinal variation or

other processes.

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