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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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Plant speciation part 2 l.jpg


Plant Speciation – Part 2







Spring 2011

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Major topics

  • Variation in plant populations and species (1)

  • Gene flow and reproductive isolating barriers (1)

  • Speciation mechanisms (modes) (2)

  • Species concepts (2)

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  • 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)

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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)

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Disruptive selection

is the beginning

of gradualistic


  • Selection is against the

  • mean values of the

  • phenotypic range.

  • ● Geographic isolation

  • is expected.

  • Reproductive barriers

  • complete the process

  • in most organisms.

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Allopatric or Local Speciation

Population A

Species A













Species B

Population B

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Allopatric or Local Speciation

allopatric speciation =

geographic isolation

followed by

reproductive isolation;

splitting of one population or

long distance dispersal

Species A



local speciation =

peripheral populations

become neospecies; these

may become fully reproductively

isolated species

Species B

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Sympatric Speciation

  • The other main mode of speciation

  • But first we need to review chromosomes and chromosomal changes

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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).

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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.

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Aneuploidy in

Claytonia virginica


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

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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

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Mitosis vs. Meiosis

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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)

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Autopolyploid Speciation

Increase in ploidy due to unreduced

gametes and selfing.

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Meiotic error &

selfing occur.

Species A?

(2n = 16; 4X)

Species A

(2n = 8; 2X)


meiosis &

selfing or



Species A

(2n = 8; 2X)

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  • 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

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Allopolyploid Speciation







Allopolyploidy l.jpg

Species A



Species D


Hybrid AD

(often infertile

& cannot cross

with either


gamete A

gamete D

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AADD(now sex


can be produced!)

Hybrid AD

Parent AA



gamete A


but could persist

through vegetative



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Allopolyploidy in Tragopogon

3) T. porrifolius

3) RR

See Box 6E

in the text

for more


4) T. mirus





2) T. pratensis


2) PP

1) T. dubius

1) DD

5) T. miscellus


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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.

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The World Famous...



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An example of hybrid, polyploid speciation

in wheat,Triticum aestivum (Poaceae)

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Polyploid Crops

  • Wheat – 6X

  • Potatoes – 4X

  • Maize – 4X

  • Sugarcane – 10X or 12X

  • Sweet Potato – 6X

  • Cotton – 4X

  • Banana – 3X

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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!

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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

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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

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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

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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!!

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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

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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

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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.