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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 • 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 • 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)
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 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 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 • The other main mode of speciation • But first we need to review chromosomes and chromosomal changes
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 • 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.
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) 2n n X Ploidy level R. sanguineus 20 10 2X diploid R. obtusifolius 40 20 4X tetraploid R. hydrolapathum 120 60 12X dodecaploid
Mitosis vs. Meiosis http://www.pbs.org/wgbh/nova/miracle/divide.html
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)
Autopolyploid Speciation Increase in ploidy due to unreduced gametes and selfing.
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)
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
AD Allopolyploid Speciation B C D E F A
Allopolyploidy Species A (AA) X Species D (DD) Hybrid AD (often infertile & cannot cross with either parent) gamete A gamete D
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 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 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.
Naturally-occurring plant hybrids: many examples Penstemon (Scrophulariaceae)
The World Famous... “Brassica Triangle”
An example of hybrid, polyploid speciation in wheat,Triticum aestivum (Poaceae)
Polyploid Crops • Wheat – 6X • Potatoes – 4X • Maize – 4X • Sugarcane – 10X or 12X • Sweet Potato – 6X • Cotton – 4X • Banana – 3X
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 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 • 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: • 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 • 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 • 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? • 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
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.