Plant Speciation – Part 2 AD ABCEDF Spring 2014

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 Population B gene flow A B geographic barrier arises Species A Species B reproductive isolation time

Allopatric or Local Speciation Species A Species B gene flow allopatric speciation = geographic isolation followed by reproductive isolation; splitting of one population or long distance dispersal local speciation = peripheral populations become neospecies; these may become fully reproductively isolated species

Sympatric Speciation The other main mode of speciation But first we need to review chromosomes and chromosomal changes

Basic Chromosome Morphoplogy

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 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) 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 Spring Beauty

Polyploidy: An example from Rumex (Polygonaceae) 2n n X Ploidy level R. sanguineus Xdiploid R. obtusifolius Xtetraploid R. hydrolapathum Xdodecaploid

Mitosis vs. Meiosis

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 Species A (2n = 8; 2X) Species A (2n = 8; 2X) Species A? (2n = 16; 4X) Meiotic error & selfing occur. Normal meiosis & selfing or out-crossing occurs.

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

Allopolyploid Speciation AD A BCEDF

Allopolyploidy Species A (AA) X Species D (DD) Hybrid AD (often infertile & cannot cross with either parent) gamete A gamete D

Allopolyploidy Hybrid AD Chromosome doubling AADD (now sex cells-AA, AD, DD- can be produced!) Parent AA X AADD AAD (infertile but could persist through vegetative reproduction) gamete A gamete AD

2

Allopolyploidy in Tragopogon 1) DD 2) PP 3) RR PR PD DR 4) DDRR 5) PPDD PR 2) T. pratensis 1) T. dubius 3) T. porrifolius 4) T. mirus 5) T. miscellus

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 probably 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

Q: “What is a species?”

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 well 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; many plant systematists apply some form of evolutionary or phylogenetic concept

But it is important to know your organisms—what biological processes might have an impact on variation? Morphological intermediacy may well be an indicator of hybridization, but could be due to clinal variation or other processes. Figure 6.18 from the text