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.
is the beginning
allopatric speciation =
splitting of one population or
long distance dispersal
local speciation =
become neospecies; these
may become fully reproductively
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).
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
2n n X Ploidy level
R. sanguineus 20 10 2X diploid
R. obtusifolius 40 20 4X tetraploid
R. hydrolapathum 120 60 12X dodecaploid
Increase in ploidy due to unreduced
gametes and selfing.
Meiotic error &
(2n = 16; 4X)
(2n = 8; 2X)
(2n = 8; 2X)
& cannot cross
can be produced!)
but could persist
3) T. porrifolius
See Box 6E
in the text
4) T. mirus
2) T. pratensis
1) T. dubius
5) T. miscellus
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.
in wheat,Triticum aestivum (Poaceae)
sympatric speciation =
reproductive isolation of
parent species from
their derivatives through
chromosome doubling (or just
without geographic isolation
Geographic or habitat
members of any other group
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