Plant Speciation – Part 2
AD
A
B
C
D
E
F
Spring 2012
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
gene
flow
Population B
A
geographic time
barrier
arises
B
Species A
reproductive
isolation
Species B
Allopatric or Local Speciation
Species A
gene
flow
Species B
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 (nonsex) 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)
R. sanguineus
R. obtusifolius
R. hydrolapathum
2n
20
40
120
n
10
20
60
X
2X
4X
12X
Ploidy level
diploid
tetraploid
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
Allopolyploid Speciation
AD
A
B C D
E
F
Allopolyploidy
Species A
(AA)
X
Species D
(DD)
gamete A
gamete D
Hybrid AD
(often infertile
& cannot cross
with either
parent)
Allopolyploidy
Hybrid AD
Chromosome
doubling
AADD (now sex
cells-AA, AD, DDcan be produced!)
Parent AA
X
AADD
gamete A
gamete AD
AAD (infertile
but could persist
through vegetative
reproduction)
2
Allopolyploidy in Tragopogon
3) T. porrifolius
3) RR
4) T. mirus
4) DDRR
PR
DR
PR
2) T. pratensis
PD
1) T. dubius
1) DD
2) PP
5) PPDD
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 smogresistant 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
Geographic or habitat
isolation accompanies
reproductive isolation
– Autopolyploidy (without hybridization)
– Allopolyploidy (with hybridization)
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?
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.