BIO 3400 Plant Systematics, Dr. L. Kelly
Evolution of Plant Diversity
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Speciation requires variation & isolation
Speciation may occur without geographic isolation
Hybridization is important cause of speciation
Polyploidy is common
I. Plant systematics
= science of diversity of plants & their phylogenetic relationships
A. Evolution is underlying process for phylogeny
B. ~ 270,000 spp of plants on Earth
II. Genetic differences in species
A. What causes differences? Darwinian evolution (& Alfred Wallace)
(struggle for existence)
Superfecundity -----------> natural variation-------------> natural selection----->Evolution
(limited resources)
(inheritance of variability)
B. What is evolution?
= changes in gene frequencies; cumulative changes in genetics;
descent with modification
C. Sources of natural variation (i.e., genetic variability)
1. mutation
2. natural selection
3. genetic drift
4. gene flow
5. genetic recombination
D. Evidence: fossil record, structural & genetic similarities, biogeography.
E. Evolution is ongoing
F. Changes may lead to speciation (or cladogenesis) = formation of new
species; requires:
1. natural variation
2. reproductive or genetic isolation
III. Speciation popn = all individuals of same species occupying same area
A. allopatric speciation = among geographically separated popns
1. most imp. type for animals
2. among plants, if separate popns look the same, we may place
them in the same species
American beech (Fagus grandifolia)
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New Brunswick to Florida
Gene flow is weak or absent
= One species based on morphology
Phragmites australis On separate continents
B. sympatric speciation = within popns
(no geographic separation)
1. requires reproductive or genetic isolation
2. may be more imp. than allopatric speciation for plants!
3. can result because of hybridization, especially if polyploidy occurs
IV. Hybridization is a major cause of plant speciation
A. hybridization = production of offspring from parents of different species
B. Common among plants
1. Examples: Betula, Quercus,Pinus, Rubus, Crataegus, etc.
2. common within genera but not between genera
3. offspring may or may not be fertile
4. offspring may reproduce vegetatively
5. infertile offspring may have apomixis (or agamospermy) = asexual
production of embryos & seeds (no fertilization); may have some genetic
variation.
Apomixis
Chromosome # is not reduced
parthenogenesis
egg cell----------------------> embryo
(2n)
(2n)
Naturally occurs in ~ 400 plant spp.
Gregor Mendel & Hieracium
Apomictic
No 3:1 and 9:3:3:1 ratios
C. Consequences:
1. reinforce reproductive isolating mechanisms
2. formation of hybrid swarm
3. introgression = permanent incorporation of genes from one species into 2nd
species; hybrid offspring backcross with one or both parent spp.
4. speciation
D. Commonly results in new species (sympatric speciation)
1. especially if polyploidy occurs
2. polyploidy = having > 2 sets of chromosomes
V. Polyploidy in plants
A. In ~ 70% of angiosperms
1. sporophyte is diploid (or polyploid)
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diploid (2n) chromosome # ranges from 4 to 250
B. Can occur since most plants lack sex chromosomes
In 95% of ferns?
2n = 1260 in Ophioglossum reticulatum
C. 2 major types:
1. autopolyploidy -- results from union of > 3 chromosome sets within same
species
2. allopolyploidy -- results from union of > 2 chromosome sets from 2 diff.
species
D. Often results from nondisjunction = failure of chromosomes to separate
during meiosis or mitosis
E. Allopolyploidy often results in allopolyploid speciation
1. may restore fertility to infertile hybrids
2. may reproductively isolate hybrids from parent species
3. resulting species is often a tetraploid = 4n
VI. Species concepts
A. biological species concept = all individuals who interbreed & produce
fertile offspring;
1. species are reproductively isolated
2. works well for animals but not for plants -- why?
B. genetic species concept = all individuals who share the same unique, gene
pool; based on genetic distance
1. excellent for phylogenetic studies
2. problematic -- why?
C. morphological species concept -- all individuals who have same
morphological characters
1. classical concept
2. most practical, widely used
3. problematic.
a. may not recognize:
sibling or cryptic species = morphologically indistinguishable but
reproductively isolated
Example: Andropogon virginicus complex
b. may recognize separate spp. when only 1 is present
c. may recognize several, morphologically distinct species that are not
genetically distinct
Example: 19 species of Bidens on Hawaii islands
How do we define plant species?
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Based on:
Morphological
Molecular
Ecological
Reproductive
Geographic
Evidence
Some species are clearly distinguishable
Other species are problematic
Allopatric speciation in Hawaiian silverswords of Asteraceae;
3 genera & 25 species; 1 founding sp.
Sympatric speciation in the absence of hybridization -- the self-pollinated
diploid Stephanomeria malheurensis
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