Plant Speciation – Part 1
Spring 2014
Major topics
• Variation in plant populations and
species (1)
• Gene flow and reproductive isolating
barriers (1)
• Speciation mechanisms (2)
• Species concepts (2)
Fig. 19.2
clade
Fig. 19.3B-E
B
C
A
B
C
D
E
F
Cladogenesis
Cladogenesis
Cladogenesis
Anagenesis
Cladogenesis
Anagenesis versus
Cladogenesis
Biological Variation
•
•
•
•
All populations of organisms have inherent variation
within them – a range of genetic variation of which part
is expressed as phenotypic variation
Influences of the environment can change how this
variation is expressed = phenotypic plasticity
Observations by biologists can be interpreted in a
number of ways to determine the similarities or
differences in groups of organisms, depending on
which characteristics are measured or emphasized
Need to understand speciation (the process) in order to
understand patterns of diversity (and vice versa) and to
provide a means to define species
Variation in Plant Populations and
Species
One P. jeffreyi
ecotype is
adapted to
serpentine soils.
This ecotype has
less overall
genetic diversity
than ecotypes
of this species
adapted to more
fertile soils.
Clinal variation within a species
Genetic variation within
and among species of
native American canes
Mutation is the ultimate source of all genetic diversity.
Types of mutations
• Point mutations (change in one base)
• Insertions, deletions, inversions,
duplications of parts of a chromosome
• Gains or losses of whole chromosomes
(aneuploidy)
• Multiples of whole genomes (the full set of
chromosomes in the nucleus) (polyploidy)
Genetic Recombination
Review of Mitosis and Meiosis
http://www.pbs.org/wgbh/nova/miracle/divide.html
Genetic Drift: chance fixation of
genes (alleles) in small populations
Generation 1
(5/10
plants
Generation
leave
offspring)
2
(2/10
plants
Generation
leave
offspring)
3
Gene flow and reproductive
isolating barriers
Figure 13.3A, B
Gene flow
• Gene flow = exchange of genes (alleles)
between populations
• In plants, occurs through the dispersal of
pollen or fruits/seeds
• Expected to occur between populations of
the same species, but in plants also
occurs between populations of different
species (hybridization)
Usually measured in meters, but can occur over longer distances.
Gene flow
• Gene flow within and between
populations of a species tends to
maintain the cohesiveness of a
species
• Lack of gene flow (due to
reproductive isolating barriers)
between populations is normally
required for speciation to occur
Reproductive Isolating Barriers
Pre-mating
Post-mating
Post-zygotic
Ecological or
habitat
isolation
Mechanical or
physiological
isolation
Hybrid
inviability
Temporal
isolation
Gametic
isolation
Hybrid
sterility
Hybrid
breakdown
Behavioral
isolation
See Table 19.1
Habitat isolation
Prairie
Woodland
White lady’s slipper
Yellow lady’s slipper
Temporal Isolation
Red = staminate plants
Blue = carpellate plants
Behavioral Isolation: Adaptation to different pollinators
Two species of orchids each with a different bee pollinator.
The labellum may be a “key” innovation driving diversification.
Variation in
the orchid labellum
Two species of
Aquilegia
(Ranunculaceae)
Habitat isolation & floral isolation
Columbines
(Aquilegia)
in California
Aquilegia formosa
mesic sites < 3,050 m
Aquilegia pubescens
exposed, xeric sites > 2,750 m
Post-mating: Mechanical or physiological isolation
See Ch. 13: 574-576.
Beetle pollination is relatively unspecialized and
probably ancestral for angiosperms.
Beetle pollination in water lilies:
http://vimeo.com/41976231
Double flowering to avoid selfing:
http://www.youtube.com/watch?v=B2S5c1s5mPA
Bee Pollination
-showy, colorful (blue,
purple, yellow) flowers
-fragrant
-day-flowering
-bilateral landing platform
-nectar and/or pollen
rewards
Moth pollination
-white or pale, usually large
flowers
-sweet, strong scent
-nectar reward
-no nectar guides but may
have nectar spurs
-night- or dusk-flowering
Yucca
and the
Yucca moth
http://vimeo.com/7048122
Pistil-packing mama
Fly pollination
-brown or maroon flowers
-fetid odor (rotting meat)
-day- or night-flowering
-usually no reward
-some operate as trap
flowers
Bird pollination
-brightly colored, often
red flowers
-no scent
-day-flowering
-usually copious nectar reward
-often tubular corolla, often
with an inferior ovary
Bat pollination
-usually large, whitish or
colorful flowers
-musky, strong scent
-night-flowering
-usually copious nectar
and/or pollen reward
Moraceae – The Fig and The Fig Wasp
Wind pollination
-small, numerous often unisexual
flowers with reduced or absent
perianth
-large quantities of pollen,
individual grains smooth
-feathery styles
-no scent
Breeding Systems
• Outcrossing = fertilization between
different individuals; depends on ability to
screen pollen by the stigma and style
(incompatibility)
• Uniparental reproduction
– Self-fertilization (pollen from a flower fertilizes
ovules of the same individual)
– Agamospermy (production of seed without
fertilization)
Breeding systems are not
necessarily mutually exclusive!
Early season, open, crosspollinated flowers in Viola
Later season, closed, selfpollinated flowers in Viola
http://www.youtube.com/watch?v=4YQ5q1cjEU4