Transcript plant 52

Phenotypic Plasticity
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Ability of an organism to express different
phenotypes depending on the biotic or
abiotic environment
Involves regulatory genes that switch on
structural genes given the appropriate
stimulus
Many trees and shrubs produce
shade leaves and sun leaves
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Shade leaves are thinner and much greater
in surface area than sun leaves
How is the type of leaf determined?
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The genes which determine the shape of a
particular leaf are sensitive to light
Specifically, the portion of the plant is
responsive to the ratio of far red light to red
light
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Red light is absorbed by leaves higher in the
canopy but far red light passes through to the
ground
Therefore, the ratio of far red:red light
increases in more shaded areas
When the FR:R ratio exceeds a critical value,
a leaf will be formed as a shade leaf, which is
much more efficient at gathering light under
darker conditions (high surface area)
“Talking trees”
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Sugar maple
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Poplar
Some vascular plants are known
to produce herbivore-deterrent
compounds
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There is evidence that the production of
these allelochemicals can be induced
Producing these compounds only when
needed presumably saves the plant energy
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Many of these allelochemicals degrade
quickly, making an inducible response even
more economical
Some researchers at Dartmouth
tested some sugar maples and
poplars to see if the production of
allelochemicals can be induced
by herbivores
The experiment involved tearing
off 7% of the leaf area of each
tree
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The level of allelochemicals was measured
before the “artificial herbivory” and 52
hours later
For both sugar maples and poplars, an
increase in the production of
allelochemicals was seen after 52 hours
But the really cool result was . . .
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Control trees in the same environmental
chamber increased their production of
allelochemicals too
This result suggests that an airborne cue
from plants that have been grazed alerts
surrounding trees to step up their chemical
defense
Hence, the “talking tree” metaphor
The dogwhelk, Nucella lamellosa, a
common rocky intertidal snail along
the western North American coast
Cancer productus, a predator on
Nucella lamellosa
Phenotypic plasticity in Nucella
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Workers exposed some Nucella to water in
which Cancer productus was present, some
Nucella to water in which the metabolites
of recently damaged Nucella was present
and a control of water without either type
of chemical signal
Dogwhelks exposed to Cancer or
to the metabolites of Nucella
increased the length of the
apertural teeth
Daphnia (water flea)
A backswimmer, a potent
predator on Daphnia
Phantom midge larva, another
potent predator on Daphnia
Helmeted and normal Daphnia
On coral reefs, competition for
space is intense
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Some corals use
particularly virulent
nematocysts (specialized
stinging organelles) to
kill competitors
The coral Agaricia grows
elongate sweeper tentacles when
a competitor comes in contact
Sweeper tentacles in the coral
Agaricia
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The sea fan
Erythropodium and
the anemone
Palythoa both induce
the growth of sweeper
tentacles in Agaricia
Sweeper tentacles from an
anemone
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Mechanisms of Evolutionary
Change
Mechanisms that change allele frequencies in
populations:
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Natural selection
Mutation
Gene flow (migration)
Genetic drift (sampling error)
Genetic Structure of Populations
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All the genes contained by the individuals of
population constitute the gene pool
To understand the genetics of evolution, we study
the gene pool of a population rather than the
genotypes of its individual members
Quantitative measures of the gene pool
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Genotype frequencies
Allele frequencies
Scarlet
Tiger
Moth
Scarlet Tiger Moth Genotype
Frequencies
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Collection from one locality in England
yielded the following numbers of
genotypes: 452 BB, 43 Bb and 2 bb for a
total of 497 moths
f(BB) = 452/497 = 0.909
 f(Bb) = 43/497 = 0.087
 f(bb) = 2/497 = 0.004
 Total
1.000
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Allele Frequencies
Used by most population geneticists
instead of genotype frequencies
 Allele frequency =
number copies of a given allele
sum of all alleles in the population
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Calculation of allele frequencies
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When two alleles are present at a locus (let’s call
it the A locus), we can use the following formula
to calculate the allele frequency of A. The
frequency of A is abbreviated as f(A) or p.
p = f(A) = (2 x number of AA homozygotes) +
(number of Aa heterozygotes)
(2 x total number of individuals)
Example calculation
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Imagine a population of 1000 individuals with 353 AA, 494
Aa and 153 aa individuals
Each AA individual has two A alleles while each
heterozygote has one A allele
p = f(A) = [(2 x 353)+ 494]/(2 x 1000 individuals)
p = 1200/2000 = 0.60
The total of all allele frequencies must be 1.0
Only two alleles are involved at this locus
We can therefore calculate q = f(a) = 1.0 - 0.6 = 0.4