Download Four Mechanisms of Evolution (PowerPoint) Southeast 2012

The four mechanisms of evolution
Today’s objective:
Define each evolutionary mechanism
and identify which is taking place in a
given scenario.
CQ: Evolution results in a better match
of an organism and the environment?
1. Yes
2. No
3. Sometimes
0%
So
m
et
im
es
0%
No
Ye
s
0%
How does evolution occur?
• Via one or more mechanisms:
– Mutation
– Gene flow
– Genetic drift
– Natural selection
• How do we measure if evolution has occurred?
Hot Chilies!
• You represent a population of the wild chili
plant, Capsicum chacoense
– native to Bolivia, Argentina, and Paraguay
– A plant can be chemically defended against insect
herbivores and fungal pathogens by producing
capsaicin… or not.
Getting started…
• Record a series of six numbers, selected from
1 to 10, in your notebook (e.g. 2, 7, 3, 3, 8, 1)
• We will run six iterations of reproduction of
your chili pepper population: 3 modeling
natural selection and 3 modeling genetic drift.
• Following each iteration, you should record
the class histogram in your notes.
Let’s look at the distribution of
chemical defense in our population.
50%
50%
CQ: Stand up and poll your color.
(sw
w
Ye
llo
Pi
n
k(
sp
i
cy
)
ee
t)
1. Pink (spicy)
2. Yellow (sweet)
Iteration 1a: if you picked 6 or less and you
have a yellow card, then sit down because
your chili plant has died.
33%
33%
33%
d
an
w
Pe
r is
he
d.
o.
..
nd
re
pr
Ye
llo
Pi
n
ka
CQ: Which one describes your
pepper?
1. Pink and reproduced
2. Yellow and reproduced
3. Perished
re
p.
..
If you survived, you successfully
produce one offspring.
Iteration 1b: if you picked 5 or more and
you have a yellow card, then sit down
because your chili plant has died.
33%
33%
33%
d
an
w
Ye
llo
Pe
r is
he
d.
o.
..
re
pr
nd
ka
Pi
n
CQ: Which one describes
your pepper?
1. Pink and reproduced
2. Yellow and reproduced
3. Perished
re
p.
..
If you survived, you successfully
produce one offspring.
Iteration 1c: if you picked 1,3,5,7,9, or 10
and you have a yellow card, then sit down
because your chili plant has died.
33%
33%
33%
d
an
w
Ye
llo
Pe
r is
he
d.
o.
..
re
pr
nd
ka
Pi
n
CQ: Which one describes
your pepper?
1. Pink and reproduced
2. Yellow and reproduced
3. Perished.
re
p.
..
If you survived, you successfully
produce one offspring.
What just happened?
• Discuss with your neighbor:
– Which of the four mechanisms just occurred?
– How would this occur in real-world terms? (i.e.
translate the instructions for which plants died
into a real-world scenario)
• What are general attributes of this
mechanism?
CQ. Why does the capsaicin trait continue
to persist in wild chili plant populations?
1. The persistence of the trait is favorable for
the species.
2. Individuals that produce capsaicin were
more likely to have offspring that survived.
3. The fungal pathogen only targets the plants
with capsaicin.
4. The capsaicin trait continues to occur due
to chance alone.
Iteration 2a: regardless of color, if you
picked 2, 4, or 6, then sit down because
your chili plant has25%
died.
25% 25% 25%
d
an
w
lo
Ye
l
d.
Pe
r is
he
re
p.
..
ro
.. .
Pi
n
ka
nd
re
p
on
e
hi
ch
CQ
:W
CQ: Which one describes
your pepper?
1. Pink and reproduced
2. Yellow and reproduced
3. Perished.
...
If you survived, you successfully
produce one offspring.
Iteration 2b: regardless of color, if you
picked 5, 7, or 9, then sit down because
your chili plant has 25%
died.
25% 25% 25%
d
an
w
lo
Ye
l
d.
Pe
r is
he
re
p.
..
ro
.. .
Pi
n
ka
nd
re
p
on
e
hi
ch
CQ
:W
CQ: Which one describes
your pepper?
1. Pink and reproduced
2. Yellow and reproduced
3. Perished.
...
If you survived, you successfully
produce one offspring.
Iteration 2c: regardless of color, if you
picked 4, 5, or 6, then sit down because
your chili plant has25%
died.
25% 25% 25%
d
an
w
lo
Ye
l
d.
Pe
r is
he
re
p.
..
ro
.. .
Pi
n
ka
nd
re
p
on
e
hi
ch
CQ
:W
CQ: Which one describes
your pepper?
1. Pink and reproduced
2. Yellow and reproduced
3. Perished.
...
If you survived, you successfully
produce one offspring.
What just happened?
• Discuss with your neighbor:
– Which of the four mechanisms just occurred?
– How would this occur in real-world terms? (i.e.
translate the instructions for which plants died
into a real-world scenario)
• What are general attributes of this
mechanism?
• Let’s compare the six histograms from our
simulation.
– What is similar in the first three and how is that
different from the second three?
Evolutionary
mechanism
Natural
selection
Genetic drift
Gene flow
Mutation
Definition
Was there a
relationship
between
phenotype
and survival?
Is a higher
percentage of
individual
protected
from
pathogens?
Has
adaptation
occurred?
Has
evolution
occurred?
There are two remaining
mechanisms…
What about when seeds disperse?
• Migration (or gene flow) refers to when
individuals (and their genes) move from one
population to another.
• How could we simulate migration with our
notecard activity?
– Write down a set of instructions and sketch the
resulting histogram.
What about if a plant mutates and produces a
new chemical that is sweet and tasty?
• Mutations can be deleterious or beneficial, and
can result in a new trait appearing in a
population.
– It can also cause an individual’s offspring to switch to
another phenotype that exists in the population.
• How could we simulate mutation with our
notecard activity?
– Write down a set of instructions and sketch the
resulting histogram.
When a population evolves,
• Gene frequencies change.
• This can be a result of
– Selection favoring one phenotype over another.
– Genetic drift causing one phenotype to increase
with or without a concomitant increase in fitness.
– Gene flow introducing (or removing) individuals to
the population.
– Mutation that converts some individuals from one
phenotype to another.
CQ: Which of the following is a key difference
between natural selection and genetic drift?
0%
0%
c. .
.
is
Dr
ift
Dr
ift
ge
n
er
di
re
at
e
...
.. .
0%
fa
vo
rs
t
Dr
ift
oc
c
ur
s
i..
.
0%
Dr
ift
1. Drift occurs in the absence of natural
selection.
2. Drift favors the phenotype that increases
survival, while selection generates a shift
in frequencies towards the more
common phenotype.
3. Drift generates a random shift in
phenotypic frequencies, while selection
generates a shift toward the
environmentally favored phenotype.
4. Drift is directed towards the less favored
phenotype, while selection is directed
toward the more favored phenotype.
CQ: Evolution results in a better match
of an organism and the environment.
1. Yes
2. No
3. Sometimes
0%
So
m
et
im
es
0%
No
Ye
s
0%