Download Summer BIO152

5/25/2005
Summer BIO152
Lectures 5 & 6
Key topics this week
|
Natural selection
Darwinian fitness (and adaptations)
z Artificial selection
z Types (directional, stabilizing,
disruptive)
z
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Alternative explanations: inheritance
of acquired traits (‘Lamarckian’)
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Natural selection
Write out the two conditions which must be
met for natural selection to occur:
1.
Individuals in a population must vary in
characteristics which are heritable.
2.
In a particular environment,
certain versions of the heritable
trait(s) help individuals survive &
reproduce more offspring.
Natural selection: 4 steps
(Chapter 23)
1.
2.
3.
4.
Individuals in a population must vary in
their characteristics (traits).
Some of these variable traits are heritable
(passed on to their offspring).
Only some offspring survive long enough
to reproduce.
Individuals with certain traits are more
likely to produce more offspring in a
given environment. (differential
reproductive success)
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Evolution
(= change in the characteristics of a
population over time)
Results when differential reproductive
success is based on heritable
variation.
Darwinian fitness
The ability of an individual to produce
more offspring relative to the ability of
other individuals in a population to
reproduce.
| Darwinian fitness is measurable.
Count the offspring
How does the meaning of Darwinian fitness
differ from common English usage of
fitness?
|
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Adaptation
A heritable trait that increases the
fitness of an individual in a particular
environment relative to individuals
without that trait.
Adaptations increase [Darwinian]
fitness---the ability to produce
offspring.
Tutorials 23.1 & 23.2
|
Check your understanding
Compare these two examples of change
in beetle populations. Which one is an
example of evolution by natural
selection?
Evolution = change in the [heritable]
characteristics of a population over
time.
http://evolution.berkeley.edu/evosite/evo101/IIIADescent.shtml
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Beetles on a ‘diet’
Imagine a year or two of drought in
which there are few plants that
these beetles can eat.
The beetles in the population are
a little smaller than the
preceding generation of
beetles
All the beetles have the same
chances of survival and
reproduction, but is the
change in size due to food
restrictions?
How would you test to see if evolution
of the beetles had taken place.
| Stated differently, how would you test
to see if the size change was due to
environmental conditions or to a
change in the genetic makeup of the
population perhaps due to natural
selection?
|
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4 steps of natural selection
Variability
| Heritability
| Some beetles have more offspring
| Beetles with a specific trait have more
offspring
|
Different coloured
beetles
Most of the beetles in a
population (say 90%)
are bright green but a
few of them (10%) are
brown.
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Some number of
generations later,
things have
changed: brown
beetles are more
common than they
used to be and
make up 70% of
the population.
Is this evolution?
What questions do you need to ask?
| Is colour heritable—how would you
find out?
| Have the proportion of heritable
traits changed in the population?
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Is this evolution?
To see if natural selection had taken
place what other questions do you
need to ask?
1.
2.
3.
4.
|
Are there variable traits (yes-green
and brown)
Is colour heritable (yes)
Do some beetles have more
offspring?
Do brown beetles have more
offspring than green?
Which example illustrates descent
with modification—a change in gene
frequency over time?
Smaller beetles following limited food
Proportion of brown/green beetles
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(e.g., 1) The difference in weight due to
environmental influences—the low food
supply—likely not because of a change in
the frequency of genes.
Example 1 is not evolution because the small
body size in this population was not
genetically determined, this generation of
small-bodied beetles will produce beetles
that will grow to normal size if they have a
normal food supply.
(2) The changing proportion of color in
the population of beetles is evolution:
the population of beetles has changed
over time from predominately green to
predominately brown.
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Level of action
Natural selection acts on
INDIVIDUALS within a population
(some individuals survive and
reproduce more than other
individuals)
| Evolution acts on POPULATIONS-the
characteristics of the population
changes over time.
|
Natural Selection in action
Chapter 23 pp 506-508
Also The Beak of the Finch, John Weirner
Peter & Rosemary Grant Galapagos Islands: Daphne Major
During severe drought in 1977:
The struggle to survive favored the larger birds with
deep, strong beaks for opening the hard seeds.
Smaller finches with less-powerful beaks perished.
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The Grants found that the offspring of
the birds that survived the 1977
drought tended to be larger, with
bigger beaks.
Hypothesis: the adaptation to a changed
environment led to a larger-beaked
finch population in the following
generation.
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Did natural selection take
place?
Variable beak size—yes
| Heritable- (yes, birds with larger
beaks had offspring with larger beaks)
| Differential reproductive success-yes,
huge die back, only birds with larger
beaks survived
|
Unusually rainy weather in 1984-85 ►
More small, soft seeds & fewer of the large,
tough ones.
The birds best adapted to eat those seeds
because of their smaller beaks were the
ones that survived and produced the most
offspring.
Evolution had cycled back the other direction.
It is possible to see evolution happen!
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More evolution in action
Chapter 23
Mycobacterial tuberculosis resistance to
antibiotics (also lab 3) plus tutorial
23.1
Alpine skypilot plants plus tutorial 23.2
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Natural Selection: Case
Study 3
http://evolution.berkeley.edu/evosite/evo101/IIIE1aTegula
.shtml
Some species live only in deep water, and
some only live much higher up the shore.
Tegula funebralis, a snail, can be found in
both (California):
In Southern California, Tegula live high up on
the shore, while in Northern California, they
live in deeper water.
NS: Snails in California cont.
Can natural selection explain the
different niches? (Michael Fawcett
study)
He found that predators, such as octopi,
starfish, and crabs, were more abundant in
southern California than in northern
California.
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Snail distribution
Hypothesis:
Intense predation in the south selected
for snails that lived higher up the
shore, out of reach of many predators.
In the north, selection might not have
been as strong, so the snails were not
selected to live high on the shore.
Snail Experiment
M. Fawcett took northern and southern
snails, released them in deep water in
the south and watched what
happened.
If predators were around, all the snails
high-tailed it towards higher ground
(snails can probably sense the
chemicals exuded by predators).
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Results
Southern snails moved
further up the shore
faster than northern
snails.
Because the northern
snails were slower and
didn’t move high
enough, they were more
likely to be eaten by
predators.
What did this experiment
show?
1.
2.
There is a difference between southern
and northern snails (i.e., some difference
that is not merely a function of being on a
southern or northern shore). This
difference is probably genetic (but we
would need to do more experiments to be
absolutely sure).
This difference can lead to differential
survival. If predation is intense, snails that
move higher faster are more likely to
survive.
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Natural selection again?
|
Variation: There is variation in a trait between and
within populations.
|
Heredity: The variation probably has a genetic basis.
|
Differential reproduction: The variants of the trait
have different probabilities of surviving to reproduction.
These three features define natural selection. Without
them, natural selection does not happen.
http://evolution.berkeley.edu/evosite/evo101/IIIE1aTegula.shtml
23.4 The Nature of Natural Selection and
Adaptation
Although natural selection appears
to be a simple process, research
has shown that it is often
misunderstood.
|
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Selection acts on individuals, but
evolutionary change occurs in
populations
Individuals do not change during
natural selection.
Those that are selected simply
produce more surviving offspring
than other individuals do, causing a
change in the genetic makeup of the
population.
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Acclimation occurs when an
individual changes in response to
changes in the environment (e.g.,
beetles changed in size from less
food)
|, but adaptation occurs only when
a population changes in response to
natural selection.
|
Preparation for Thursday
Lab 2: print, read, prepare table (2
copies!) also Major Project- copy of
log Friday-Wednesday; write up your
method
| Read Chap 24.2 types of selection:
|
Directional
z Stabilizing
z Disruptive
Do tutorial 24.2 modes of Natural
Selection
z
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