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Natural selection
Outline of the day
1. Turn in your lab reports at the front
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Both the tidepool trip report and the horticulture
tour worksheet
More than 10 minutes late = bad
Any questions on last week’s lab?
Quiz
Introduction to the lab
Lab!
Check out
2.
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6.
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Get a stamp
Make sure I mark you down for attendance
Quiz
• Ends 8 minutes after it’s started
– Ends at: ____
Lab this week!
• Looking at evolution by natural selection
– Brief introduction
– Modeling natural selection with tools
– Selecting on alleles
A question
How might we be able to explain the
diversity of life around us?
--OR-How do you think all the species on earth
came to be?
Evolution
• “Descent with modification” - Darwin
• “The change in a lineage of populations
between generations” - Ridley 1993
• “Any genetic and resulting phenotypic
change in organisms from generation to
generation” - Purves et al. 1995
Evolution is change in a
population over time …
• But how does this change over time
occur?
Natural selection
• Selection on traits that have heritable variation
– Those individuals that produce the most surviving offspring
pass on their traits to more individuals than those individuals
producing fewer offspring
• Thus certain traits increase in prominence in a population
• In other words:
– The organisms that are best able to survive and reproduce
• Have the most offspring
– And thus more of the next generation of that population is made
of up organisms with the genes of the best surviving and
reproducing individuals
» So the population has changed over time.
» Lather, rinse, repeat (and combine with a heterogeneous and
changing environment) to cause speciation, etc.
A catch-phrase we’ve all heard …
• “Survival of the fittest”
• More accurately, “Reproduction of the fittest”
– Survival is NOT the key, reproduction is
• Individuals that have the highest reproductive
output have the highest fitness
– Fitness ranges from 0 to 1
• 0 = no offspring
• 1 = tons of surviving (and reproducing) offspring
A key point
• Evolution occurs at a population level, not an
individual level
– An individual’s genetic makeup is fixed; it’s not going
to change.
– But the genes contained in a population can change
over time (as less fit individuals die / reproduce less,
and more fit individuals survive and reproduce quickly)
• So a population of squid can evolve in response
to a new predator being introduced …
– But a single squid can not.
– Let’s watch squid respond to a predator that
selectively kills only small squid :)
Population with variation;
Lots of small and big squid
(all of them happy)
New predators start eating small squid
Squid!
(A quick,
hypothetical
example)
Small squid get eaten
Large squid survive and
reproduce
Over time all small squid are killed;
only large squid are left in the population.
The population of squid has evolved!
Images CC from: population - http://flickr.com/photos/bluesmoon/290785871/l;
small - http://flickr.com/photos/maynard/842182839/; giant http://flickr.com/photos/checco/328347060/
For natural selection to occur
three components are needed
1. Selection – some sort of selective force
that alters organisms’ fitness
2. Heritable traits – the selective force
needs to select on some trait that is
heritable
3. Variation – there needs to be variation in
heritable traits on which the selective
force can differentially act
Natural selection
• “The process by which individuals with
certain heritable traits tend to produce
more surviving offspring than individuals
without those traits, resulting in a
change in the genetic makeup of the
population.” – Freeman 2002
What might cause natural
selection?
• Anything that can cause a change in the
reproductive output (fitness) of an organism!
• Environmental pressures
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Droughts, flooding, fires
Heat waves / changing temperatures
Amount / type of food available
Number of mates available
Predation
• Internal pressures
– Circulatory / respiratory system’s ability to provide O2 /
nutrients
– Balance of reproduction with other body maintenance
tasks (do you grow or reproduce?)
– Ability of an organism’s skeleton to support its body
What are we doing in lab today?
• Modeling evolution by natural selection
• We’re going to become predators
– We’ll be competing for prey
– Our population will start out with a variety of
mechanisms of capturing prey
– The more prey you eat, the more you’re going to be
able to reproduce
• We’ll go through three generations of selection
How will this work?
1. Each person will start out with a different mouthpart
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Represented by tools: forceps, pliers, rules,
screwdrivers, spoons, etc.
2. We’ll go outside to capture “prey” (toothpicks)
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Must pick up prey using your mouthpart (tool) and move
it into your stomach (beaker)
Cannot steal food from another person’s stomach
(beaker), but can steal food at any other time
Do not harm your classmates!
3. We’ll come back in and count how much food
everyone captured
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Those mouthparts that captured the most food will
reproduce the best; those that captured the least will die
out
Let’s go do this!
What we want to do now:
1. Count the number of toothpicks you
captured
2. Add up the total number of toothpicks
that were captured by those with your
mouthpart type
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Give me this number!
3. Reassign mouthpart types and do it
again!
Questions:
• Were the three components of natural
selection (selection, heritability, and
variation) present in our model?
• How would our results have changed if the
food-type changed?
– (e.g., if we changed eating to turning a screw
or filling up a cup with water)
Next: Do the marble selection
• This models selection at a population genetics level
– Black and white marbles represent different alleles
• An individual has two alleles
– Those two alleles will be drawn at random from a population of
alleles
• This is much like how real reproduction work – you randomly
combine two alleles from your parents!
– The chart indicates which individuals live, and which have lethal
mutations
• Carry out this selection for 5 generations for both
scenarios (recessive lethal gene or dominant lethal
gene)
• Remove any individuals who have lethal mutations from the
population before doing the next generation
Before you leave
• Clean up your work area
• Show me your lab report so I can stamp it
– Need to have all data fields filled in
– Complete at home and then turn in at the beginning of next lab
• Remember that we’ll have a quiz at the beginning AND
end of the next class
– First quiz:
• 6-7 questions on today’s lab
• 3-4 questions on the lab we’ll do next week
– Second quiz:
• On next week’s lab
Notes for the instructor:
• I add quite a bit of introduction to this lab, as well
as discussion afterwards, as I feel that our
simulation isn’t intuitively linked to evolution by
natural selection, especially if the students
haven’t thought about evolution for many weeks.
Thus, I think the intro and wrapup discussion
help clarify that link, as well as help provide
additional clarification on the topic. I may be
going overboard, though.
• Add any relevant cleanup instructions to the final
slide (that slide is a generic one I’m adding to
each presentation).
License information
• This work is licensed under the Creative Commons AttributionNonCommercial-ShareAlike 3.0 License. To view a copy of this
license, visit http://creativecommons.org/licenses/by-ncsa/3.0/us/ or send a letter to Creative Commons, 171 Second
Street, Suite 300, San Francisco, California, 94105, USA.
• The slides in this presentation were originally created by Marc
C. Perkins (http://faculty.orangecoastcollege.edu/mperkins).
• You are free to use, modify, and distribute these slides
according to the terms of the Creative Commons license (e.g.,
you must attribute the slides, no commercial uses are allowed,
and future distributions must be licensed under a similar
license).
• Attribution should be given to Marc C. Perkins (and any later
editors), including a link back to Marc’s current website. This
applies both while distributing the slides and during use of the
slides; attribution during use can be satisfied by, for instance,
placing small text on at least one of the slides that has been
shown (see below for an example).
History
• August 2007: Marc Perkins released first
version.
http://faculty.orangecoastcollege.edu/mperkins
(If you modify these slides and redistribute them, add your information to the list)