Natural Selection
1. Hand in your Simulation Stations labs
Homework
• Test+ due Friday 4/29
• Quiz tomorrow on what we do today
• If you received a parent letter, remember to
bring it back signed tomorrow
Objectives
• Be able to explain changes in allele
frequencies in terms of natural selection
– The outcome of variation, inheritance,
overreproduction, and limited resources
Recap
•
•
•
•
Individuals and groups of individuals vary.
Varied traits come from varied alleles.
These variations are heritable.
Variations can confer reproductive
advantages or disadvantages.
• Organisms reproduce more than can possibly
survive.
– Competition, struggle for life, is the inevitable
result.
Differential Reproductive
Success
• Generalize: What did you observe in
the different simulations? What was
happening and how?
Differential Reproductive
Success
• You saw differential reproductive
success, which means that some are
more successful at reproducing than
others.
Natural Selection
• Inherited variation + Differential
reproductive success + Competition =
Natural Selection.
– Whenever you put these three elements
together, whether in a computer chip’s
design, a satellite’s orbit, or a group of
living things, this is the result…
Natural Selection
• Natural Selection = The process by organisms
bearing favorable heritable traits produce more
offspring. (The reverse also applies: unfavorable
heritable traits yield fewer offspring.) N.S. can
increase or decrease genetic diversity.
– The term was originally coined by Darwin, who decided
later he didn’t really like it. He didn’t favor it because
organisms don’t “select” themselves and nature doesn’t
“select” them, it’s just something that happens.
– Take out the flowchart, please…
Flowchart
• On average,
the organisms
best suited for
survival and
reproduction
leave the most
offspring =
natural
selection
Flowchart
• Natural selection
changes the
genetic
composition of a
population over
time. This
change in gene
frequency =
evolution.
Natural Selection
• 1. The laptop simulation
– What was the favorable heritable trait?
– Did genetic diversity increase or decrease?
– Were the grabbers “choosing” to live or “choosing”
to sacrifice themselves? Did the population
“choose” to develop longer grabbers?
– Why did almost all of the grabbers have long arms
200 generations later?
– Notice, no grabber developed a longer arm, they
just had babies with longer arms. Groups (allele
frequencies) change over time, groups evolve,
individuals don’t!
QuickTime™ and a
decompressor
are needed to see this picture.
Natural Selection
• 2. Peppered Moths (cool true story!)
– Did the hawks back in 1847 say to themselves, “I’ll
make that moth species stronger by picking off the
weak ones?”
– Why did they eat more white moths back then and
black moths now?
– Important lesson from that story: A trait that’s
favorable in one environment can be unfavorable
in another place or time. Traits aren’t inherently
beneficial, it all depends on the context.
Natural Selection
• 3. Candy Dish
– Explain how natural selection was
evidenced in this simulation.
Natural Selection
• 4. Hairless Bunnies
– Did genetic diversity increase or decrease?
– Did the red beans become more common
because they were dominant or because they
were beneficial?
• Alleles don’t increase or decrease because of
dominant/recessive!
– Apply evolutionary thinking: Could the
environment change to make hairlessness the
beneficial trait and hair the unfavorable one?
Natural Selection example
• Dr. Sean Carroll teaches a 10th-11th
grade group about the Rock Pocket
Mouse in 2005 (Ch 28-35).
Antibiotic Resistance
• Real life example of the consequences
of not taking natural selection into
account…
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Fitness
• A term we’ll be using will be fitness, though it
DOESN’T mean “strength” or “endurance”
like it does in the gym.
• (Reproductive) Fitness = An organism’s ability
to pass its genes down to future generations.
Often measured in number of grandchildren.
– An organism can be weak and wimpy and “unfit”
by the gym definition, but “fit” by the biology
definition if it has more grandchildren than its
competitors.
– For instance, “sneaker” crickets.
http://evolution.berkeley.edu/evolibrary/article/0_0
_0/sneakermales_01
Fitness
• Other examples of biological fitness that isn’t “gymfit.”
– Some males of marine isopods are extremely tiny, putting
their energy into making extra sperm instead of making
extra body size. They let bigger tougher males fight it out
over a female, then release a cloud of sperm when he’s
mating with her.
– Unhealthy male salamanders undermine the competition
by playing the role of a female in courtship dances, then
destroying the tougher male’s sperm packet.
– As mountain average temperatures rise, mice that used
to be too thin now have an advantage in temperature
regulation.
Natural Selection
• We’re going outside to play another
natural selection game, and when we
return, you will explain what happens
using the following vocab: Variation,
allele frequency, inheritance,
overreproduction, competition, natural
selection, fitness.
Worms of a Different Color
• You are birds, and your diet consists of worms (toothpicks).
The worm population is 250.
• Your goal is to reproduce.
– If you don’t get enough food (10 worms), you’ll die outright.
– If you get enough food to live, you still may not have enough left over to
feed any young.
• Of those who get enough worms to live, only the top 50% will be able
to find a mate and reproduce.
• You may not touch other birds, or steal worms they’ve caught.
You may not leave the area. When I say stop, you put your
hands in the air and freeze.
• Write a prediction in your notebook, what will happen to the
worms and the birds?
Worms of a Different Color
• Starting worm numbers by color:
– Red: 50
– Orange: 50
– Yellow: 50
– Green: 50
– Blue: 50
Worms of a Different Color
• Everyone please count up your worms by color.
Keep your personal total in mind, and report
back to me the number of worms by color.
• Worms eaten:
– Red:
– Orange:
– Yellow:
– Green:
– Blue:
• Which phenotype conferred the greatest fitness
advantage to the worms?
• What could happen to the environment to
change that?
Evolution
• So we’ve covered variation, inheritance,
competition, natural selection… but
what’s evolution?
• We’re going to fill in that central
statement at the bottom of the flowchart.
Evolution
• _____ _______ ______ over _______ =
_____________
Evolution
• Allele _______ ______ over _______ =
_____________
Evolution
• Allele frequencies ______ over _______ =
_____________
Evolution
• Allele frequencies change over _______ =
_____________
Evolution
• Allele frequencies change over
generations = _____________
Evolution
• Allele frequencies change over
generations = Evolution
• Evolutionary thinking takes some work to
acquire, but the basic definition is simple.
It’s just “change over time.”
Evolution by Natural
Selection
• http://www.pbs.org/wgbh/evolution/librar
y/11/2/quicktime/e_s_4.html
Evolution
• Natural selection is a major source of evolution.
There are others, like genetic drift, that we’re not
going to dwell on.
– The MCAS is going to give you a ton of scenarios and
ask you to either 1) predict what’s going to happen as a
result of evolution by natural selection, or 2) explain how
things got to be as they presently are through evolution
by natural selection.
• And there are some kinds of wrong answer that
can look right at first that they love to tempt you
with. I’m going to teach you how to overcome.
Misconceptions & Common
Mistakes
• Look at the giraffe example on the reverse of
your flowchart.
• “A population of giraffes lives on the Serengeti.
The best leaves that provide the most nutrition are
at the tops of the trees. In one generation, the
average giraffe’s neck is 1.4 meters long. Fifty
generations later, the average giraffe’s neck is 1.55
meters long. How did this happen?”
• Let’s talk about why each of these answers is
wrong:
Misconceptions
• 1. “The more the giraffe stretches its neck
to get to the tops of the trees, the longer its
neck becomes. It passes this longer neck on
to its babies.”
– What’s wrong with this? (an example of
Lamarck’s hypothesis, by the way)
Misconceptions
• 1. “The more the giraffe stretches its neck to get to the
tops of the trees, the longer its neck becomes. It passes
this longer neck on to its babies.”
– Groups evolve, individuals don’t. Your DNA doesn’t
change over your lifetime.
– You don’t pass on to your children things that happen to
you in your lifetime. If you break your arm, your children
are not born with broken arms or scarred armbones,
because it doesn’t affect the genes in your gametemaking cells.
• Famous mouse experiment (Weismann 1889) Called
“inheritance of acquired characteristics,” idea rejected by
scientists 100+ years ago but high schoolers still fall for it on
their exams all the time!
Misconceptions
• 2. “All giraffes with shorter necks die. Only
the longer-necked giraffes survive to have
babies.”
• What’s wrong with this?
Misconceptions
• 2. “All giraffes with shorter necks die. Only the
longer-necked giraffes survive to have babies.”
• Do you really think that having a neck one inch
shorter automatically results in death?
– Most of the time, natural selection is just about who
has MORE babies. If you have more babies, the next
generation has a higher frequency of your alleles, and
the generation after that will have an even higher
frequency. Examples where it’s either “survive and
reproduce, or don’t survive at all” are fairly rare.
Misconceptions
• 3. “Because giraffes are supposed to have
long necks.”
• What’s wrong with this?
Misconceptions
• 3. “Because giraffes are supposed to have
long necks.”
– Evolution has no destined endpoint.
– The direction that evolution takes depends
on the environment at the time, and that
direction is constantly changing. There’s no
ideal giraffe that all giraffes aspire to.
Misconceptions
• 4. “A longer neck is dominant, so over
time, the dominant longer neck allele
becomes more and more common.”
• What’s wrong with this?
Misconceptions
• 4. “A longer neck is dominant, so over
time, the dominant longer neck allele
becomes more and more common.”
– Alleles do not ever become more common
because they are dominant or recessive. They
only become more common if they confer a
reproductive advantage.
Misconceptions
• 5. “They got longer necks because they had to. If
the giraffe species doesn’t get a longer neck, then
it will eventually go extinct because there will be
no leaves left at low levels of the tree. So the
giraffes adapted themselves to the new
environment and evolved longer necks to keep the
species alive.”
• What’s wrong?
Misconceptions
• 5. “They got longer necks because they had to….”
• Several major problems:
– 1. Evolution isn’t a choice. Species don’t have a
will, they don’t “decide” to do it. You can’t alter
your DNA sequence.
– 2. There’s no “supposed to happen” or “has to
happen” or “needs to happen.” Never use these
phrases.
– 3. Species don’t say to themselves “if I don’t pass
that allele on to my babies, my species will go
extinct!”
Natural Selection
• The format for answering a natural selection
question:
–
–
–
–
Here’s what varies.
That variation has a genetic basis, so it’s inherited.
Here’s what the limited resource is.
Look, more babies are being born than can
survive.
• There is competition as a result.
– This trait gives its bearer an advantage in
reproducing.
– The individuals with that trait have more babies
than others, so in the next generations, that trait is
more common.
Misconceptions
• So, what’s an accurate explanation for
why the average giraffe neck was .15
meters longer after fifty generations?
– With your partner, write a short paragraph
explaining this. Write as though it’s one of
the MCAS essays (and it very well may
be…)
Natural Selection
• In the original giraffe population, the mean neck length was
1.4 m but individuals varied in terms of the length of their
necks. Those individuals with the allele/s for longer necks
were able to reach more nutritious leaves, and so they had
more energy to produce more and healthier offspring.
These offspring inherited the longer necks of their parents.
Because there were more babies of longer-necked
parents, the next generation had a slightly longer neck on
average. This trend continued, with longer-neck alleles
giving a reproductive advantage to the giraffes that carried
them. These giraffes continued to out-reproduce their
competitors, and their alleles appeared more and more
frequently in the gene pool. Thus, in time, natural selection
produced a giraffe generation with an average neck length
of 1.55 m.
Natural Selection
• Depending upon the environmental
conditions and the nature of the trait,
natural selection can have different
outcomes. These are three “varieties”
of selection…
• Directional Selection =
Occurs when natural
selection favors a single
phenotype.
– Example: Hairless
Bunnies, most of our
simulations so far
because it’s easiest to
visualize.
QuickTime™ and a
decompressor
are needed to see this picture.
• Stabilizing Selection =
Occurs when natural
selection favors an
average, middle-of-theroad trait.
– Example: Birth weight.
Being born very light
leads to survival and
reproductive
disadvantage, and so
does being born very
heavy.
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
• Disruptive Selection =
Occurs when natural
selection favors both
extremes.
– Example: Some reef fish. If
you can blend in with your
surroundings, you’re more
likely to evade predation and
make more baby fish. If
there’s blue coral and yellow
coral but no green coral,
selection favors the two
extremes.
Natural Selection
• A common misconception is that natural
selection is random.
– If it were random, you wouldn’t have been
able to make predictions about the
outcomes of those simulations.
– I’m also going to illustrate this through a
card game.
Natural Selection
• We have two suits from a deck of cards (2ace). You thoroughly shuffle the cards, and
then your goal is to get them into a stack that
is in order (2, 3, 4, 5, 6, 7, 8, 9, 10, J, Q, K,
A). The suits are racing against each other to
reach this goal with the fewest shuffles, and
these are the rules:
– One deck will be the “Full Shuffle” deck. The way
it works is, check and see if this deck is in order. If
it isn’t, reshuffle it thoroughly. Check it again. If it
isn’t, reshuffle it again. Continue until you have a
deck in order.
Natural Selection
• The other deck is the “Selective Shuffle” deck.
Check to see if the TOP CARD ONLY is correct (a
two, to begin with). If it isn’t, shuffle and check
again. Once the top card is correct, set it aside and
reshuffle. Now, you’re going to keep shuffling and
checking until the top card is a 3. Repeat until you
have a deck in order.
• Any predictions? Which deck will achieve the goal
faster? How many shuffles will this take?
Card Game
• The “Selective Shuffle” deck is almost always
done first (try it at home!). Why?
– The probability calculation for getting the Full Shuffle
deck in order is 1313 shuffles = 3.029 x 1014, or
302,900,000,000,000 shuffles. Every card has to be
in the right position at the same time in order to win.
– The probability calculation for getting the Selective
Shuffle deck in order is 13 + 12 + 11 + 10 + 9 + 8 + 7
+ 6 + 5 + 4 + 3 + 2 + 1 = 91 shuffles on average.
• This serves as a decent metaphor for natural
selection…
Card Game
• The Selective Shuffle deck was resolved
predictably faster because even though every
shuffle was random, it had a non-random element:
the rule of picking off the top card.
– The shuffles are like mutation. Mutation happens
regularly, and exactly what the mutation will be is
random.
– The rule, however, is like differential reproductive
success. It’s non-random: for a particular environment,
a trait clearly provides an advantage, disadvantage, or
neutrality.
– A non-random rule acting on random fuel still gives you
a non-random outcome. Mutation is random; natural
selection isn’t.
Summary
• Definitions of natural selection, fitness, evolution.
– Natural selection happens whenever there’s variation,
inheritance, and competition.
– Natural selection can increase or decrease diversity.
– Traits can provide a reproductive advantage in one
environment and a disadvantage in another.
• Three kinds of natural selection - directional,
stabilizing, disruptive.
• Mutation is random, natural selection is not.
• Misconceptions = easy traps, recognize them to
avoid them.