Transcript Natural Selection

Natural Selection
Homework
• Reminder, paper due 4/2
Recap
• Who can explain to me what we learned
about variation?
Recap
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Individuals and groups of individuals vary.
Varied traits come from varied alleles.
These variations are heritable.
Variations can confer reproductive
advantages or disadvantages.
• Who can explain to me what we learned
about reproduction rates?
Recap
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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
• What did you observe in the different
simulations?
Natural Selection
• Heritable variation + Differential
reproductive success + Competition +
Time = 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 which
favorable heritable traits become more
common in successive generations. (The
reverse also applies: unfavorable heritable
traits become less common in successive
generations.)
– The term was originally coined by Darwin, who
didn’t really like it but struggled with thinking of a
better one. He didn’t favor it because organisms
don’t “select” themselves and nature doesn’t
“select” them, it’s just something that happens.
– All of the stations simulated natural selection…
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.
This change in
gene frequency
= evolution.
Natural Selection
• 1. The laptop simulation
– What was the favorable heritable trait?
– Were the grabbers “choosing” to live or
“choosing” to sacrifice themselves?
– Why did almost all of the grabbers have
long arms 200 generations later?
Natural Selection
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2. C. plastica
What was the favorable heritable trait?
What was the unfavorable heritable trait?
Did anyone select which of you would live,
reproduce, or die, or was it just the result of
how many beans you got?
• What were your results? Why did things turn
out that way?
Natural Selection
• 3. Pepper 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
• 4. Candy Dish
• Explain how natural selection was
evidenced in this simulation.
Natural Selection
• 5. Hairless Bunnies
• Did any groups end the simulation with
some f alleles still in existence? Why
were they still there?
• Apply evolutionary thinking: Could the
environment change to make
hairlessness the beneficial trait and hair
the unfavorable one?
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, “unfit” by
the gym definition, but have many more
grandchildren than its competitors and so it’s
highly fit by the biological definition.
– For instance, “sneaker” crickets.
Natural Selection
• We’re going outside to play another
natural selection game, and when we
return, you must write a paragraph
(graded) explaining what happens using
all the following vocab: Variation, allele
frequency, inheritance,
overreproduction, competition, natural
selection, fitness.
Worms of a Different Color
• You will all be birds, and your diet consists of
worms (toothpicks).
• Your goal is to reproduce.
– If you don’t get enough food, 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 can try to sabotage the others, though
you’re not allowed to make physical contact.
• 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:
– Green
– Yellow
– Red
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:
– Green
– Yellow
– Red
• Which phenotype conferred the greatest
fitness advantage to the worms?
• What could happen to the environment to
change that?
Worms of a Different Color
• You must write a paragraph (graded)
explaining what happens using all the
following vocab: Variation, allele
frequency, inheritance,
overreproduction, competition, natural
selection, fitness.
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
• 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.”
– 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. “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 evolved
longer necks to keep the species alive.”
• What’s wrong with this?
Misconceptions
• 3. “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 evolved longer necks to keep the
species alive.”
• A species doesn’t have a will, it doesn’t
“decide” or “choose” to survive or evolve, and
it certainly doesn’t choose the direction of its
survival or evolution. Evolution is just that
babies are unlike their parents, and some
parents have more babies than others. It’s
not a decision.
Misconceptions
• 4. “The giraffes need long necks to eat, so
nature drove or allowed them to develop
longer necks.”
• What’s wrong?
Misconceptions
• 4. “The giraffes need long necks to eat, so nature
drove or allowed them to develop longer necks.”
• “Mother Nature” is a folk saying, there’s no
physical thing that is Nature. The planet
Earth doesn’t have a giant rock brain that’s
handing out directions and restrictions when it
feels like it or sees a “need.”
Misconceptions
• 5. “A giraffe tries to adapt itself to the new
environment, and it passes that adaptation
on to its babies.”
• What’s wrong?
Misconceptions
• 5. “A giraffe tries to adapt itself to the new
environment, and it passes that adaptation
on to its babies.”
• Two major problems:
– 1. Individuals don’t evolve, only groups do.
The giraffe can’t change its genetic code
when it feels like it.
– 2. Evolution isn’t a choice. Critters don’t
“decide” or “choose” to do it.
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
• Ticket to leave: To be able to leave the
classroom, you must explain how
natural selection works to the
satisfaction of Ms. Weekes or myself.
• You may do this verbally, or in writing.
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
• Each small group gets two suits from a deck
of cards (2-ace). You’ll 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.
• To ensure randomness, have two people working
each deck: one person checks and keeps track of
the number of shuffles, and the other shuffles
blindly.
• Any predictions? How many shuffles will this take?
Card Game
• The “Selective Shuffle” deck is almost always
done first. 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. Every card has to
be in the right position 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.
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
• You need to hone your ability to apply
evolutionary thinking and your
knowledge of natural selection to
scenarios from the natural world.
• For remainder of period, you have time
to work on your worksheet. Let’s go
through one of the problems together,
be prepared to explain using this week’s
concepts.