Natural Selection

Homework

• Reminder, paper due 4/2

Recap

• Who can explain to me what we learned about variation?

Recap• 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• 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• 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 gamete-making 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.

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• Stabilizing Selection = Occurs when natural selection favors an average, middle-of-the-road trait.– Example: Birth weight. Being born very light leads to survival and reproductive disadvantage, and so does being born very heavy.

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• 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.

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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.