Link back to the Teacher’s Guide Table of Contents

Contents

1. Natural and artificial selection; population genetics
2. Speciation
3. Evidence for evolution
4. Phylogeny
5. Origin of Life

Essential Links

1. The College Board’s AP Bio Course and Exam Description.
2. My Condensed Version of the Course and Exam Description: takes the objectives, Enduring understandings, and key ideas of the 230-page CED and renders it into 40 pages.
3. My AP Exam Review Outline: Takes the Course and Exam description and renders it into student (and teacher) friendly language.
4. 2023-24 AP Bio Scope and Sequence Calendar: A spreadsheet that lays out the entire course.

Topics 7.1 to 7.5: Natural and Artificial Selection; Population Genetics

Unit 7: Introductory Thoughts

First, a gripe about the College Board’s name for Unit 7. It’s “Natural Selection.” That’s way too narrow. One of the major ideas of topics like population genetics and extinction is that not all evolutionary change is selective. There’s a big dose of randomness, which is what genetic drift is all about. There’s also a big dose of contingency in evolutionary history. Who dies in a mass extinction is often about being in the wrong place at the wrong time. The survivors (like our mammalian ancestors who survived that asteroid impact that ended the reign of the dinosaurs 65 million years ago) were often just lucky. Remember to emphasize both ideas to your students.

Topic 7 is a big chunk of our AP Biology course, and the Course and Exam Description allots Topic 7 more class periods and a higher AP exam weighting than any other unit. It’s also, to my mind, the apex of our course: it brings together what you’ve previously taught in every other unit, and it sets the stage for ecology. The College Board suggests about 20 – 23 class periods, and I think that’s about right.

However, I make a few modifications to the CB sequence.

First, I start the year teaching about natural selection and adaptation. Why? Well, if you agree with Dobzhansky that “nothing in biology makes sense except in the light of evolution,” then you need to teach natural selection and adaptation right at the start of your course. Otherwise, phenomena like the fit between enzymes and substrates don’t make sense. The same is true for the complementary match between receptors and ligands. In addition, teaching just a little bit about natural selection sets you up for teaching about heterozygote advantage when you teach about sickle cell disease in units 5 and 6.  If you didn’t do so this year, consider a day or two about natural selection at the start of next year.

Second, I shift the order.

The College Board’s course and exam description lists its evolution topics in this order

1. Natural Selection, Artificial Selection (Topics 7.1 – 7.3)
2. Population Genetics and Hardy Weinberg (Topics 7.4 and 7.5)
3. Evidence of Evolution, Common Ancestry, Continuing Evolution (Topics 7.6-7.8)
4. Phylogeny (Topic 7.9)
5. Speciation and Extinction, Variations in Populations (Topics 7.10 – 7.12)
6. Origin of Life (Topic 7.13)

That’s a good start and end, but I think you’ll get better results if you do the following.

1. Natural Selection, Artificial Selection (Topics 7.1 – 7.3)
2. Population Genetics and Hardy Weinberg (Topics 7.4 and 7.5)
3. Speciation and Extinction, Variations in Populations (Topics 7.10 – 7.12)
4. Phylogeny (Topic 7.9)
5. Evidence of Evolution, Common Ancestry, Continuing Evolution (Topics 7.6-7.8)
6. Origin of Life (Topic 7.13)

That’s because Evidence of Evolution is, to a large degree, about homology. Homology only makes sense in the context of ancestral species splitting apart into descendants, which is speciation and phylogeny. And phylogeny only makes sense if you understand speciation.

That’s the order that I’ll be sharing in my discussion of the unit (below).

Finally, I love teaching about the origin of life. But most years, just to save a bit of time, I have my students complete our Origin of Life module over Spring Break or Winter Break. The only prerequisite is that students understand the central dogma. I’ll share more thoughts about the Origin of Life below.

Learning Objectives: Topics 7.1 to 7.5 (Natural Selection and Population Genetics)

The College Board’s original objectives (see above) for these topics are extensive. Here’s a condensed and guided version of what you need to cover.

Topic 7.1: Introduction to Natural Selection

1. Define adaptation.
2. Explain how while evolution is non-random, the mutations that lead to adaptation are themselves random.
   • Selection is non-random. But If it weren’t for mutation (which is random), all that selection could do would be to cull the population of less adaptive phenotypes. Mutation is what makes the process of natural selection creative and open-ended.
3. Explain how natural selection works:
   •  Inherited variation, followed by selection for beneficial traits and against harmful traits, shifts the average phenotype in a population, leading to adaptation.
4. Describe how evolutionary fitness can be measured
   • As an organism’s ability to survive and reproduce.

Topics 7.2 and 7.3: Natural Selection and Artificial Selection

1. Explain the importance of phenotypic variation to natural selection.
   • Natural selection acts on phenotypic variation in populations. If selective pressure is maintained in the same direction over multiple generations, different phenotypic variations will become more or less common, depending on fitness.
2. Explain how artificial selection works.
   • During artificial selection, humans select favored phenotypes within plant or animal gene pools, shifting the average phenotype in the desired direction.
3. Explain how natural selection acts on phenotypes to shift allele frequencies within populations.
   • The result of selection (artificial or natural) is a shift in allele frequencies. But what’s being selected are phenotypes.
4. Explain the relationship between environmental change and selective pressure
   • As environments change, so do selective pressures. If selective pressure continues in the same direction, there’s directional selection for specific phenotypes. If change is fluctuating, then so will the average phenotype in a population.
5. Distinguish between directional, disruptive, and stabilizing selection*

Topics 7.4 and 7.5: Population Genetics and Hardy Weinberg

1. Define gene pool
2. Define allele frequency
3. Describe evolution in terms of a population’s gene pool.
   • Evolution is a change in the genetic makeup of a population over time.
4. Describe the Hardy-Weinberg equilibrium model.
   • The Hardy-Weinberg model is a mathematical model of a non-evolving population. That means a population in which allele frequencies stay constant over time.
5. Be able to solve problems related to the two Hardy-Weinberg equations. [Note: you’ll be giving the formulas, so you don’t need to memorize them). The formulas are:
   • p + q = 1, and
   • p² + 2pq +q² = 1
6. State the five conditions associated with non-evolving populations in Hardy Weinberg equilibrium
   • Large population size
   • isolation (no outside alleles coming into or leaving the gene pool)
   • no net mutation
   • random mating (no sexual selection or assortative mating).
   • no beneficial or harmful alleles
7. Define genetic drift.
   • Genetic drift is a change in gene frequencies caused by random sampling of alleles in small populations.
8. List and describe two ways in which genetic drift can occur.
   • The founder effect
   • The bottleneck effect.
9. State the conditions that lead to evolution (change in allele frequencies). These are all violations of the Hardy-Weinberg conditions listed above
   • Violate large population size, and you have genetic drift, shown either by population bottlenecks or by the founder effect.
   • Violate isolation, and you have gene flow.
   • Violate no net mutation, and you have alleles changing frequency as they mutate from one form to another.
   • Violate random mating, and you have evolution caused by sexual selection or assortative mating.
   • Violate no harmful or beneficial alleles, and you have evolution caused by natural selection.

Natural Selection and Population Genetics Teaching Tips

Here are a few ideas to emphasize when teaching about natural, artificial, and sexual selection.

1. Randomness and design: The philosopher Daniel Dennett called natural selection “the best idea anybody ever had…It unites the two most disparate features of all of reality. On the one side purposeless matter and motion…on the other side meaning, purpose, and design.”

One of the most widely shared misconceptions about evolution is the idea that evolution is a random process.  Of course, it’s not. The mutations that underlie variation are random. But the selection that leads to adaptation is not random. Selection is about fitness, which is a concept that can be rigorously quantified. A great way to help your students see this is through the HHMI video and activities about the rock pocket mouse.

2. Selection can unleash positive feedback loops that accentuate selection The result of selection (artificial or natural) is a shift in allele frequencies. But what’s being selected are phenotypes. When you use illustrative examples like the peppered moth or the rock pocket mouse, you can talk to your students about how predatory birds (in both cases) are selecting against poorly camouflaged individuals. When the birds eat those individuals, they’re (literally) eating the genes for poor camouflage. Because the genes for good camouflage are left in the bodies of the prey, the paradoxical result is that predation increases the frequency of alleles for good camouflage. And the result of that is that superior camouflage selects for improved visual discrimination in predators…which feeds back to the prey. The results can be astonishing, as in this picture of a leaf insect.

Positive feedback loops are also at work in sexual selection, especially in intersexual selection (where, in mammals and birds, females choose which male they wish to mate with). The male with the most attractive trait passes that trait on to its male offspring. And the female passes her preference for that trait onto her female offspring. That dynamic, over time, results in the accentuation of both the trait and the preference for the trait, with results like the peacock’s tail.

3. Artificial selection is a useful bridge to understanding natural selection. Though the CED lists natural selection before artificial selection, I teach artificial selection first. It’s concrete and easy to understand. For most students, understanding how selective breeding enabled dog breeders to create various breeds of dogs, or how plant breeders were able to convert wild Brassica oleracea into cultivated varieties like broccoli, cauliflower, cabbage, Brussels sprouts, etc., is pretty straightforward. A great lecture prop is to have examples of each of these vegetables to show to your students. But if you can’t you can use this image from my website.

4. As environments change, so do selective pressures. A great example of this is the rapid evolution of beak depth observed in the Galapagos finches on Daphne Major in the Galapagos Islands, as famously studied by Peter and Rosemary Grant. The Grants were able to track yearly shifts in the mean phenotype of the medium ground finch in response to the amount of rainfall on the island, which changed the type of vegetation. This is all covered in HHMI’s video about finch evolution, which you can access at https://www.biointeractive.org/classroom-resources/origin-species-beak-finch. For a summer read, you (or your students) can read The Beak of the Finch, by Jonathan Weiner.

And here’s a similar list of ideas and tips for teaching population genetics and Hardy-Weinberg.

1.  The terms “dominant” and “recessive” have nothing to do with “common” and “rare.”

Students often enter this unit with a big point of confusion.  If your students are like mine, there are always a few students who will ask something like “If dominant alleles are dominant, then why doesn’t everyone in a population have that allele?” Those students are often my most confident students…and my guess is that most students naively come into a population genetics unit with some version of this incorrect idea in their minds. This is a deeply rooted misconception that I’ve been battling against for years. Overcoming it involves showing students that the meaning of the term “dominant” in “dominant” allele doesn’t make that allele like a dominant army that will conquer all else in its path, and spread like a virus. Ultimately, you want your students to understand that the frequency of an allele has to do with the qualities of the phenotype that it codes for and whether that phenotype is harmful or beneficial.

2. When you teach the Hardy-Weinberg equations, use cross-multiplication tables.

If you’re a gifted math instructor, you can teach students how to manipulate the Hardy-Weinberg equations.

1. p + q = 1, and
2. p² + 2pq +q² = 1

If (like me) you’re not, you’ll want to consider adopting a trick I learned years ago. Use modified Punnett squares called “cross-multiplication tables.” Here’s what they look like:

	A (0.3)	a (0.7)
A (0.3)	AA (0.09)	Aa (0.21)
a (0.7)	Aa (0.21)	aa (0.49)

Note how if you know the proportion of recessives in a population (aa, above), you can take the square root of that to figure out the frequency of the recessive allele. Since p + q = 1, subtracting the frequency of “q” from 1 gives you the frequency of the dominant allele. And from there you can figure everything else out.

Natural Selection and Population Genetics tutorials on Learn-Biology.com

Here’s a list of tutorials that cover all of the objectives above.

All of these are pretty short. Your students can probably handle two each night, allowing you to get through them all in a week.

1. Topics 7.1 – 7.3, Part 1: Natural and Artificial Selection
2. Topics 7.1 – 7.3, Part 2: Sexual Selection
3. Topics 7.4 -7.5, Part 1: Understanding Allele Frequencies
4. Topics 7.4 -7.5, Part 2: The Hardy-Weinberg Equation
5. Topics 7.4 -7.5, Part 3: The Hardy-Weinberg Principle
6. Topics 7.4 -7.5, Part 4: Natural Selection in Gene Pools 
7. Topics 7.4 -7.5, Part 5: Mutation, Heterozygote Advantage, and Genetic Drift
8. Topics 7.4 -7.5, Part 6: Natural Selection and Population Genetics Cumulative Flashcards and Quiz (Speed Challenge)

Additional Resources for Natural Selection and Population Genetics.

If you didn’t start your year with Natural selection, then please read my notes from Week 1, Days 3 and 4. They’ll point you to two great activities that you’ll be able to use with your students.

A classic population genetics demonstration is to hand out PTC paper and identify the percentage of tasters and non-tasters in your class population. From there, you can calculate the frequency of the recessive allele (the non-tasting allele) in your class gene pool. To learn more about PTC, read this article at the University of Utah’s Genetic Science Learning Center.

The folks at Flinn have a  POGIL on Hardy Weinberg.

I mentioned above HHMI’s Video about The Galapagos Finches. When I show that video, I also use this student guide (a google doc version of their PDF, with some of their instructor resources thrown in). Here’s the original teacher’s guide (with the answers).

Since mutation is one of the violations of Hardy-Weinberg equilibrium, this unit is a great time to review it. If you need to, you can use Flinn’s POGIL on mutation, or HHMI’s handout about Mutations and M1CR Signalling in the Rock Pocket Mouse. That handout lets you review natural selection, mutation, and cell signaling in one activity. Highly recommended.

HHMI also has two additional activities that are excellent for kicking off natural selection and population genetics.

1. HHMI: Skin Color Evolution. This focuses on the work of Nina Jablonski relating to the evolution of skin color variation in human populations. The resources include
   1. Video
   2. Educator and Student Materials
2. Allele and Phenotype Frequencies in Rock Pocket Mouse Populations. This activity takes what your students previously learned about the evolution of different color phenotypes among rock pocket mice, and grounds it in the Hardy-Weinberg equations.

HHMI has a fantastic activity about speciation (which also introduces phylogeny). It’s called Using DNA to Explore Lizard Phylogeny. This activity goes with the video The Origin of Species: Lizards in an Evolutionary Tree.  It’s a very rich activity, involving speciation, natural selection, sexual selection, convergent evolution, DNA sequencing, and phylogeny. It requires a bit of setup (printing out or otherwise making available photos of Anole lizards on various Caribbean Islands for an opening card sort activity), but it’s well worth the effort. Also, the phylogeny program the activity uses can be a bit hit or miss. Use the teacher’s guide to have the output ready to go in case the program hangs up.

If you want to go even deeper, then use HHMI’s Lizard Evolution Virtual Lab. The benefit of this activity is that your students will do some of the science (measuring lizard limb length, analyzing toe pads, etc.) to get morphological data about the different lizard ecomorphs. The drawback is that this will take you 3-4 class periods, as opposed to what I described immediately above, which takes about 90 minutes.

You can also use Flinn’s POGILs on Selection and Speciation.

For teaching mass extinction, here are a few more resources.

1. Flinn’s POGIL on Mass Extinction
2. I have put together a six-page reader on mass extinction. The images are from Adrienne Zilman’s Human Evolution Coloring Book. In terms of instructional delivery, I usually hand it out, and, as a class, we read it in sections and discuss it.
3. I also love showing the impact scene from the 1990s movie Deep Impact (which I also include in my tutorial, but your students will not mind watching it again). 
4. If you want to get into the details of the Cretaceous extinction (the one that wiped out the dinosaurs), you can watch this episode of Radiolab.

Along the way, students get lots of practice in analyzing (and even building) phylogenetic trees. Here’s an example of a sample exercise.