Phenomenon
There is variation in phenotypes within populations, and relative frequencies of phenotypes can differ between populations.
We observe the distribution of several traits within our class, then observe the global distribution of several other traits.
Question
What determines whether a trait is common or rare?
Why does the distribution of a trait differ from place to place?
Model Ideas
- Frequencies of variants (phenotypes) in populations reflect the frequencies of the alleles for those traits in the gene pool of the population.
- Frequencies of alleles in a population stay the same from one generation to the next if all phenotypes have equal chances to survive and reproduce (i.e. no selection, migration or mutation).
- Frequencies of alleles change over time when there is differential survival and reproduction of phenotypes in the population. Alleles associated with advantageous phenotypes increase in frequency, and those associated with disadvantageous phenotypes decrease.
3a. Environmental factors determine which phenotypes are advantageous or disadvantageous, so distributions of phenotypes often differ geographically.
3b. Alleles associated with disadvantageous phenotypes decrease in frequency but may never completely disappear because of heterozygotes.
3c. In an environment where heterozygotes have an advantage over homozygotes, the frequency of disadvantageous alleles may remain unexpectedly high.
(You may also develop some ideas about the role of mutation and migration using the resources provided.)
Overview
Transition in: After exploring the models of classical genetics, we are ready to return to populations to think about the frequency of traits within populations and the distribution of traits across populations. The phenomena and questions for this model return our attention to evolutionary thinking and to once again consider the role of natural selection in shaping the diversity we see on the planet today.
Overview
We have explored the genetic and environmental factors that contribute to variation in individuals, but now ask students to step back and think about the distribution of variations within and across human populations. In developing a model to explain phenomena at the population level, we go deeper into the processes underlying natural selection and the distribution of traits in populations over time and space. Later, in the next model, we will extend this thinking to help us understand how new species develop. (This deeper understanding of natural selection ultimately positions us to address our overarching question from the beginning of the year about the origins of biodiversity and unity.)
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We begin reorienting our thinking from the individual to the population level by looking at the occurrence of several traits within the class. One idea that invariably surfaces is that more common traits are probably dominant. Our data allows us to easily dispel this idea. Students will also want to attribute frequency differences to selection. Though we cannot necessarily eliminate this idea, we recognize that we do not yet have all the ideas we need to explain why traits are more or less common in a population.
At this point we introduce the terms gene pool and frequency, and use the class data to consider the chances of different combinations in the next generation.
Now that we have done some initial thinking about traits in populations, we look at maps showing the geographic distribution of phenotypes for several human traits – Huntington’s disease. height, lactose intolerance, cystic fibrosis, and sickle cell anemia. We ask students first what they notice, and then what questions they have after looking at the maps. We hope they notice that in each case there are differences in the frequencies of phenotypes across different areas. We hope to eventually arrive at a driving question along the lines of, “What determines frequencies of phenotypes in a population, and why do the frequencies differ across different populations?”. We elicit student ideas about the answer to this question more formally now, and generate a class list of initial model ideas.
To get a better understanding of how gene pools behave over time, we introduce an activity that uses different colors of beans to represent alleles in a gene pool. Through simulation, students work to generate a couple of model ideas about populations: 1) the frequencies of phenotypes in a population reflect the frequencies of alleles for the trait in the gene pool, and 2) if there is no selection, the frequency of alleles in the gene pool of a population does not change from one generation to the next. In debriefing the bean activity, students are asked to hypothesize what would happen to gene frequencies between generations if one of the genotype combinations had an advantage over others. They readily predict that the frequency of advantageous alleles will increase and that of disadvantageous alleles will decline.
For the remainder of the unit, we explore sickle cell anemia--and eventually, the geographic correlation between the frequency of the mutant allele and the incidence of malaria. As a result of our work to make sense of the patterns, we add new ideas to our model: Frequencies of alleles change over time when there is differential survival of phenotypes. The frequency of alleles associated with disadvantageous phenotypes decreases (but usually never disappears completely because of heterozygotes), and the frequency of advantageous alleles increases. Environmental factors determine which phenotypes are advantageous or disadvantageous, so distributions of phenotypes often differ geographically. (Although it isn’t necessary, students may also want to include a model statement about selection that favors heterozygotes, as in sickle cell: In an environment where heterozygotes have an advantage over homozygotes, the frequency of a disadvantageous allele may remain unexpectedly high.)
Before we finalize the model, we address any initial model ideas that we haven’t taken up. Students will often assert the import of both mutation and migration in determining the distribution of traits. We address these specifically with sickle cell anemia using additional geographic data (including some data about the incidence of the trait in North America, a segment for which you should thoughtfully prepare--especially for the whole-class discussion) before deciding how to add them to our final model.
We end the unit by returning to our driving question. Students write explanations about the distribution of traits and alleles using the specific example of sickle cell anemia. It time allows, students can share their answers in groups and come up with a consensus “best” explanation.
Transition out: Now that we understand how frequencies of phenotypes and alleles across different population become different, we ask how such differences over time might lead to the rise of new species. The next model will explore the mechanisms of speciation, and inform our understanding of biodiversity. This will position us to finally return to the big question of Unity and Diversity.
Overall Time: 5-7 days
(Length depends on whether whether or not students design their own extension to the bean lab for modeling selection in sickle cell anemia, and whether they actually try out the procedures they design. )
View, Copy or Download Resources from Google Drive!
1. We begin the unit with reframing our conversation about traits by “zooming out” to the population level. Though we’ve spent a lot of time understanding how traits are passed along and expressed in individual organisms, how does this help us to understand frequencies of traits in populations? We explore this question using frequency data we’ve collected from our classroom population.
In refocusing on traits in populations, we’ve asked the question, “What makes a trait more common or rare?” Since our ideas about dominance didn’t help us to understand frequencies in our classroom, we are still wondering about this question.
2. We work in groups to make sense of maps showing the global distribution of 5 traits: Huntington’s disease, lactose intolerance, height, cystic fibrosis, and sickle cell anemia. After reporting out what we notice and what we wonder, we refine our driving question for the unit.
After looking at international frequency data for a number of human traits, we’ve settled on a driving question (a variant of “What makes traits more common or rare?” or “Why do trait frequencies differ across the world?”).
3. We continue our work in groups—brainstorming our initial thoughts about the driving question and then compiling them into class list of initial model ideas.
We have an initial model but recognize that we need to explore further the ideas about how traits “behave” in populations.
4. To help illuminate how variations in populations are related to the genetic makeup of its individual members, we use beans to simulate what happens to the gene pool of a population when all individuals survive and reproduce (i.e. there is no selection).
*The simulation takes 55 minutes and the debriefing an additional 10 minutes.
We’ve concretely explored the concept of the “gene pool” and how it relates to the frequency of traits in the moment and over time. In doing so, we have begun to formalize one model idea about the relationship between allele frequencies and genotype frequencies and a second idea about how those remain stable in populations where there is no selection at play. We have also begun to think about how selection might work in populations / gene pools.
5. We return to the map of the HbS allele for Sickle Cell Anemia and learn a little about the disease and underlying traits so that we can explore ideas about how selection changes allele, genotype and trait frequencies.
*Time depends on a few different options in this learning segment.
We’ve extended our model to include differential survival and have tied the concepts of the gene pool and allele frequencies back to our Natural Selection model.
6. We look yet again at the map for the HbS allele and wonder about its distribution, sharing out our ideas about selection or other processes. We then inspect a parallel map for the incidence of malarial infection and notice a relationship. This leads us to wonder about the link, to learn more through a video, and finally to work in groups to design a simulation (using the bean lab introduced earlier) that might explain the distribution of HbS.
*Time depends on a few different options in this learning segment.
We’ve essentially recognized (through our examination of sickle cell and malaria) that selection may act differently among populations, resulting in different patterns of allele and trait frequencies between different populations.
7. We revise our model and work toward a final version by asking which of our current model ideas are relevant to – or help explain – the case of sickle cell. We might also add ideas, for example, about mutation (imported from our genetics models about variation) and also migration. (If not already offered into the classroom, the contribution of migration can be generated through an exploration of North American sickle cell frequencies.)
We now have a relatively solid model that connects the environment to frequencies through selection. We also have an idea that states that alleles may be spread geographically by migration.
8. We return to the driving question and answer in writing as it applies to Sickle Cell Anemia. If time allows, we share and write consensus “best” explanations in our work groups.
*This segment can run longer if you allow time for the group work.
We used the model to answer our driving question and explain the phenomenon of the distribution of frequencies in sickle cell anemia!
IR. Return to Isle Royale for an exploration of inbreeding, migration and genetic rescue*. Plus bottlenecks and the spread of harmful traits in populations. A number of relevant materials are available on the Wolves and Moose of Isle Royale website.
*This module has not yet been developed in an appropriate, open source form as of the current version of this unit.
Download Resources
| Attachment | Size |
|---|---|
| All Microsoft Office Resources for MBER-Bio Population Variation (Download) | 13.77 MB |