Download GENE_AYT_GizmoMicroevo_V01

Teacher Guide: Microevolution
Learning Objectives
Students will …
 Understand the concept of fitness.
 Observe how natural selection affects allele frequencies over time.
 Compare the effects of a deleterious dominant allele, a deleterious recessive allele, and
a situation in which the heterozygous phenotype is the fittest.
 Model how natural selection operates on real-world genetic disorders.
Vocabulary
allele, cystic fibrosis, deleterious, dominant allele, fitness, genotype, heterozygote superiority,
heterozygous, homozygous, incompletely dominant, malaria, predator, recessive allele, sickle
cell anemia
Lesson Overview
If no natural selection is acting on a population, the
proportions of the different alleles and genotypes in the
population will tend to remain relatively constant. The
expected genotype ratios are described by the HardyWeinberg principle.
Natural selection can favor dominant alleles, recessive
alleles, or heterozygous genotypes. The Microevolution
Gizmo™ allows students to model each of these situations
and observe how allele and genotype frequencies change
over time. Students can apply their observations to realworld genetic conditions, such as cystic fibrosis and sickle
cell anemia.
Which parrots are most successful
at blending in with the background?
The Student Exploration sheet contains three activities:
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Activity A – Students explore the case of a deleterious dominant allele.
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Activity B – Students explore the case of a deleterious recessive allele.
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Activity C – Students explore the case of heterozygote superiority.
Suggested Lesson Sequence
1. Pre-Gizmo activities
( 10 – 20 minutes)
The Microevolution Gizmo explores how natural selection affects allele and genotype
percentages in a population. Before beginning, review basic genetic terms such as
allele, genotype, phenotype, dominant, recessive, homozygous, and heterozygous.
The Hardy-Weinberg Equilibrium Gizmo has a similar scenario and explores a situation
in which no natural selection is occurring. The Microevolution Gizmo works well as a
follow up to the Hardy-Weinberg Equilibrium Gizmo but also can be used on its own.
2. Prior to using the Gizmo
( 10 – 15 minutes)
Before students are at the computers, pass out the Student Exploration sheets and ask
students to complete the Prior Knowledge Questions. Discuss student answers as a
class, but do not provide correct answers at this point. Afterwards, if possible, use a
projector to introduce the Gizmo and demonstrate its basic operations. Demonstrate how
to take a screenshot and paste the image into a blank document.
3. Gizmo activities
( 15 – 20 minutes per activity)
Assign students to computers. Students can work individually or in small groups. Ask
students to work through the activities in the Student Exploration using the Gizmo.
Alternatively, you can use a projector and do the Exploration as a teacher-led activity.
4. Discussion questions
( 15 – 30 minutes)
As students are working or just after they are done, discuss the following questions:

Which is more likely to persist in a population, a deleterious dominant allele or a
deleterious recessive allele? Why?

How can you explain the continued persistence of inherited genetic disorders
such as cystic fibrosis and hemophilia in human populations?

If the heterozygous genotype is most fit, why will the less-fit homozygous
genotypes persist in the population?

What will occur if both of the homozygous genotypes have a higher fitness than
the heterozygous genotype? [Test student ideas with the Gizmo.]
5. Follow-up activity: Microevolution in action
( variable)
Evolution is occurring all the time, and often it can happen quite quickly. Frequently, a
single mutation can spread through a population, significantly altering behavior and
increasing chances of survival.
Divide your class into groups of 2–4 students, and have each group research and
present a “news broadcast” to announce a recent evolutionary innovation. Possible
topics include the following:

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Evolution of quiet crickets in Kauai
Evolution of lactose tolerance in humans
Appearance of hemophilia in Queen Victoria’s descendents
Evolution of U.S. mussels in response to invasive Asian shore crabs
Evolution of “superbacteria” that are resistant to antibiotics
Adaptation of squirrels, mosquitoes, and songbirds to global warming
Development of a new fruit fly species in a lab
See the Selected Web Resources on the next page for helpful websites.
Scientific Background
Microevolution refers to changes in the frequency of alleles within a species or population. It is
contrasted with macroevolution, which refers to evolution at the level of species or higher. Both
microevolution and macroevolution occur by processes that include natural selection, mutation,
gene flow (immigration and emigration), and genetic drift (chance).
Microevolution also describes the intersection between genetics and evolutionary theory. The
effects of natural selection can be very different on dominant and recessive alleles. For
example, a dominant allele that is harmful will be expressed in every individual that possesses
the allele. As a result the deleterious dominant allele is quickly removed from the population by
natural selection. A harmful recessive allele will only be expressed if an individual has two
copies of the allele. Deleterious recessive alleles thus can “hide” in a population for many
generations without being expressed. Genetic disorders such as cystic fibrosis, sickle cell
anemia, albinism, and Tay-Sachs disease are caused by recessive alleles.
Health Connection: Inbreeding depression
If a deleterious allele is dominant, it will be expressed in every individual that carries the allele.
As a result, deleterious dominant alleles are typically weeded out quickly by natural selection
and are extremely rare. Recessive alleles, on the other hand, can persist indefinitely because
they are rarely expressed. An individual must have two recessive alleles to express the genetic
disorder, an extremely unlikely circumstance if the recessive allele is rare.
If the breeding population is very small, however, close relatives are likely to mate and produce
offspring. This greatly increases the likelihood that both parents are carriers of the deleterious
allele, and the probability of offspring that express the disorder goes up. In general, the health of
a population tends to be inversely correlated to the amount of inbreeding, a phenomenon known
as inbreeding depression.
In most human cultures, incest between family members is considered taboo. The problem of
inbreeding also affects populations of animals that are reintroduced into the wild or are
expanding in population after being driven nearly extinct. For example, cheetahs are extremely
inbred because of a population crash several thousand years ago. This lack of genetic diversity
may harm the endangered cheetah’s ability to survive into the future.
Selected Web Resources
Microevolution: http://evolution.berkeley.edu/evosite/evo101/IVMicroevolution.shtml
Microevolution and macroevolution: http://www.infoplease.com/cig/biology/microevolutionmacroevolution.html
Inbreeding depression: http://evolution.berkeley.edu/evolibrary/article/_0_0/conservation_03,
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1629062/
Websites related to the follow-up activity:
Quiet crickets in Kauai: http://evolution.berkeley.edu/evolibrary/news/061201_quietcrickets
Lactose tolerance: http://evolution.berkeley.edu/evolibrary/news/070401_lactose
Hemophilia, the royal disease: http://www.sciencecases.org/hemo/hemo.asp
Mussels repel crabs: http://evolution.berkeley.edu/evolibrary/news/060901_mussels
Superbacteria: http://www.pbs.org/wgbh/evolution/educators/lessons/lesson6/act1notes.html
Adapting to global warming: http://evolution.berkeley.edu/evolibrary/news/060701_warming
New fruit fly species: http://www.time.com/time/magazine/article/0,9171,843527,00.html
Related Gizmos:
Natural Selection: http://www.explorelearning.com/gizmo/id?447
Evolution: Mutation and Selection: http://www.explorelearning.com/gizmo/id?554
Mouse Genetics (One Trait): http://www.explorelearning.com/gizmo/id?449
Hardy-Weinberg Equilibrium: http://www.explorelearning.com/gizmo/id?517