Developing Understanding Through Model-based Inquiry
James Stewart
Jennifer L. Cartier
Cynthia M. Passmore
Department of Curriculum & Instruction,
University of Wisconsin-Madison
National Center for Improving Student Learning & Achievement in Mathematics
and Science
Developing Understanding Through Model-based Inquiry
| Understanding in science includes being familiar with
a discipline's concepts, theories, and models, an appreciation for
how knowledge is generated and justified, and the ability to use such
knowledge to engage in inquiry. |
Teaching for, and learning with understanding are desirable educational goals
and have been the centerpiece of two significant national reform efforts in
science education (NRC, 1996; AAAS, 1993). The positions in the National Research
Council and the American Association for the Advancement of Science documents
reflect the view that understanding involves a set of interrelated components
that form the basis for knowing about and interacting with the natural world.
These components includes a familiarity with a discipline's concepts, theories,
and models, an understanding of how knowledge is generated and justified in
various disciplines, and the ability to use such understandings to engage
in inquiry.
The view taken in this chapter is that scientific practice begins with the
recognition (mediated by the conceptual lenses provided by appropriate explanatory
models) of some phenomena warranting explanation. From data about natural
phenomena, scientists then must establish patterns related to the data and
then create, revise, or use explanatory models to account for these data patterns.
Clearly, data collection, pattern recognition, and proposing, revising, or
defending explanatory models in science is, in reality, both a highly discipline-specific
process and a seamlessly integrated one; that is, data collection and analysis
proceeds concurrently with the use and production of explanatory models. However,
for education purposes, the generalized process describe above has proven
to be quite powerful. While disciplinary constraints dictate just what constitutes
a phenomenon and what types of patterns are sought we have found that this
framework can be useful for identifying important phenomena, patterns and
models in various disciplines and developing curricula around these central
ideas. The specificity of disciplinary practice comes into play in such curricula
when students begin to discuss and defend their models with one another. The
structure of their explanatory models, as well as the body of scientific ideas
in relation to which their models are judged, is discipline-specific.
| In this chapter we illustrate how high school students
have come to understand sophisticated science concepts by participating
in model-based inquiry. |
Given the importance of models and modeling in science classrooms (and in science),
it is essential that students learn about scientific models (in addition
to learning the concepts relevant to particular models) and how to
communicate with one another regarding the use and evaluation of such models.
In this chapter we will illustrate how high school students have been able
learn about science as model-based inquiry generally while also coming to
understand significant discipline-specific concepts and inquiry procedures
through participation in inquiry in genetics and evolution. The introduction
to the chapter begins with a classroom vignette. Interestingly the vignette-while
it portrays a classroom in which students are generating data, seeking patterns,
and proposing and revising explanatory models-does not showcase students engaged
in learning new scientific concepts. Don't be alarmed! The vignette illustrates
important aspects of a model-based inquiry classroom : such "science-free"
modeling activities are used to introduce students to science as a modeling
game, to develop classroom norms for interacting with other students, and
to establish scientific norms of argumentation concerning data, claims, explanatory
models, and their relationship. Following similar introductory activities
students then engage in similar intellectual pursuits in the context of sophisticated
disciplinary content.
An Initial Modeling Experience
| A class of high school students is examining
a half-gallon milk-carton shaped container that pours a pre-measured
amount of liquid laundry soap each time it is tipped. They are
engaged in what they consider to be a central activity of scientists-making
observations.
| Mrs. S: |
Making observations is
important in science. I want you to observe this carton.
Just call out what you notice and I will write it
on the board. |
The students respond with a variety of observations.
| Ian: |
The box is white with
blue lettering. |
| Delia: |
The contents
slosh around and it looks like liquid soap when we
pour it. |
| Sarah: |
Hey it stopped
coming out! Try to pour it again so we can see what
happens. |
| Owen: |
It always
pours about the same amount then stops. |
After several minutes of listening to the students, their teacher
stops them and invites them to take a closer look at the carton,
prompting them to identify patterns associated with their observations.
Their reflection on these patterns leads the students to propose
manipulations of the container-these in turn produce more observations.
The teacher now interrupts them to comment on the high quality
of their work, saying,
| Mrs. S.: |
Okay, you've
made some wonderful observations, ones that you are
going to be using in just a few minutes. But, there
is more to science than making observations. Scientists
also develop ideas of what is not visible in order
to explain that which is. These ideas are called models. |
She goes on to challenge them,
| Mrs. S.: |
Imagine what is inside
the container that could explain your observations.
I want you to make drawings of what you think might
be in there and maybe some groups will have time to
develop a three-dimensional representation too. And,
one last thing, I want each group to develop at least
one test of your model. Ask yourself, 'If the world
inside the carton is as I imagine it and I do X to
the carton what result would I expect?' |
Over the next two class periods, they work in animated groups to
develop models that can account for their observations. They describe,
draw, and build three-dimensional representations of what they
think is in the carton. They argue. They negotiate. They revise.
Then they share drawings of their models with one another.
| Sarah: |
Hey Scott,
you have a different idea than ours. How does that
flap work? |
| Scott: |
The flap is
what stops the detergent from gushing out all at once
when you tip it. |
| Delia: |
Yeah, I get
that, but does your design allow the same amount of
detergent to come out every time? Because we tried
a flap, too, but we couldn't figure out how to get
the amount to be the same. |
The students also propose tests of their models.
| Sarah: |
Well, Scott is saying
that the flap is like a trapdoor and it closes to
keep the detergent in. But I think that if there is
a trapdoor-like thing in there, then we should be
able to hear it close if we listen with a stethoscope,
right? |
| Delia: |
Hey, Mrs.
S. Can we get a stethoscope? |
| A visitor to Mrs. S.'s
classroom would notice that she listens attentively to
the descriptions that each group gives of its model and
the observations that the model is designed to explain.
Through her comments, she demonstrates how to question
the models of others and how to present a scientific argument. |
A visitor to the classroom would notice that Mrs. S. listens attentively
to the descriptions that each group gives of it's model and the
observations that the model is designed to explain. She pays special
attention to their interactions with other groups, and is skillful
in how she converses with them during their presentations. Through
her comments she demonstrates how to question the models of others
and how to present a scientific argument. To one group she says:
"I think I follow your model, but I am not sure how it explains
why you get 90 milliliters of liquid each time you tip the box."
To another she comments, "You say that you have used something
similar to a toilet bowl valve. But I don't understand how your
valve allows soap to flow in both directions." And to a third
group she asks, "Do you think that Ian's model explains the data?
What question would you ask his group at this point?" By the end
of the three-day activity the students are explicit about how
their prior knowledge and experiences influence their observations
and their models. They also ask others to explain how a proposed
model is consistent with the data, and challenge them when a component
of a model, designed to explain patterns in observations, doesn't
seem to work as it has been described. |
Curriculum Design Principles
Introduction
| We consider a scientific model
to be an idea or set of ideas that explains the causes
of particular natural phenomena. Models are complex constructions
that consist of conceptual objects (e.g., alleles, populations)
and processes (e.g., selection independent assortment) in which
the objects participate or interact.
Hafner & Culp, 1991
Stewart & Hafner, 1991
Carter, et al., 2001:
http://www.wcer.wisc.edu/ncisla/
publications/reports/Models.pdf |
Over the course of several days, the students in this classroom have experiences
that, in important respects, are similar to those of scientists, including:
- using prior knowledge to pose problems and generate data.
- searching for patterns in data.
- developing scientific (or explanatory) models to account for patterns.
- using patterns in data and their models to make predictions.
- revising initial models in light of anomalous data and in response to
critiques of others.
- making their ideas and their thinking public.
In short, they are participating in a classroom scientific community. Such
participation is a hallmark of classrooms where model-based inquiry curricula
are used. In this chapter we will describe three design principles that are
central to developing such curricula and the ways in which these principles
have been implemented in high school instruction in genetics and evolutionary
biology. The core design principles, consistent with three major themes of
the National Research Council Book How People Learn (NRC, 1999), are:
| We encourage readers to visit the web site (www.wcer.wisc.edu/ncisla/muse/)
that serves as a companion to this chapter. For each of our multi-week
curricula, the site includes—
- discussions of student knowledge and reasoning
- intended learning outcomes
- instructional activities
- instructional notes
- assessments
- examples of student work
- teachers' reflections
- connections to the National Science Education Standards (NAS,
1996) and Benchmarks for Scientific Literacy (AAAS, 1993).
|
Attend to Significant Disciplinary Knowledge-such knowledge includes
the conceptual structures (concepts and explanatory models) of a discipline
as well as how such structures are developed and the role they play in
inquiry.
Attend to Student Knowledge-students may have significant prior
knowledge, including misconceptions, related to scientific inquiry and
science concepts. Their prior knowledge needs to be considered when developing
curricula and related instructional materials.
Engage Students in Reflective Scientific Practice-classrooms should
be places where all students are full participants in learning science
by participating in realistic inquiry. Moreover, such participation leads
to rich opportunities to reflect and learn about knowledge development
in science.
This particular approach to curriculum development has emerged as a result
of collaborative work with high school teachers and their students (our collaborative
group is known as MUSE, or Modeling for Understanding in Science
Education) As a part of that collaboration we have conducted research
on student learning, problem solving, and reasoning. This research has led
to refinements in the instruction, which in turn have led to improved student
understanding. The remainder of this chapter is organized so that Section
One provides an overview of the three design principles, while Sections Two
and Three contain descriptions of these principles as they are instantiated
in MUSE genetics and evolution curricula. Section Four describes some common
elements in both of these classrooms that enable students to participate in
reflective scientific practice.
Section One: An Overview of the Design Principles
Attending to Significant Disciplinary Knowledge
| Disciplinary knowledge and ideas about the nature of
scientific inquiry are instantiated in classrooms where students learn
by "doing science." |
For well over a decade we have developed science curricula where both disciplinary
knowledge and knowledge about the nature of science comprise the student learning
outcomes. Such learning outcomes are realized in classrooms where students
learn by "doing science" in ways that are similar to the work that scientists
do in their intellectual communities. We have created classrooms in which
students are engaged in discipline-specific inquiry (empirical and conceptual
investigations) as they learn and employ the explanatory models and the reasoning
patterns of the discipline. In the descriptions of the genetics and evolution
courses that follow it will become apparent that, while explanatory models
are central in both disciplines, there are very different reasoning patterns
involved in the use or construction of such models. This comes about largely
because a primary activity in the practice of evolution, the reconstruction
of past events, is not common in the practice of genetics.
Attending to Student Knowledge
| Understanding students' prior knowledge has influenced our work
in the areas of—
- Curriculum; selection and sequencing learning outcomes;
- Instructional design; and
- Assessment of student achievement.
|
Our work has benefited from the general research on student knowledge (outlined
in Chapter Z). However, we have benefited even more from research about student
learning in the fields of genetics and evolution and research about students'
knowledge of aspects of the "nature of science" (especially that which speaks
directly to models and modeling). An understanding of student prior knowledge
has had a significant influence on our decision making in three areas:
- curriculum (selecting and sequencing student learning outcomes);
- instruction (designing and selecting tasks and activities for students);
and
- assessment (determining the extent to which students have realized the
intended learning outcomes).
Curriculum (Selecting and Sequencing Learning Outcomes): The
primary basis for selecting learning outcomes is the practice of the appropriate
discipline. That is, each discipline has at its core a set of key explanatory
models, assumptions, and particular argument structures. These form the basis
of MUSE curricula in various disciplines. Even so, the choice of learning
outcomes is informed by the literature on student understanding of specific
disciplinary concepts and from our knowledge of student performance in previous
versions of the genetics and evolution courses. For example, the evolution
education literature has made it clear to us that many students: (1) believe
that evolution results from teleological processes; (2) do not consider evolution
to be truly scientific (it lacks experimentation and directly observable events);
and, (3) that science need not be limited to naturalistic, mechanistic explanations
(see for example, Demastes, Trowbridge, & Cummins, 1992; Jensen, &
Finley, 1995; Rudolph, & Stewart, 1998; Wirtz, 1993). With these understandings
in mind we have focused upon the metaphysical, methodological, and epistemological
perspectives of students and scientists in addition to the conceptual components
of evolutionary biology as we developed the model-based inquiry evolution
curricula.
| Students' Prior Knowledge
• Many high school students do not understand the interrelationship
of genetic, meiotic, and bio-molecular models, relationships that
are key to a deep understanding of inheritance phenomena.
Cartier, 2000a:
http://www.wcer.wisc.edu/ncisla/
publications/reports/RR98-1.pdf
Kindfield, 1994
• Even after instruction, students may still believe that
mutations or "abnormal" genes are recessive, and that
dominant traits are more frequent in the population and more likely
to be inherited and/or selected for.
Allchin, 2000
• Students misunderstand the primacy of naturalistic explanations
in science.
Rudolph & Stewart, 1998
• Students have difficulty in understanding the concept of
variation that is a foundation of the natural selection model.
Bishop & Anderson, 1990
• Students have misunderstanding about the origin, function,
and very nature of scientific models.
Cartier, 2000b:
http://www.wcer.wisc.edu/ncisla/
publications/reports/RR99-1.pdf
Grosslight, et. al, 1991
|
While the selection of learning outcomes is informed by research on
student prior knowledge we have found that it is equally important to take
that research into consideration when sequencing those learning outcomes.
For example, many high school students do not understand the interrelationship
of genetic, meiotic, and bio-molecular models, relationships that are key
to a deep understanding of inheritance phenomena (Cartier, 2000a; Kindfield,
1994; Wynne, Stewart, & Passmore, in press). To offset this problem we
have identified learning outcomes that address the conceptual connections
among families of models at the cytological, genetic, and bio-molecular levels
and to introduce these models in a sequence that emphasizes their relatedness.
In the evolution course, knowing that students misunderstand the primacy of
naturalistic explanations, we designed experiences that require students to
identify the assumptions about mechanism (not all naturalistic) contained
in the writings of William Paley, Jean Baptiste Lamarck, and Charles Darwin.
Through these activities, which occur early in the instruction, students have
opportunities to consider and debate the reasons why scientists feel it is
important to limit themselves to naturalistic explanations. From this point
on, we are able to focus only on Darwin's naturalistic idea of evolution by
natural selection. In addition, this introductory comparison also allows us
to address learning outcomes designed to eliminate teleological explanations
(evident in Lamarckian-like explanations that students give for evolutionary
phenomena) that have been widely reported in the literature (see for example,
Clough & Wood-Robinson, 1985a, 1985b; Jensen & Finley, 1996). Details
of how understanding student prior knowledge of the conceptual content of
particular disciplines, about models and modeling, and about what it means
to learn science will be returned to in Sections Two and Three.
Instruction (Development of Student Tasks and Classroom Structure):
An awareness of typical misconceptions and difficulties that students may
experience when learning particular content is important to the development
of instructional tasks. Throughout our work we have carefully designed instructional
activities that address students' prior knowledge and misconceptions related
to understanding in genetics and evolutionary biology.
In addition to our concerns with student knowledge in particular content areas
we have also found, as have others (Grosslight, Unger, Jay, & Smith, 1991),
that students have misunderstandings about the origin, function, and very
nature of scientific models. Because we expect students to develop understanding
of the nature of models and of appropriate norms of argumentation in modeling
classrooms we have developed introductory activities to address "nature of
science" learning outcomes explicitly. For now it is enough to say that we
begin instruction with a "science free" activity. For example, the genetics
instruction begins with a black box activity (similar to the detergent-container
described in the vignette that opened the chapter) that introduces students
to the nature of explanatory models—they are ideas used to guide investigations.
Similarly, the evolution course begins with a cartoon sequencing activity
that provides students with a sense of what is involved in reconstructing
the past (a central goal of inquiry in evolutionary biology). Both of these
activities serve a common purpose-to introduce students to important ideas
about the nature of knowledge generation and justification in particular disciplines.
| Because we expect students to develop understanding
of the nature of models and of appropriate norms of argumentation
in modeling classrooms, we have developed introductory activities
to address "nature of science" learning outcomes explicitly. |
| By having tasks that require students to attend to
knowledge across domains and by structuring classrooms so that students
have to make their thinking about such integration public we have
seen improvements in their understanding of genetics. |
Of course, there are many instances in our curriculum development work where
an understanding of student conceptual knowledge related to a particular discipline
is influential. For example, in early versions of the genetics instruction,
it became apparent that students were solving problems, even sophisticated
ones, without adequately drawing on an integrated understanding of meiotic
and genetic models (Cartier, 2000a, Wynne, Stewart & Passmore, in press).
In response, we designed a set of data analysis activities and related homework
activities that required students to integrate across cytology, genetics,
and molecular biology when conducting their genetic investigations and when
presenting model-based explanations to account for patterns in data that they
had generated. By having tasks that require students to attend to knowledge
across domains and by structuring classrooms so that students have to make
their thinking about such integration public we have seen improvements in
their understanding of genetics (Cartier, 2000b).
| Knowing that students often perceive evolution to be
an individual rather than a population-level process can provide teachers
with an ear for listening to student discussions. We have seen, time
and again, teachers who, when they become aware of the common struggles
of students, begin to "hear" their own students differently. |
Assessment: Assessing the extent to which students realize intended
learning outcomes needs to be undertaken with an eye to the various types
of prior knowledge just described (misconceptions of science concepts, ideas
about what science is, and the extent to which students' knowledge is integrated).
It is fairly obvious how, at least in general terms, this might be accomplished.
The identification of prevalent student knowledge, including misconceptions,
can influence both formal and informal assessment activities. For example,
knowing that students often perceive evolution to be an individual organism
rather than a population-level process can guide the development of exam questions
as well as provide teachers with an ear for listening to student discussions.
We have seen, time and again, teachers who, when they become aware of common
struggles of students, begin to "hear" their own students differently. Similarly,
in genetics, test questions, or questions of students as they work in research
groups solving problems can be structured to provide evidence of their understanding
of the relationship of genetic and meiotic models. Thus, an important feature
of instructional activities that provide students with opportunities to make
their thinking and knowledge public, and therefore "visible" to teachers,
is that they make assessment and instruction seamless. This becomes possible
when students articulate the process of arriving at solutions and not simply
the solution itself.
It is clear that knowledge of student understanding can be influential in the
informal and formal assessment work of teachers. However, we believe that
students also benefit from assessments that provide them with opportunities
to see how their understanding may have changed over the duration of some
unit of study. One method that we have found to have such potential is to
require each student to critique her own early work based on what she knows
at the conclusion of a course. Not only does this approach provide teachers
with insights into student knowledge but it provides students with a glimpse
at how much their knowledge, and their ability to critique arguments, has
changed.
Promoting Reflective Scientific Practice
| Reflective practice involves an understanding of central
explanatory models, the ability to use such models to conduct inquiry,
and an understanding of and ability to engage in the assessment of
scientific models. |
By considering authoritative accounts of scientific practice and research on
students' knowledge it is possible to develop instruction that enable students
to engage reflectively in scientific practice. Reflective practice involves
an understanding of central explanatory models, the ability to use such models
to conduct inquiry, and an understanding of and ability to engage in the assessment
of scientific models. We have found that these outcomes can be realized in
classrooms where students are full participants in a "scientific community"
(Cartier, 2000b, Passmore & Stewart, 2000) Interestingly, one unexpected
outcome of structuring classrooms so that students are expected to participate
in the "intellectual work" of science has been an increased involvement and
achievement by students who had not previously been identified as successful
in science. Creating contexts for reflective practice will be illustrated
in the descriptions of the genetics and evolution classrooms and will also
be returned to in the conclusion to the chapter.
| For students to understand science content as well
as what it means to make and justify claims in modeling classrooms,
they need to be placed in situations where they are expected to think
about their own thinking. |
To conclude Section One, we repeat that in order for students' understanding
of science content and of how to make and justify claims in modeling classrooms
to progress, they need to be placed in situations where they are expected
to think about their own scientific thinking. Each of the MUSE classrooms
has been carefully constructed to provide just such opportunities. Sections
Two and Three include detailed illustrations of how this occurs in genetics
and evolutionary biology.
Section Two: Developing Models of Inheritance in High School Genetics
| Student Inquiry in Genetics
Nineteen students are sitting at desks and lab tables in a small
and cluttered high school biology classroom. The demonstration
desk at the front of the room is barely visible under the stacks
of papers and replicas of mitotic cells. A human skeleton wearing
a lab coat and a sign labeled "Mr. Stempe" stands in a corner
at the front of the room and the countertops are stacked with
books, dissecting trays, and cages holding snakes and gerbils.
During the previous few days the students in this class studied
the work of Gregor Mendel, the nineteenth century monk who has
come to be known as the "Father of Genetics." Mendel grew generation
after generation of pea plants in an attempt to understand how
certain traits were passed from parent plants to their offspring.
Years of Mendel's work resulted in the publication of Experiments
on Plant Hybridization, (1865), a paper in which Mendel put
forth his model explaining inheritance of discrete traits in plants1.
The students read an edited version of this paper and refer to
Mendel's idea as the "simple dominance model" because it explains
the inheritance of traits derived from two alleles (or pieces
of genetic information) when one of the alleles is completely
dominant to the other. (see Figure 1A)
During class on this day, the students' attention is drawn to the
cabinet doors along the length of the room. These doors are covered
with student drawings of family pedigrees labeled Summers:
Marfan (See Figure 2), Healey: Blood Types, Jacques:
Osteogenesis Imperfecta, and Cohen: Achondroplasia.
The teacher is standing at the side of the room facilitating a
discussion about these family pedigrees.
| Teacher: |
Now that we've learned
about Mendel's model, the simple dominance model,
can we use it to explain any of the patterns in our
pedigrees? |
| Kelly: |
Well, I think Marfan is
dominant. |
| Teacher: |
Okay Since we are using
1's and 2's to show alleles in the Mendel model, can
you put some numbers up there so we can see what you're
talking about. |
Kelly pulls a stool over to one of the cabinets at the side of
the room, climbs up, and begins to label each of the circles and
squares on the pedigree with two alleles: Some are assigned the
genotype 1,2 (heterozygous, or possessing two different alleles)
and others 2,2 (homozygous recessive, or possessing two recessive
alleles) (see Figures 2A and 2B).
| Teacher: |
Kelly thinks that the
allele that causes Marfan syndrome is dominant and
she's put some genotypes up there to help us see her
idea. What do you all think about that? |
| Chee: |
Yeah, that's kayK. That
works. |
| Jamie: |
Yeah, because
all the filled in ones, the ones who have Marfan,
are all 1,2's, so it's dominant. |
| Curtis: |
Well, but we started off
by saying that it's dominant. I mean, we made that
assumption. If we say that the Marfan allele is recessive
and switch all the affected genotypes to 2,2's then
that would work too. Do you know what I'm saying? |
| Teacher: |
Wow! That's quite an idea.
I think we need help thinking about that, Curtis,
so can you write your genotypes next to Kelly's in
a different color. |
Curtis proceeds to label the same pedigree consistent with his
idea that the Marfan allele is actually recessive (see Figure
2C).
| Teacher: |
Well, that's very interesting. |
| David: |
I don't get it. Both of
them work. |
| Teacher: |
You think they both work.
Marfan could be dominant or recessive. |
| Lucy: |
Well, we can't tell right
now. |
| Sarah: |
But if we could take two
people with Marfan, like the grandmother and the son
and find out what kind of kids they'd have, then we
could tell for sure. |
| David: |
Gross! |
| Sam: |
That's sick, man! |
| Teacher: |
Wait a minute. Wait a
minute. What's Sarah saying here? |
| Sarah: |
That if you got children
from two affected people- |
| Curtis: |
-that you could tell if
it was recessive or dominant. |
| Teacher: |
What would you see? |
| Sarah: |
Well, if it's recessive,
then all the kids would be Marfan too. But if it's
dominant, then some of the kids might not be Marfan
'cause they could get like a 2 from both parents. |
| Sarah's Thought
Experiment
In Sarah's thought experiment, two individuals with
Marfan's syndrome would produce sex cells and those
sex cells would recombine during fertilization. Looking
at the children from such a mating would enable the
students to determine whether Marfan's syndrome is
inherited as a dominant or recessive trait because
the only situation in which you would expect to see
both unaffected and affected children would be if
Marfan's is inherited as a dominant trait.
A Punnett square is a representation that shows how
genetic information is passed from parents to offspring
through sex cell formation and then fertilization.
The Punnett squares to the left represent possible
offspring when two individuals with Marfan's syndrome
reproduce. In the first scenario, Marfan's is inherited
in a dominant fashion, while in the second, it is
a recessive disease.
|
| The students in this
high school biology class are engaged in genetic inquiry:
they are examining data and identifying patterns of inheritance
for various traits. They are also attempting to use a
powerful explanatory model, Mendel's model of simple dominance,
to account for the patterns that they see. |
| Teacher: |
Do you all see that? Sarah
is saying that if the parents had what genotype? |
| Sam: |
They'd have to be a 1,2,
right? |
| Teacher: |
A 1,2. Then if these parents
had kids, their kids could be what? |
| Kelly: |
1,2 or 1,1 or 2,2 |
| Teacher: |
Right. So Sarah is actually
proposing an experiment that we could do to find out
more. |
| Teacher: |
Now what about the Healey
pedigree? Can Mendel explain that one? |
| Chee: |
I don't think so. |
| Chris: |
Why not? |
| Chee: |
Because there's four things.
And Mendel only saw two. |
| Teacher: |
Four things? |
| Sarah: |
Yeah. Like phenotypes
or traits or whatever. |
| David: |
There's people who have
type A and people who have B and some who have AB
or O. |
| Tanya: |
But isn't AB the most
dominant or something? |
| Teacher: |
What do you mean by "most
dominant," Tanya? |
| Tanya: |
I don't know. It's just
like- |
| Chee: |
-like it's better or stronger
or something. |
| Tanya: |
Like you're gonna see
that showing up more. |
| Lee: |
Well even if that's true,
you still can't really explain why there're A's and
B's, too. It's not just AB is dominant to O, right?
You still have four different things to explain and
Mendel didn't see that. |
| Teacher: |
OK, so Mendel's model
of simple dominance isn't going to be enough to explain
this pattern is it? |
| Chris: |
Nope. |
|
Figure 1


Figure 1: (A) Student's Representation of Mendel's Simple Dominiance
Model. The simple dominance model accounts for the inheritance
of discrete traits for which there are two variants (designated
A and B). Each individual in the population possesses two alleles
(designated 1 and 2) for the trait and one allele (1) is completely
dominant over the other (2). There are only two different alleles
in the population. For example, for plant height there are two
phenotypic variants, Short and Tall. There are also two different
alleles in the population, designated allele 1 and allele 2. Plants
whose genetic makeup consists of (1,1) or (1,2) will be Tall,
whereas plants whose genetic makeup is (2,2) will be Short. (B)
Meiotic Processes governing Inheritance. The underlying
processes governing simple dominant inheritance are Mendel's law
of segregation (the meiotic process of sex cell formation
during which half of all parental genetic information is packaged
into sperm or egg cells) and fertilization (during which
genetic information from both parents combines in the offsprint). |
Figure 2

Figure 2: Pedigrees representing inheritance of Marfan Syndrome
in the Summers family. (A) The original Pedigree represented
in the inheritance pattern within the Summers family without specifying
individual genotypes. (B) Kelly's genotype assignments,
assuming that Marfan Syndrome is inherited as a dominant trait.
(C) Curtis' genotype assignments, assuming that Marfan
Syndrome is inherited as a recessive trait. |
The students in the high school biology class that we just 'peeked into' are
engaged in genetic inquiry: They are examining data and identifying patterns
of inheritance for various traits. They are also attempting to use a powerful
explanatory model, Mendel's model of simple dominance, to account for the
patterns that they see. And just as practicing scientists do, they recognize
the limitations of their model when it simply cannot explain certain data
patterns. These students are poised to continue their inquiry in genetics
by revising Mendel's model such that the resulting 'new' models will be able
to explain a variety of inheritance patterns, including the multiple allele/codominance
pattern within the Healey pedigree.
Attending to Significant Disciplinary Knowledge
| The genetics unit in this biology class reflects important
aspects of authentic genetic practice, particularly the use of essential
explanatory models to account for and predict complex inheritance
patterns in organisms. |
The genetics unit in this biology class reflects important aspects of authentic
genetic practice, particularly the use of essential explanatory models to
account for and predict complex inheritance patterns in organisms. We have
already discussed the basis for emphasizing in our curricula particular ideas
about the nature of science-these include how and why scientists use models
to explain natural phenomena and the general criteria they apply when judging
the utility of such models. In this section, we will focus on the genetics
concepts that are foundational in our biology unit and how we engage students
in model assessment and revision in this context.
| In choosing which genetics concepts to place at the
core of our curriculum, we attempted to identify a set of important
disciplinary concepts that would provide a rich context for illustrating
how those concepts were generated and justified through inquiry. |
The choice of which genetics concepts to place at the core of our curriculum
was governed by our belief that whatever specific subject matter we chose
should not only be central to understanding in the discipline of genetics,
but should also lend itself to be studied from the perspective of model-based
inquiry; that is, we wanted a set of important disciplinary concepts that
would provide a rich context for illustrating how those concepts were generated
and justified. Thus, we selected the set of concepts associated with an understanding
of classical Mendelian inheritance as a foundation to our genetics unit. In
fact, the development of modern genetic theory from its classical Mendelian
origins has been the subject of much historical and philosophical analysis.
In Theory Change in Science (1991), philosopher Lindley Darden draws
on historical evidence to identify a set of strategies scientists utilized
to generate and test ideas while conducting early inquiries into the phenomenon
of inheritance. She traces the development of a number of inheritance models
that were at least originally seen to be at odds with those underlying a Mendelian
(i.e. simple dominance) explanation of inheritance. Among these are the notions
of linkage and multiple forms (alleles) of a single gene. In short, Darden
provides a history of model-based inquiry into the phenomenon of inheritance-from
a classical genetics perspective. Drawing on Darden's work and our own experiences
as teachers and researchers in genetics, we were able to see how engaging
students in building and revising Mendel's simple dominance model could provide
rich opportunities to learn important genetics concepts as well as key ideas
about the practice of genetics.
| Early instruction in the genetics class includes a
few days during which students learn about the meiotic model and what
phenomena this model can explain. |
Early instruction in the genetics class includes a few days during which students
learn about the meiotic model and what phenomena this model can explain. This
instruction will be discussed in more detail below in the section about Attending
to Student Knowledge and is described only briefly here. However, we wish
to point out that, although Mendel did not have the advantage of a basic meiotic
model with which to begin his work, it has proven fruitful to start here with
students. In an introductory activity, students look at sets of pictures and
are asked to determine which are members of the same families. The bases for
their decisions include physical similarities between parents and children
and between siblings. Thus, instruction about meiosis focuses on how the meiotic
model can account for these patterns: Children resemble their parents because
they receive information from both of them; and siblings resemble each other
but are not exactly alike because of the random assortment of parental information
in meiosis.
| Meiosis is the process
by which sperm and egg cells are formed. During meiosis, chromosomal
replication is followed by two rounds of cell division. Thus,
one cell undergoing meiosis produces four new cells each of which
contain half the number of chromosomes of the original parent
cell.
See figure 1B for a representation of meiotic processes that underlie
Mendel's simple dominance model of inheritance.
|
After students have developed some understanding of meiosis, they "create",
with guidance from the teacher, a representation of Mendel's model of simple
dominance (see Figure 1A) in an attempt to further explain why offspring look
like parents. First, "Mendel" (a teacher dressed in a monk's robe and wearing
antique spectacles) pays the class a visit and tells them that he'd like to
share some phenomena and one important model from his own research with them.
In character, "Mendel" passes out three packets of peas representing a parental
generation and the F1 and F2 generations (the first and second filial generations,
respectively). He asks students to characterize the peas according to color
and shape. For example, the parental generation includes a round green pea
and a wrinkled yellow pea. The F1 generation contains only round yellow peas.
Finally, the F2 generation contains a mixture of round yellow, wrinkled yellow,
round green, and wrinkled green peas in a ratio of approximately 9:3:3:1.
Using what they already know about meiosis-particularly the fact that offspring
receive information from both parents-they reconstruct Mendel's model of simple
dominance to account for these patterns (see Figure 1A).
| After students have developed some understanding of
meiosis, they "create", with guidance from the teacher,
a representation of Mendel's model of simple dominance (see Figure
1A) |
While Darden's work aided in the identification of important inheritance models
and strategies scientists used to judge those models, it was the work of philosopher
Philip Kitcher (1984, 1993) that put the simple dominance model developed
by students into context with comparable models of geneticists. According
to Kitcher, genetic 'schemata'-which we take to be analogous to explanatory
'models'-provide the following information:
| The genetic models of both scientists and students
provide information about different alleles for a given trait and
the ways in which they combine to give rise to particular phenotypic
variants. |
(a) Specification of the number of relevant loci and the number of alleles
at each locus; (b) Specification of the relationships between genotypes
and phenotypes; (c) Specification of the relations between genes and chromosomes,
of facts about the transmission of chromosomes to gametes (for example,
resolution of the question whether there is disruption of normal segregation)
and about the details of zygote formation; (d) Assignment of genotypes
to individuals in the pedigree. (1984, p.356)
Moreover, Kitcher describes how such schemata (models) might be used in inquiry:
[A]fter showing that the genetic hypothesis is consistent with the data
and constraints of the problem, the principles of cytology and the laws
of probability are used to compute expected distributions of phenotypes
from crosses. The expected distributions are then compared with those
assigned in part (d) of the genetic hypothesis. (p.356)
With their teacher's guidance, students represent the simple dominance model
of Mendel in a way consistent with Kitcher's description of the models of
geneticists. They pay particular attention to (b) and (d) above: Specifying
the relationships between genotypes and phenotypes and identifying the genotypes
of individuals in their experimental populations. Because our unit doesn't
address multi-gene traits, one locus per trait is assumed (thus part of criterion
(a) above is not applicable in this case) and students focus on determining
the number of alleles at that locus. Finally, the students' prior understanding
of meiosis-developed earlier in the unit-enables them to specify chromosomal
transmission of genes for each particular case (letter (c) above). A representation
of Mendel's model of simple dominance is shown in Figure 1A.
| The software program GENETICS CONSTRUCTION KIT or GCK
(Calley & Jungck, 1997) enables students to study populations
of "virtual fruit flies," noting patterns in the inheritance
of various traits such as eye color, wing shape, etc. Students use
their genetic models to explain data that they generate using GCK
as well as other types of data pertaining to the inheritance of human
diseases. |
Once the students have represented and used the simple dominance model to explain
some phenomena, such as inheritance of characteristics in peas and disease
traits in humans, the students use the model to explain data that they generate
using the software program genetics construction kit or GCK (Calley &
Jungck, 1997). This program enables students to manipulate populations of
virtual organisms (usually fruit flies) by performing matings (or crosses)
on any individuals selected. Each cross produces a new generation of organisms
whose variations for particular traits are described. Thus, the students develop
expertise using the simple dominance model to account for new data and they
also design and perform crosses to test their initial genotype to phenotype
mappings within these populations. Figure 3 is an excerpt from one student's
work with GCK and the simple dominance model.
| The next step is to revise Mendel's simple dominance model to
account for anomalous inheritance patterns such as—
- codominance;
- multiple alleles; and
- linkage.
|
Next, the teacher presents or revisits phenomena that the simple dominance
model cannot explain. For example, students realize during the application
of the simple dominance model to explain their human pedigrees that it is
inadequate in some cases: It can't be used to account for inheritance of human
blood types or achondroplasia. The next step for the class is to study these
"anomalous" inheritance patterns using GCK. They begin with achondroplasia,
a trait for which there are three variations rather than two. Students revise
the simple dominance model to account for the codominant inheritance pattern
observed for this trait. While solving GCK problems such as this one, students
propose models that specify some or all of the information (a through d) listed
above and then test their models for fit with existing data as well as for
the ability to accurately predict the results of new experiments.
| Geneticists must assess a new inheritance model in
part based upon how well it fits within an existing family of related
models such as meiosis (including cytological data) and molecular
biology (which specifies the relationships between DNA and proteins
as well as protein actions in cells). Students must also judge their
models based upon their consistency with previous models or ideas. |
Since most students ultimately explain the inheritance of achondroplasia using
a codominant model (one where each possible genotype maps to a distinct phenotype),
they must also revise their understanding of "dominance" and "recessiveness."
Up to this point, most students tend to associate recessiveness either with
(a) a phenotype; or (b) any genotype that contains a recessive allele (designated
with the number 2); or (c) both. It is quite common for students to conclude
that the phenotypes associated with (1,1) and (1,2) genotypes are both "recessive."2
However, this is inconsistent with their prior concept of recessiveness as
it was developed in the simple dominance model. Thus, it is at this point
in the unit that we emphasize the need for models to be consistent with other
knowledge in a scientific discipline. In other words, geneticists must assess
a new inheritance model, in part, based upon how well it fits within an existing
family of related models such as meiosis (including cytological data) and
molecular biology (which specifies the relationships between DNA and proteins
as well as protein actions in cells). After explicit instruction about DNA
transcription, translation, and protein function, students attempt to reconcile
their codominance models with this new model of protein action in cells. In
the case of codominance, this requires them to conceptualize recessiveness
at the level of alleles and their relationships to one another rather than
at the level of phenotypes or genotypes. In so doing, students construct meanings
for dominance and recessiveness that are consistent across various inheritance
models (i.e. simple dominance, codominance, multiple alleles, etc.) as well
as models of meiosis and molecular biology.
For the final GCK inquiry, the students are organized into two research teams,
each of which consists of four small research groups. Each team is assigned
a population of virtual fruit flies and told to explain the inheritance of
four traits within this population. The work is divided such that each research
group studies two of the traits. Consequently, there is some overlap of trait
assignments among the groups within a team. The teams hold research meetings
periodically and a minimal structure for those meetings is imposed: Two groups
present some data and tentative explanations of the data, one group moderates
the meeting, and one group records the proceedings. The roles of individual
groups alternate in successive meetings.
| The entire class then gathers for a final conference
during which students create posters that summarize their research
findings, take turns making formal presentations of their models,
and critique their classmates' models.
|
Each of the fly populations in this last problem contains traits that exhibit
the following inheritance patterns: (1) Mendelian simple dominance; (2) codominance;
(3) multiple alleles (specifically, three different alleles with varying dominant/codominant
relationships between pairs of alleles); and (4) X-linkage. After about a
week of data collection, model testing, and team meetings, each small research
group is usually able to describe a model of inheritance for at least one
of the traits in their population, and most groups can describe inheritance
models for both of the traits on which they chose to focus. The entire class
then gathers for a final conference during which students create posters that
summarize their research findings, take turns making formal presentations
of their models, and critique their classmates' models.
This high school biology curriculum is designed to give students opportunities
to learn about genetic inquiry in part by providing them with realistic experiences
doing inquiry in the discipline. Since a primary goal of practicing scientists
is to construct explanatory models to account for natural phenomena, involving
students in the construction of their own explanatory models provides a major
emphasis in the classroom. The students work in groups structured like scientific
communities to develop, revise, and defend explanatory models for inheritance
phenomena. The overall instructional goals include helping students to understand
mechanistic explanations for inheritance patterns in fruit flies and humans
and to appreciate the degree to which scientists rely on empirical data as
well as broader conceptual knowledge to assess explanatory models. The role
that research about students' ideas has played in helping to achieve these
instructional goals is the focus of the next section of this chapter.
Attending to Student Knowledge
The collaboration between high school science teachers and educational researchers
that has recently come to be known as MUSE really began over a decade ago
with a nine week genetics course in a biology curriculum for juniors and seniors.
That original genetics course provided the setting for many studies about
student learning, with a particular focus on how students used, revised, and
judged scientific models. What we have learned about students in our own classrooms,
as well as what others have shared in related studies (Kindfield, 1991 &
1994; Grosslight, et al., 1991; also see Allchin, 2000), has informed us as
we-
- sequence concepts within the curriculum.
- design tasks for students & structure classroom instruction.
- design and implement formative and authentic assessments.
| Students begin their exploration of Mendelian inheritance
in pea plants with a firm understanding of a basic meiotic model and
continue to refer to this model as they examine increasingly complex
inheritance patterns in humans. |
Curriculum. Students' difficulty in thinking about and discussing
issues related to the consistency of particular genetics concepts with one
another informed the sequence of elements within the genetics unit. In our
early studies (as well as those in similar studies on problem solving in genetics-see
Kindfield, 1994) students often did not examine their inheritance models to
see whether they were consistent with meiosis. In fact, students proposed
models where offspring received unequal amounts of genetic information from
their two parents or had fewer alleles at a particular locus than did their
parents (Cartier, 2000a). Because of their struggles and the fact that meiosis
is central to any particular model of inheritance, we placed this model first
in the revised curricular sequence. Students now begin their exploration of
Mendelian inheritance with a firm understanding of a basic meiotic model and
continue to refer to this model as they examine increasingly complex inheritance
patterns. Another example of how the curricular sequence pushes students to
examine ideas for consistency is the placement of instruction about molecular
biology and protein function after students develop a codominant model of
inheritance (described in detail in the previous section).
Instruction. Students' ideas about models. One early study
of student learning in the genetics unit focused on identifying the criteria
that students used when assessing their models for inheritance phenomena (Cartier,
2000a). The study was predicated on a commitment to developing the idea of
consistency as a basis for model assessment with students early in the course.
Thus, students read a mystery scenario involving a car accident and evaluated
several explanations of the cause of the accident. Each explanation was problematic
because it was either (1) inconsistent with some of the information they'd
been given; (2) inconsistent with their prior knowledge about the world; or
(3) unable to explain all of the information mentioned in the original scenario.
Students discussed these explanations and their shortcomings and the teacher
provided the language for talking about model assessment criteria: she instructed
them to seek-
- explanatory power.
- predictive power (which was discussed but not applied to the accident
scenario).
- internal consistency (among elements within the model).
- external consistency (between a model and one's prior knowledge or other
models).
| Studies have shown that students view models in
a "naïve realistic" manner (as objects or replicas
of actual phenomena) rather than as conceptual structures that
scientists use to explain data and ask questions about the natural
world.
Grosslight, 1991
Cartier, 2000a
Also see Harrison and Treagust, 1998 |
Throughout the genetics unit, students were prompted to use these criteria
to evaluate their own assessment of inheritance models. Despite the explicit
emphasis on consistency as a criterion for model assessment, however, we found
that very few students actually judged their models this way. Instead, students
valued explanatory adequacy, visual simplicity, and "understandability" more
strongly. A closer look at the work of students in this study showed that
most of them-7 of the 8 interview participants-did not view models as conceptual
structures, but as physical replicas, instructional tools, or visual representations.
Other studies have also shown that students view models in this "naive realistic"
manner rather than as conceptual structures that scientists use to explain
data and ask questions about the natural world (Grosslight, 1991; also see
Harrison and Treagust, 1998). Given this understanding of models, it was not
surprising that our genetics students neither applied nor discussed the criterion
for conceptual consistency within and among models. Moreover, the students
in this study also participated in a "black box" activity (similar to the
detergent box task described on page XX) where they proposed explanations
for a data pattern associated with an imaginary box (described in Cartier,
2000a). When attempting to apply the model assessment criteria to their explanations
for this pattern, several students treated "internal consistency" and "external
consistency" literally: They evaluated the box's proposed internal components
and the external phenomena (observations) separately. This confusion stemmed
from students' prior understanding of concepts associated with vocabulary
that we provided: Clearly "internal" and "external" were already meaningful
to students and their prior knowledge took precedence over the new meanings
with which we attempted to imbue these terms.
| Teachers present sample models that purport to explain the phenomena
at hand and ask students to evaluate these models. Most commonly,
students will describe the need for a model to-
- explain all the data;
- predict new experimental outcomes; and
- be realistic.
Throughout the course, the instructor will return to these assessment
criteria in each discussion about students' own inheritance models.
|
Following this study, we altered the instruction in the genetics unit to take
into consideration students' prior knowledge about models and particular vocabulary
used to describe their attributes. Most importantly, we recognized the powerful
prior ideas students brought with them about models as representational entities
and explicitly addressed these ideas at the outset of the unit. In the genetics
unit currently taught by members of our collaborative, teachers employ tasks
early on that solicit students' ideas about scientific models (including a
black box task and a reading/homework reflection about models) and explicitly
define the term "model" as it will be used in the science unit. Additionally,
teachers provide model assessment tasks to students and allow them to develop
their own vocabulary for discussing shortcomings of particular explanations.
Frequently, teachers will present sample models that purport to explain the
phenomena at hand (most often the black box data patterns) and ask students
to evaluate these models. Teachers create the models to have particular shortcomings
in order to prompt discussion by students. Most commonly, students will describe
the need for a model to explain all the data, predict new experimental
outcomes, and be realistic (their term for conceptual consistency).
Throughout the course, teachers will return to these assessment criteria in
each discussion about students' own inheritance models. A subsequent study
has shown that these instructional modifications (along with other curricular
changes in the genetics unit) helped students to understand the conceptual
nature of scientific models and learn how to evaluate them for consistency
with other ideas (Cartier, 2000b).
| In early versions of the genetics course, students
paid attention more to empirical issues than to consistency among
models when assessing their new inheritance models. When the course
was revised, we situated the study of inheritance patterns within
the context of a single population of organisms. This emphasized the
need for each inheritance model to be basically consistent with other
models within genetics. |
Students' model revision heuristics.Several early studies of students'
GCK work in our genetics unit revealed that students assessed their tentative
models based primarily on empirical criteria rather than conceptual criteria
(Cartier, 2000a & 2000b). Even when conceptual inconsistencies occurred
between their proposed models and other models or biological knowledge, their
primary focus was usually on how well a given model could explain the data
at hand. Students frequently had difficulty recognizing specific inconsistencies
between their models and meiosis or other biological knowledge such as the
method of sex determination in humans. In some instances, students recognized
that their models were inconsistent with other knowledge but were willing
to overlook such inconsistencies when their models were judged to have adequate
explanatory power. (For example, students sometimes proposed models to account
for x-linkage inheritance patterns wherein a male organism simply could never
be heterozygous. No explanation consistent with independent assortment in
meiosis was given for this model.) Thus, students paid more attention to empirical
than conceptual issues and tended to value empirical power over conceptual
consistency in models where both criteria were brought to bear. In summary,
the students' methods of model assessment differed from those of practicing
geneticists
White & Frederiksen (1998) have described a middle school science curriculum
designed to teach students about the nature of inquiry generally and the role
that modeling plays in specific scientific inquiries. One aspect of the curriculum
that had a measurable effect on its success was the emphasis on reflective
assessment on the part of the students. Following modeling activities, students
were asked to rate themselves and others in various categories, including
"understanding the science", "understanding the processes of inquiry", being
systematic", and "writing and communicating well" (p. 25). Involving the students
in such an explicit evaluation task helped emphasize the importance of learning
"about" inquiry and modeling in addition to learning how to "do" inquiry.
| Influenced by our research in the genetics unit, we built into
the curriculum more tasks that require students to reflect, write,
and discuss conceptual aspects of genetic modeling. These tasks
include-
- journal writing
- written self-assessments
- homework assignments that require students to explain their
reasoning (see Figure 3)
- class presentations (both formal and informal).
|
Our work developing tasks for students is also predicated on the importance
of metacognitive reflection on their part. Influenced by our research in the
genetics unit, we built into the curriculum more tasks that require students
to reflect, write, and discuss conceptual aspects of genetic modeling. These
tasks include journal writing, written self-assessments, homework assignments
that require students to explain their reasoning (see Figure 3), and class
presentations (both formal and informal). Most importantly, we created a complex
problem, involving several different inheritance patterns, and asked the students
to account for this new data while working in cooperative laboratory teams.
As described above, the regular team interactions required students to be
critical of their own thinking and that of others. Moreover, situating the
study of these inheritance patterns within the context of a single population
of organisms helped to emphasize the need for each inheritance model to be
basically consistent with other models within genetics. We have found that
in this new context, students are more successful at proposing explanatory
models and have a better understanding of the conceptual nature of such scientific
models (Cartier 2000a & 2000b).
Assessment Design. Related to the design of tasks is the design
of good assessments; That is, we attempt to employ formative and authentic
assessments throughout the unit. Because students struggle with conceptual
problems in the genetics unit, we incorporate a number of assessments that
require students to describe the relationship between models or ideas that
they have learned (Figure 4A). Whenever possible, we design formal assessments
as well as written classroom tasks that reflect the structure of students'
work in the classroom. Our students spend a lot of their class time working
in groups, pouring over data, and talking with one another about their ideas.
Thus, assessments also require them to look at data, propose explanations,
and describe the thinking that led to particular conclusions (Figure 4B).
Figure 4a
Below is a concept map that represents the relationships between
specific models, models in general, and data. Use the map to answer
the questions below.
a. Remember that a line in a concept map represents a relationship
between two terms (concepts, ideas, etc.) in the map. Write a
few sentences that describes the numbered relationships between
the terms given. Be as specific as you can: use the appropriate
vocabulary of genetics to make your point as clearly as possible.
b. Draw a line (not necessarily a straight one) to separate the
world of ideas from that of observations on this map. Please label
both sides. Justify your placement of that line.
|
Figure 4b
Inheritance of PKU in the Samson Family

1. Use Mendel's simple dominance model to assign genotypes to the
individuals in this pedigree.
2. Do the affected individuals in this pedigree show a dominant
or recessive variation of the trait? Pick two family groups (a
group is one set of parents and their offspring) and describe
how those groups helped you make that decision.
3. Describe how you would convince another student, who had no
knowledge of how PKU is inherited, that you understand the inheritance
of this trait. As the student is not easily convinced, you must
carefully show how the Mendel model can be used to support your
idea.
Figure 4B: Simple Dominance homework assignment. Students
are asked to use Mendel's simple dominance model to explain a
realistic data pattern. They are also asked to explicitly justify
their reasongng similar to the way in which they argue in support
of their ideas in regular classroom activities. |
Summary
The structure of the genetics class that we have described reflects important
aspects of scientific practice: Students are engaged in an extended inquiry
of inheritance through which they collect data, seek patterns and attempt
to account for those patterns using explanatory models. The models proposed
by students also bear a great deal of resemblance to those of practicing geneticists
in that they specify allelic relationships and genotype to phenotype mappings
for particular traits. In the next section, we will describe a course in evolutionary
biology that provides opportunities for students to participate in realistic
inquiry within another sub-discipline of biology. In the final section, we
will discuss some important instructional and curricular elements that these
two courses share and that are crucial to the learning success of students
in these contexts.
Section 3: Developing Darwin's Model of Natural Selection in High School Evolution
| Developing a Darwinian Explanation
Hillary and Jerome are sitting next to each other in their sixth
hour science class waiting for the bell to ring.
| Jerome: |
What are
we doing in here today? |
| Hillary: |
I think we will be starting
the next case study. |
The bell rings and their teacher announces that the class will
start work on the last of three case studies designed to allow
them to continue to develop and use the Darwinian model of natural
selection. She tells the students that there are two parts to
this third case. First, they will need to use their knowledge
of the natural selection model to develop an explanation for the
bright coloration of the male ring-necked pheasant. And secondly,
they will have to write a research proposal that will then be
considered by the rest of the students in a research grant competition.
| Casey: |
What kind
of research proposal? |
| Mrs. J. : |
Each of you has seen during
the past two cases that there are aspects of your
explanation that you would like to explore further
or confirm in some way. This is your opportunity to
imagine how you might do that. Each group will need
to think about their explanation and identify areas
that could use a bit more evidence. |
As Mrs. J. passes out the eight pages of case materials she asks
the students to get to work. Each group receives a file folder
containing the task description and information about the natural
history of the ring-necked pheasant. There are color pictures
that show adult males, adult females, and young. Some of the pages
contain information about predators, mating behavior and mating
success. Hillary, Jerome and their third group member, Grace,
begin to shuffle through the pages in the folder.
| Hillary: |
The males
look completely different from the females! |
| Jerome: |
Okay, so what
are we supposed to be doing here? |
| Grace:
|
It is similar
to the last case. We need to come up with a Darwinian
explanation for why the males look brighter than the
females. |
| Hillary: |
How can this be? It seems
like being bright would be a problem for the males,
so how can it fit with Darwin's ideas? |
| Grace: |
Well, I guess we need
to look at the rest of the stuff in the folder. |
The three students spend the remainder of the period looking over
and discussing various aspects of the case. By the middle of the
period on Tuesday this group is just finalizing their explanation
when Casey, a member of another group, asks if she can talk to
them.
| Casey:
|
What have
you guys come up with? Our group was wondering if
we could talk over our ideas with you. |
| Grace: |
Sure, come
over and we can each read our explanations. |
These two groups have very different explanations. Hillary's group
is thinking that the males bright coloration distracts predators
from the nest, while Casey's group has decided that the bright
coloration confers an advantage on the males by helping them attract
more mates. A lively discussion ensues.
| Ed: |
But wait, I don't understand.
How can dying be a good thing? |
| Jerome: |
Well, you have to think
beyond just survival of the male himself. We think
that the key is the survival of the kids. If the male
can protect his young and give them a better chance
of surviving then he has an advantage. |
| Claire: |
Even if he
dies doing it? |
| Grace: |
Yeah, because he will
have already passed on his genes and stuff to his
kids before he dies. |
| Casey: |
How did you come up with
this? Did you see something in the packets that we
didn't see? |
| Grace: |
One reason
we thought of it had to do with the last case with
the monarchs and viceroy. |
| Hillary: |
Yeah, we were thinking
that the advantage isn't always obvious and sometimes
what is good for the whole group might not seem like
it is good for one bird or butterfly or whatever. |
| Jerome: |
We also looked at the
data in our packets on the number of offspring fathered
by brighter versus duller males. We saw that the brighter
males had a longer bar. |
| Grace: |
See look on page 5, right
here. |
| Jerome: |
So they had more kids,
right? |
| Casey: |
We saw that table too,
but we thought that it could back up our idea that
the brighter males were able to attract more females
as mates. |
|
The groups agree to disagree on their interpretation of this piece of data
and continue to compare their explanations on other points.
| Because of the key role that natural selection plays
in understanding evolution and biology more generally we devote a
significant period of time to allow students to develop a deep understanding
of the process. |
The students in the above vignette are using Darwin's model of natural selection
and realistic data to create arguments about evolution in a population of
organisms. In doing so they attend to and discuss ideas such as selective
advantage and reproductive success that are crucial components of the model.
Because of the key role that natural selection plays in understanding evolution
and biology more generally we devote a significant period of time to allow
students to develop a deep understanding of the process. Early in the course
students have opportunities to learn about natural selection, but as
the course progresses they are required to use their understanding
to develop explanations (as illustrated in the vignette). This structure provides
students with a setting in which to clarify their understanding of natural
selection and it also gives them a context for learning by and about scientific
inquiry. Section 3 will describe how the three design principles discussed
earlier were influential in the development of this nine week course.
Disciplinary knowledge
"There is probably no more original, more complex,
and bolder concept in the history of ideas than Darwin's mechanistic
explanation of adaptation."
-Ernst Mayr (1982, p. 481) |
The choices we make when designing curricula are determined in part by examining
the discipline under study. In the case of evolution it is clear that a solid
understanding of natural selection provides a foundation upon which further
knowledge can be built. But that foundation is hard won and takes time to
develop because the concepts that comprise the natural selection model are
difficult for students to understand and apply. Indeed, biologist Ernst Mayr
has said that "there is probably no more original, more complex, and bolder
concept in the history of ideas than Darwin's mechanistic explanation of adaptation"
(1982, p. 481). Many courses or units in evolutionary biology at the high
school level require far shorter periods of time than the nine weeks described
here, and also include additional sophisticated concepts such as genetic drift
and speciation. It seems clear to us, however, that with a large number of
concepts covered in a short period of time the likelihood that students will
develop a deep understanding of any of them is diminished-a survey of content
is not sufficient to develop lasting understanding. Furthermore, we have expanded
the notion of content to include opportunities for students to engage in and
think explicitly about disciplinary inquiry. With these considerations in
mind our teacher/researcher collaborative designed a course that requires
students to not only learn about the natural selection model, but also to
use it in ways that are analogous to those of evolutionary biologists.
| Our concept of a Darwinian explanation draws together
the components of the natural selection model (shown in figure 5)
with a narrative structure that demands attention to historical contingency. |
Central to our approach is a commitment to looking at authoritative accounts
of scientific practice for guidance. Kitcher's (1994) concept of a Darwinian
history assisted us in thinking about how students could use the natural selection
model in authentic ways. He describes a Darwinian history as a "narrative
which traces the successive modifications of a lineage of organisms from generation
to generation in terms of various factors, most notably that of natural selection"
(p. 20-21). The use of narrative explanation is a key way in which evolutionary
biology can be distinguished from other scientific disciplines. "Narratives
fix events along a temporal dimension, so that prior events are understood
to have given rise to subsequent events and thereby explain them" (Richards,
1992). Thus, our concept of a Darwinian explanation draws together the components
of the natural selection model (shown in figure 5) with a narrative structure
that demands attention to historical contingency. Frequently textbook examples
of explanations for particular traits take the form of what O'Hara (1988)
terms "state-explanations"-that is, they explain the present day function
of particular character states without reference to their history. In contrast,
what we call a Darwinian explanation attempts to explain an event, or as O'Hara
says, "how-possibly" a trait came into being. This type of explanation is
summarized by Mayr (1997),
When a biologist tries to answer a question about a unique occurrence such
as 'Why are there no hummingbirds in the Old World?' or 'Where did the
species Homo sapiens originate?' he cannot rely on universal laws.
The biologist has to study all the known facts relating to the particular
problem, infer all sorts of consequences from the reconstructed constellation
of factors, and then attempt to construct a scenario that would explain
the observed facts in this particular case. In other words, he constructs
a historical narrative (p. 64).
Although, our focus is limited in scope to the development and use of the natural
selection model, these considerations of the form of explanation in the discipline
played a key role in the design of the course described here.
| Providing opportunities for students to use the natural
selection model to develop narrative explanations is a central goal
of the course. |
Providing opportunities for students to use the natural selection model to
develop narrative explanations that are consistent with the view described
above is a central goal of the course. During the final weeks of the course,
students are engaged in creating Darwinian explanations. In essence, students
are asked to use the components of the natural selection model to make sense
of realistic data that they have been given concerning an extant organism.
Each scenario is presented to students as a case study and they are provided
with materials that describe the natural history of the organism. Photographs,
habitat and predator information, mating behavior and success, and phylogenetic
data are examples of the types of information that may be included in a given
case (complete case materials can be found on the MUSE website). Students
then weave the information into a narrative that must take into account all
of the components of a natural selection model and describe the change over
time that may have occurred (see Figure 5 for a sample student Darwinian explanation).
As students hone their abilities to develop and assess evolutionary arguments
over three successive case studies, they are able to participate in realistic
evolutionary inquiry.
Figure 5
Monarchs and viceroys are very similar in appearance, although
this has not always been true. The brightness in both butterflies
is viewed as an advantage in their environment—where
a main predator is the blue jay—an advantage that may
be explained by the Darwinian model.
Each butterfly lays many more eggs than can survive on the
limited resources in its environment. As a result of this
limit, there is a struggle among the offspring for survival.
As within all species, there exists natural variation among
the populations of monarchs and viceroys, including variations
of color. In the past populations, some butterflies were brightly
colored and others were dull. Blue jays, a main predator of
the monarch, rely on movement and coloration to identify their
prey when hunting. They can vomit up bad tasting or poisonous
food, and exhibit an ability to learn to avoid such food in
the future.
As caterpillars, monarchs' source of food is milkweed leaves,
which contain cardenolides—poisonous or unpalatable
substances. As the larva is growing, they ingest a great amount
of cardenolides. As butterflies, these substances remain in
their bodies, making them unpalatable to their predators.
When blue jays eat the monarchs, they react to the cardenolides
by vomiting up their prey. They learn by this experience that
they should avoid the brightly colored monarchs to avoid the
cardenolides. The dull monarchs, although poisonous, were
still consumed by their predators more because they more closely
resembled non-poisonous prey such as moths, grasshoppers,
and lacewings. The brightly colored monarchs survived more
than the dull ones and were more prolific. After many generations,
most monarchs were bright because of their success in the
environment. Because of the blue jays' association of bright
colors with bad food, the brightly colored viceroys, although
not poisonous like the monarch, were also avoided, and this
advantageous variation was passed on as with the monarch.
Figure 5. Darwinian explanation written by a group of students
at the end of the Monarch/Viceroy case |
| By the end of our course students are able to reason
in sophisticated ways about evolutionary phenomena. |
By the end of our course students are able to reason in sophisticated ways
about evolutionary phenomena. We believe that careful consideration of the
discipline is an important part of realizing that goal, but also recognize
that there are several other key areas that must be considered. In the sections
that follow we describe how we structured the class to take seriously student
ideas and how we developed a community in which students were required to
make their own thinking explicit.
Student Knowledge
| Studies have confirmed what teachers already know-students
have very tenacious misconceptions about the mechanism of evolution
and its metaphysical assumptions. |
Anyone who has ever taught evolution can attest to the fact that students bring
a wide range of conceptions and attitudes to the classroom. During the past
two decades there have been several studies that document student ideas both
before and after instruction (see for example Bishop and Anderson, 1990; Demastes,
S., Good, R., Peebles, P. (1995, 1996) and Trowbridge & Cummins, 1992).
These studies have confirmed what teachers already know-students have very
tenacious misconceptions about the mechanism of evolution and its metaphysical
assumptions. Our knowledge of the ideas students bring with them about both
evolutionary biology and the nature of science has been important in our curriculum
design efforts.
Curriculum: In the evolution course there are several specific ways
in which we have considered student ideas when choosing and sequencing course
content. For example, the course begins with an extended examination of argumentation
in science and in evolution in particular. In early versions of the course
we found that students were unable to engage in discussions about particular
arguments because they did not have a framework with which they were all familiar.
Our course has since been modified to provide opportunities for students to
develop such a framework by participating in a cartoon sequencing activity
(described in more detail below). Our decision to place this at the beginning
of the course was a relatively easy one as we felt students needed to have
the a framework for critiquing arguments for virtually every subsequent experience.
We have also found that students have difficulty in understanding the concept
of variation that is a foundation of the natural selection model. These difficulties
have been documented in the literature (Bishop & Anderson, 1990) and we
have found them to be present in our own classrooms. Because of the experiences
students have with variability in most genetics instruction-in which they
usually examine traits with discrete variations-the concept of continuous
variation can be a significant challenge for students. We have seen that an
incomplete understanding of variation in populations promotes the student
idea that adaptations are a result of a single dramatic mutation, or that
selection is an all-or-none event operating on one of two to three possible
phenotypes. Recognition of these problems has led to explicit instruction
on variability in populations and perhaps more importantly the inclusion of
opportunities for students to examine and characterize the variability present
in real organisms before they begin using the concept in constructing Darwinian
explanations.
One of the activities used for this purpose is a relatively simple one, but
it provides a powerful visual representation that students can draw on later
when thinking about variation in populations. Typically, students do not recognize
the wide range of variation that is present even in familiar organisms. In
order to give students experience thinking about and characterizing variation,
we have them examine sunflower seeds. Their task is to count the stripes on
a small sample, but even this simple direction is less than straightforward
as the class then has to negotiate what counts as a stripe, whether to count
one side or two, or if the edges should be included. Once they have come up
with common criteria and have sorted their sample into small piles the teacher
has them place their seeds into correspondingly numbered test tubes. The result,
once the test tubes are lined up in a row, is a clear visual representation
of a normal distribution. The subsequent discussion centers on ways to describe
distributions using concepts like mean, median, and mode. This activity takes
place prior to times when students will need to draw on their understanding
of variation to construct explanations using the Darwinian model of natural
selection.
Careful thought has been given to how instruction can be carried out
to allow student ideas to be the focus in the classroom. The three
major sections of the course are each do this is their own way:
• First, the students work with the teacher to develop a
framework for examining arguments that focuses in part on prior knowledge.
• Second, they examine historical ideas that correspond to
common alternate conceptions students have.
• Third, they work on case studies that require them to develop
original explanations and defend them to their peers. |
| Students realize that they are all examining the same data but that
they also each bring a lifetime of experience to the table. Together
they establish that the process of making inferences is influenced
by what they observe (the data) and by their own prior knowledge and
beliefs. |
Instruction: Consideration of student knowledge is not simply a commitment
we have when making curricular decisions; it also plays into the design of
instructional tasks. An important feature of the evolution course is explicit
discussion among students about their own ideas. Furthermore, we have designed
instruction to address some of the common misconceptions that students bring
to the study of evolution. We have given careful thought to how the actual
instruction could be carried out to accomplish our goal of making student
ideas a focus in the classroom.
The first activity in the evolution course is designed to develop a framework
for examining arguments. This initial experience takes place in a non-science
context in order to increase student comfort with the task. Students are given
a set of cartoon frames that have been placed in random order. Their task
is to work with their group to reconstruct a story using the information they
can glean from the images. Students are enthusiastic about this task as they
imagine how the images relate to one another and how they can all be tied
together to create a story. The whole class then comes together to compare
their 'stories' and discuss how they made decisions. The sequences presented
by different groups usually vary quite a bit from group to group (see Figure
6 for examples of two sequences). This provides a context for a discussion
of how inferences are made. The initial conversation centers on the observations
they made of the images. However, it quickly becomes apparent that each person
did not place the same importance on specific observations and that even though
groups may have observed the same thing they may not have made the same decisions
about the order of the cards. What ensues is a conversation about particular
considerations that entered into their decision-making. Students realize that
they are all examining the same images (the data) but that they also each
bring a lifetime of experience with cartoons and stories to the table. Together
they establish that the process of making inferences about the order of the
cards is influenced by what they observe (the data) and by their own prior
knowledge and beliefs. This notion is then generalized and students see that
all inferences can be thought of as being based on these two things. They
discuss how scientific arguments are usually a collection of several inferences
that are all dependent on data and prior knowledge and beliefs. And finally
they discuss the importance of questioning all three areas when critiquing
an argument.
Figure 6
Group One
  
"We think that in this first frame little red riding hood
is telling the pigs that she is going to visit her sick grandmother.
In the second scene, the pigs are telling the wolf about little
red riding hood and her sick grandmother and showing him which
way she went. In the next frame, the pigs see that the grandmother
is tied up in the woods and they feel bad that they gave the wolf
the information earlier."
Group Two
  
"The pigs have discovered grandma tied up in the woods and
they try to throw the wolf off the track by telling him that he
must get away before the hunter comes. In the last frame, little
red riding hood is thanking the pigs for saving the grandmother
and they feel bashful."
Figure 6. Two different groups of students interpreted
the same images in different ways. There are thirteen images in
the complete set that the students worked with for this activity.
|
The cartoon activity described above provides the foundation for examining
arguments that is used for the remainder of the course. It is important to
note that an explicit part of that framework requires students to consider
the prior knowledge and beliefs that they and others bring to bear on inferences
made about data. In this way, the students are encouraged to continue to examine
their own knowledge throughout the course.
| Students examine three historical models that account for species
adaptation and diversity:
- Paley's model of intelligent design
- Lamarck's model of acquired characteristics
- Darwin's model of natural selection.
Once students have developed an understanding of the explanation
that each author proposed and some familiarity with the observations
on which it was based, they examine the prior knowledge and beliefs
that each author may have held. |
The second major section of the evolution course engages students in examining
three historical models that account for species adaptation and diversity.
The students must draw on the framework established during the cartoon activity
in order to accomplish this comparison. This means that as they examine each
argument they also identify the major inferences made and the data and prior
knowledge and beliefs that formed the basis of the inferences. The three models
are: (a) William Paley's model of intelligent design that asserts that all
organisms were made perfectly for their function by an intelligent creator,
(b) Jean Baptiste de Lamarck's model of acquired characteristics that is based
on a view that adaptations can result from the use or disuse of body parts
and that changes accumulated during an organism's lifetime will be passed
on to offspring, and (c) Charles Darwin's model of natural selection. The
models of Paley and Lamarck were chosen because they each represent some of
the common ideas that students bring with them to the classroom. Specifically,
it is clear that many students attribute evolutionary change to the needs
of an organism and believe that extended exposure to particular environments
will result in lasting morphological change. Many students are also confused
about the role of supernatural forces in evolution. Darwin's model is included
in the analysis so that students can see how the underlying assumptions of
his model compare to those of Paley and Lamarck.