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Conceptual Change in the Fly Room:
A Lesson for Undergraduate Biology Education
presented at
Ninth International History, Philosophy, and Science Teaching Conference
Calgary, Alberta
Canada
August 25, 2007
M. Frances Rowe, Ph.D
Edgewood College
1000 Edgewood College Drive
Madison, Wisconsin 53711
[email protected]
Conceptual Change in the Fly Room:
A Lesson for Undergraduate Biology Education
M. Frances Rowe
Department of Natural Science
Edgewood College, Madison Wisconsin
Abstract
This paper describes the merit of linking the history of science and the study of biology in
undergraduate biology classrooms. Historical cases, here Thomas Hunt Morgan’s conversion to
chromosomal theory and natural selection, provide an opportunity to teach biological concepts,
create a classroom climate that supports metacognition, and develop students’ understanding that
biology is an accumulated, yet impermanent body of knowledge. Moreover, a case is made for
inclusion of the history of biology in the biology classroom to enhance the learning environment
and open the door to facilitate student conceptual change by illustrating the difficulty biologists
often have had in giving up their beliefs, ideas, and assumptions.
Introduction
Teaching undergraduate students the foundational concepts of genetics and evolution has proven
to be a formidable task for biology educators. Students often complete their biology education
with a poor understanding of the processes, mechanisms, and applications of genetics and
evolution. The science education literature reports multiple obstacles to successful genetics
teaching, including: students’ inability to relate meiosis to allele segregation (Stewart & Dale,
1989), students’ poor comprehension of genetic terms (gene, allele, chromosome, gamete, trait),
confusion between “dominant” and “frequent” when referring to alleles, and the perception that
genetic ratios are determinate, not probabilistic (Collins & Stewart, 1989). Understanding the
concepts of mitosis and meiosis, the application of probability to reproduction, and the
relationship between genes and chromosomes are all important concepts in understanding the
mechanisms of inheritance.
There is a large body of literature pointing to biology students’ (including prospective biology
teachers) lack of understanding of evolution (Clough & Driver, 1986; Good, Trowbridge,
Demastes, Wandersee, Hafner & Cummins, 1992; Greene, 1990). Brumby (1984) reported that
medical biology students do not complete their education with a Darwinian understanding of the
diversity of life on Earth. Bishop and Anderson (1990) found that students fail to distinguish
between variation and natural selection. They reported that students failed to make a distinction between
the appearance of traits in a population and the trait’s survival over time. “Rather, students think there is
a single process in which characteristics of the species gradually change" (p. 422). Bishop and
Anderson stated that the problems they observed were characteristic of both high school and
college students; they concluded that “most presently used methods of teaching about evolution
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by natural selection are ineffective” (p. 425). In their replication of the Bishop and Anderson study,
Demastes, Settlage and Good (1995) also found a poor level of student understanding with regard to the
origin and survival of new traits in populations.
Halldén (1988) reported that students had conceptual difficulties with both genetics and evolution.
Problems he reported included students’ use of the term "adaptation" as an act of will, lack of distinction
between the terms "adaptation", "natural selection," and "mutation”, and an inability to differentiate
between an individual and a population as the unit of evolutionary change.
Students' understanding of the origin of variation in populations was also studied by Clough and WoodRobinson (1985). They found that students had a poor understanding of mutation as a source of
variation. Their students proposed explanations that credited defects in development as an explanation of
variation. Jensen and Finley (1995) found that students frequently cited a "need" to explain the origin of
new traits. Jensen and Finley (1996) identified additional student difficulties with explanations for
variation. They reported examples of students professing that genes go from recessive to dominant, that
genes become dominant over time, and that different species breeding with each other produce variations.
The research on misconceptions cited above points to significant problems in teaching genetics
and evolutionary biology. One of these is that students bring to class firmly rooted ideas
regarding genetics and evolution processes and phenomena. Some of this information is dead
wrong, strongly held, and severely handicaps students’ pursuit of correct explanations and
understandings. For students to adopt appropriate explanations for natural phenomena these
naive misconceptions must be abandoned or changed.
Misconceptions are one element of a student’s conceptual ecology, the knowledge a person holds
and the interactions of that knowledge. Students rely upon this wealth of personal information to
explain the workings of the natural world outside the classroom, and often inside the classroom
as well. Misconceptions as part of a student’s conceptual ecology are powerful factors in his/her
learning. Therefore, in planning and executing instruction, it is essential to find strategies that
will enable students to identify their ideas, compare them with the ideas of others, and wrestle
with their status. To confront naïve, often incorrect ideas, it is not only essential for students to
identify them, but it is also essential that students bring their ideas openly into classroom
discussions (Rowe, 2001; Hewson & Hennessey, 1992; Zietsman & Hewson, 1986; Hewson &
Hewson, 1984). This is where history of science enters the stage.
The history of science offers biology instructors an opportunity to engage students in the
evolution of an idea in a non-threatening way. It is rich with examples of scientists making
conceptual changes when confronted with new knowledge: Einstein (relativity), Darwin (natural
selection), Wegener (plate tectonics), and Thomas Hunt Morgan (genetics and evolution). As the
20th century opened, Morgan was an outspoken critic of both Mendelian genetics and Darwinian
natural selection (Allen, 1968). He did not accept Mendelian genetics as a plausible explanation
for the origin of variations observed in living organisms and for ten years he fought the SuttonBoveri announcement that genes were located on chromosomes (Lederman, 1989). Morgan
viewed Darwin’s work as incomplete and of little value in understanding the origin of species.
All this changed by 1927, Morgan was forced to proclaim, “The investigator must . . . cultivate a
skeptical state of mind toward all hypothesis –especially his own- and be ready to abandon them
the moment the evidence points the other way.” (Morgan, 1927).
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At the turn of the century, Morgan was in the same situation as many biology students are today;
his conceptual ecology interfered with his ability to embrace a new idea. Morgan was an
experimental embryologist, for him laboratory experimentation provided the authority to accept
or reject a hypothesis. The Drosophilia experiments in the Fly Room demonstrated to him that
factors affecting eye color, body color, wing shape, and sex determination all segregated with the
X chromosome. These results changed his point of view regarding chromosomes and Mendel’s
Laws (Morgan, 1911).
In addition to being an experimentalist, Morgan was an embryologist. His conceptual ecology
included belief in epigenesis, the cytoplasmic control of development. For Morgan to adopt a
nuclear control of cell functions would also require him to adopt nuclear control of development,
for Morgan this was problematic.
This story repeats itself with regard to evolution via natural selection. In 1903 Morgan
repeatedly stated that the theory of natural selection had no merit. However, by the 1930’s he
embraced Darwinian natural selection, primarily because the mechanisms of heritable variations
had become both intelligible and plausible to him (Allen, 1968).
Morgan’s conversion to both Mendelian and Darwinian thought provides a useful example for
biology instruction, as a case study in how scientific knowledge evolves and as a model for
conceptual change. It demonstrates the strength of prior experience and belief on learning new
concepts, the role of dissatisfaction in changing an idea, the role of metacognition in learning,
and the change in status of an idea. Morgan’s story offers a useful “hook” for students to learn
genetics and evolution content knowledge, and it demonstrates that biology is an impermanent
body of knowledge, though not easily changed. Moreover it illustrates that “giving up” ones
beliefs is no trifling matter.
Conceptual change theory (Posner, Strike, Hewson, & Gertzog, 1982) provides a framework for
examining how we learn concepts that are counter-intuitive to our personal, conceptual ecologies
or that challenge our personal beliefs. It speaks to the status of the ideas we hold, where we
place authority, and the interconnected nature of knowledge and understanding. Because
conceptual change theory enables us to understand how we learn, it also provides us with a
strategy for teaching science (Hewson, Beeth, & Thorley, 1997). I am not claiming that
conceptual change theory dictates a unique set of teaching sequences, but rather that it provides a
framework that can be used to inform instructional design and the development of teaching
strategies. Thomas Hunt Morgan’s conversion to Mendelian and Darwinian thought
(specifically, conversion to chromosomal theory and natural selection) offers biology instructors
an opportunity to engage students in his conceptual change and in so doing open the door for
students to struggle with the status of their own ideas.
I have found sharing his story with my students useful in two additional ways. First, it illustrates
the connected nature of biology by highlighting the convergence of embryology, genetics, and
evolution. Biology is too often taught as a basket of facts with little attention paid to the
interwoven strands of the discipline. Morgan’s story can change that approach. Second,
Morgan’s story highlights a historical change in biology methodology. During his lifetime
biology moved from a discipline with a primarily descriptive methodology to one where
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experimental inquiry played a major role. Discussion of this transition offers biology instructors
an opportunity to explain the concept of argument construction based on data.
Thomas Hunt Morgan
Thomas Hunt Morgan was born 1866 in Lexington, Kentucky, the same year Gregor Mendel
published his famous paper, Experiments on Plant Hybridization. Morgan’s parents were from
“old” southern families. His father and uncle distinguished themselves as famous Confederate
solders, his uncle led “Morgan’s Raiders” and his mother was the granddaughter of Francis Scott
Key. Morgan took to biology early, as a child and as an undergraduate at the State College of
Kentucky. Morgan completed his graduate work at John Hopkins under the direction of William
K. Brooks, a morphologist; Morgan’s dissertation centered on the phylogeny and embryology of
marine invertebrates (Allen, 1978).
The Status of Morgan’s Ideas Before 1910
In 1894, following the completion of his Ph.D., Morgan spent a year in Naples studying marine
embryology under the guidance of Hans Driesch. His work in Naples not only strengthened his
knowledge of embryology, but also introduced him to the German flavor of experimental
biology, Entwicklungsmechanik, a mechanistic experimental approach to biology that became
his mantra for the rest of his life.
“The working hypothesis is . . . an attempt to find an answer to some feature of a
complex situation in terms of accepted chemical or physical principles. This is
conceded in physics and chemistry and enough has been done even in biology to
warrant their employment.” (Morgan, 1927).
Morgan’s year in Europe has a second profound influence upon him, it introduced him to
epigenesis, the concept that development was governed by the interactions between the nucleus,
the cytoplasm (called protoplasm in 1894), and the environment. European embryologists at the
time were engaged in debate, the epigenesists (biologists that believed in the cytoplasmic control
of development) against the preformationists (biologists that believed that adult structures were
present in miniature form in the sperm or egg before conception) wrestling with the question of
how development was directed. Morgan landed firmly in the camp of the epigenesists and never
wavered.
The second great biology debate of the late 1800’s was over evolution, Hugo de Vries’ Mutation
Theory against the neo-Darwinians. Although there had been continuous debate over the merits
of natural selection and common descent since 1859, the late 1800’s witnessed a resurgence of
doubt. Hugo de Vries’ Mutation Theory filled the void. De Vries’ “work threw a new light on
the problem which Darwinian selection had left unclear in many minds; the origin of hereditary
variations” (Allen, 1969, p. 66). Morgan and his fellow embryologists, Hans Driesch, Jacques
Loeb, and Hans Spemann embraced de Vries’ theory. These men found de Vries’ Mutation
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Theory an attractive alternative to Darwin’s natural selection (Allen, 1969). It provided an
explanation for variation that Darwin’s work did not, a solution to the mystery of incipient stages
of new traits, discrepancies in the geologic record, and offered an opportunity for laboratory
investigation. Stimulated by his interest in evolutionary biology and his interactions with Hugo
de Vries, Morgan’s initial work in the Fly Room focused on finding experimental evidence to
support Mutation Theory.
Morgan’s pre-1910 point of view on the work of Mendel and Darwin was grounded in his
conceptual ecology. He had been trained as an embryologist and during his time in Naples he
had adopted epigenesis as an explanatory theory for embryonic development. As he began his
Drosophila studies, Morgan was an outspoken critic of both Mendelian genetics as an
explanation for inheritance and Darwinian natural selection as an explanation for evolutionary
change. Morgan opposed Mendelian genetics because:
• He believed chromosomes were uniform and that their difference in size did not
indicate a difference in chemical make-up or any other respect (Morgan, 1910).
• The behavior of chromosomes at meiosis was a mystery, he questioned whether
chromosomes before and after synapse were the same (Morgan, 1910).
• The number of chromosomes is small for a given organism, while the number of traits is large,
therefore many trait origins must be on the same chromosome. This contradicted Mendel’s
claim of independent assortment (Morgan, 1910).
• Mendel’s theory of dominance and recessive variations could not account for the
inheritance of sex in the observed one-to-one ratio. Which sex factor was dominant
and which recessive? (Allen, 1978, p. 53).
• Mendel’s “dominant” and “recessive” categories were not always as clear-cut as “tall”
and “short” in pea plants; offspring often showed what seemed to be intermediate
conditions between the supposed dominant and the supposed recessive character
(Allen, 1978, p. 53).
• There was no experimental evidence to support the existence of Mendel’s postulated
“factors,” thus, the Mendelian scheme was a hypothetical construct that had no basis
in reality (Allen, 1978, p. 53).
• Mendel’s “laws” might work for pea plants, but they were not demonstrated in a large
variety of other organisms, especially animals (Allen, 1978, p. 53).
In addition to the points made above, Morgan’s commitment to epigenesis would not allow him
to accept the idea that the nucleus was the control center for a developing embryo (Morgan,
1910; Gilbert, 1978). According to Gilbert, “Morgan was against Mendelism because he thought
it was preformationist . . . he attacked those who posited morphological elements as controlling
heredity from the nucleus” (p.338-339). Morgan believed that sex and other traits were
determined by physiological events, not discrete particles. As late as 1910, Morgan could not
understand how anyone could believe that sex was due to a nuclear element.
Morgan was also a vocal critic of Darwinian natural selection. He viewed Darwin’s work as
incomplete and of little value in understanding the origin of new species (Allen, 1968). Morgan
had eight particular problems with natural selection:
•
Darwin had provided no acceptable explanation for the origin of variation in organisms
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•
Morgan believed that species were a human construction; to him the only real unit in
nature was the individual organism. He wrote:
“We should always keep in mind that the fact that the individual is the
only reality with which we have to deal, and that the arrangement of these
into species, genera, families, etc. is only a scheme invented by man for
purposes of classification.” (Morgan, 1903. p. 33)
•
•
•
•
He felt that Darwin was confused about the types of variations (continuous vs.
discontinuous) on which natural selection could act. Morgan could not accept the idea
that natural selection acted on continuous variation and he did not believe continuous
variation could be explained by Mendelian principles (Morgan, 1903, p. 267).
Morgan believed natural selection could only sort out the negative, not preserve the
positive or explain the origin of new traits and variations (Morgan, 1903, 426).
He did not believe natural selection could act on small adaptations to improve the
function of particular organs, like the eye. (Morgan, 1903, 131-132).
Morgan found Darwin’s reliance on Lamarckian principles unacceptable. Morgan said,
By falling back on the theory of inheritance of acquired characters Darwin
tacitly admits the incompetence of natural selection to account for the
evolution of the flatfish. (Morgan, 1903, p. 138).
•
•
Darwin’s work was not supported by experiments.
Morgan questioned the role of chance in the process of natural selection (Morgan, 1910).
Morgan’s lack of support for Darwin’s work led him to consider alternate explanations, such as
De Vries’ Mutation Theory, for changing life on Earth.
The Fly Room
Fall 1904 Morgan left Bryn Mawr College to take a position at Columbia University. His
mentor and friend, Edmund Beecher Wilson, was a faculty member at Columbia and had
encouraged Morgan to join him. At Columbia, Morgan continued his work on embryology;
initially he focused his research questions on regeneration. In 1908 Fernandus Payne, one of
Morgan’s graduate students, began experiments with Drosophila to determine if rearing flies in
the dark would cause their eyes to atrophy from disuse and then disappear in future generations,
a test of Lamarckian theory. After sixty-nine generations in the dark, no blind flies appeared
(Shine & Wrobel, 1976). Payne’s investigation introduced Morgan to Drosophila as an
experimental animal, one that was easy to breed, feed, house, and study. It wasn’t long before
Morgan’s 16 x 23 foot laboratory in Schermerhorn Hall was filled with half-pint milk bottles
containing flies.
Morgan’s second experiment with Drosophila focused on the production of mutations. Payne
and Morgan attempted to mutate flies by exposing them to a wide variety of substances: salt,
sugars, acids, bases, and radium. Their goal was to induce de Vriesian-type mutations and in
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turn produce offspring that would be significantly different from their parents, saltation.
Morgan embraced the work of Hugo de Vries as his theoretical framework. Because he did not
find in de Vries’ work the pitfalls he saw in Darwin’s, Morgan felt that it could be the
explanation for speciation.
Some time between January and May of 1910 a male white-eye mutant emerged from its pupa,
was observed, bred, and placed Drosophila forever in the annals of genetic history. By the end
of 1912 the Fly Room contained 40 Drosophila cultures exhibiting mutants all demonstrating
inheritance pattern ratios that supported Mendel’s Laws (Shine & Wrobel, 1976). Calvin
Bridges and Alfred Sturtevant joined Morgan as undergraduates in the Fly Room shortly
thereafter. They worked in the Fly Room as undergraduates, graduate students, and all of their
professional life. The fourth member of the Drosophlia core was Hermann Muller. Mueller
joined the group in 1912 as a graduate student. Together the four men worked on fly genetics
contributing knowledge of chromosome structure, alleles, linkage groups, chromosome
recombination (crossover), gene mapping, non-disjunction, and much more. Their early work
culminated in the publication of The Mechanism of Mendelian Inheritance in 1915.
From 1910 to 1928, the year Morgan left Columbia, the Fly Room was a hub of activity and
productivity. It is famous for the relaxed atmosphere its human inhabitants enjoyed while
supporting a rigorous commitment to experimental inquiry. Allen described the environment as
one that encouraged team work, “There was little consideration of priority in new ideas or
discoveries, and everyone was free to criticize each other openly, and sometimes vehemently”
(1972, p. xix). According to Sturtevant, the Fly Room group demonstrated cooperation, not
competition, and as a result worked through problems as a group (Sturtevant, 1965). He
described the lab atmosphere in his memoirs,
This group worked as a unit. Each carried on his own experiments, but each
knew exactly what the others were doing, and each new result was freely
discussed. There was little attention paid to priority or to the source of new ideas
or new interpretations. What mattered was to go ahead with the work. There was
much to be done; there were many new ideas to be tested, and many new
experimental techniques to be developed. There can have been few time and
places in scientific laboratories with such an atmosphere of excitement and with
such a record of sustained enthusiasm. This was due in part to Morgan’s own
attitude, compounded with enthusiasm combined with a strong critical sense,
generosity, open-mindedness and a remarkable sense of humor. (Sturtevant, 1959,
p. 295).
The Status of Morgan’s Ideas Post-1910
Morgan’s Conversion to Mendelism. Morgan’s discovery of the white-eye mutant changed
Morgan’s research program. His attention turned away from evolutionary questions to those
associated with inheritance. Morgan’s first breeding experiment with the white-eyed male
produced offspring that supported Mendel’s Laws. However, he noted a serious discrepancy; all
the white-eyed offspring were male. When Morgan combined his observation with the
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cytological work of Nettie Stevens on sex chromosomes, he began to see that there was a
relationship between the X factor (his term for the white-eye gene) and the X chromosome.
Data collected through further experiments continued to support the ratios theorized in Mendel’s
Laws. Eventually, Morgan came to the conclusion that Mendel’s factors were real entities
located on chromosomes that could be consistently found in the same position on a specific
chromosome (Allen, 1972). His change of heart was motivated and supported by the breeding
experiments he and his students carried out in the Fly Room. Morgan’s 1910 paper printed in
Science, “Sex Limited Inheritance in Drosophila,” described his conceptual change regarding
Mendel’s laws. In the paper, he claimed:
•
•
•
•
Mendel’s factors must reside on chromosomes
Each factor resides on a particular chromosome
The eye color trait is positioned on the X chromosome
The red eye color variation is dominant to the white variation.
As a whole, these findings became known as the Chromosomal Theory of Heredity.
Morgan’s second epiphany, the possibility of chromosomes exchanging parts (crossover),
occurred to him in 1911. “He reasoned that the strength of linkage between any two factors must
be related in some way to the distance between them on the chromosome. The farther apart any
two genes [were], the more likely that a break could occur somewhere between them.” (Allen,
1972, p. xviii).
The third significant breakthrough in the Fly Room, chromosome mapping, belonged to Alfred
Sturtevant. In 1911, as an undergraduate, Sturtevant conceived and drew the first chromosome
map. It was of the X-chromosome and included five genes.
The research conducted in the fly lab provided Morgan with the experimental evidence necessary
to change his position on Mendel’s Laws. Morgan was an experimentalist and a materialist. For
him, hereditary material had to have a basis in structure and function to be real. Allen describes
Morgan’s position,
To Morgan and his group, the establishment of the chromosome theory of
heredity marked a triumph for the application of quantitative, rigorous, and
experimental methods to an area of biology which had largely been qualitative
and descriptive.” (Allen, 1972, p. xxiv)
The publication of The Mechanism of Mendelian Inheritance by Morgan, Sturtevant, Muller and
Bridges in 1915 marked the conversion of Morgan to a Mendelian. In the Preface of the book
Morgan claims,
Chromosomes furnish exactly the kind of mechanism that the Mendelian laws call
for; and since there is an ever-increasing body of information that points clearly to
the chromosomes as the bearers of the Mendelian factors, it would be folly to
close one’s eyes to so patent a relation. Moreover, as biologists, we are interested
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in heredity not primarily as a mathematical formulation but rather as a problem
concerning the cell, the egg, and the sperm.” (p.viii-ix)
Conceptual change for Morgan was not easy or swift he was firmly grounded in his belief that
Mendel’s work deserved little attention and openly hostile to Mendel’s Laws. He was
particularly critical of the lack of experimental evidence to support the existence of Mendel’s
“factors.” The white-eye mutant changed Morgan’s thinking. When he saw the link between the
white-eye phenotype and the X-chromosome, the vast majority of his white-eye flies were male,
Morgan could no longer dismiss Mendel’s factors a simply theoretical constructs.
Morgan’s discovery of the white-eyed mutant changed his life and the discipline of biology
forever. It sent a tidal wave through the biological community that is still rippling today. In
1933, Morgan was awarded the Nobel Prize for Physiology and Medicine for his work on
Drosophila genes and chromosomes, he received several honorary degrees, and in 1939 was
offered an appointment to the Royal Society. Drosophila also became a star. Drosophila
cultures are currently found in teaching and research laboratories around the world. Fruit flies
appear on the covers of magazines and books (both research and the popular press), are found in
computer simulations build around its inheritance patterns, and appear in most introductory
biology classrooms. Drosophilia is the best known model organism on the planet.
Morgan’s Conversion to Darwinism. Morgan’s conversion to Darwinian thinking took longer.
It was the result of three factors: his acceptance of the genetic origin of variation as worked out
in the Fly Room, Theodosius Dobzhansky Drosophila research that united field and laboratory in
the study of natural populations, and his students, principally Sturtevant, unending efforts to
promote the merits of natural selection to Morgan year after year.
The experimental work in the Fly Room made intelligible to Morgan how traits and variation
were inherited. The experimental data collected supporting Mendelian patterns of inheritance,
the ability of chromosomes to recombine, and mutation. This work challenged Morgan’s long
held ideas about the origin and heritability of variation. With the problem of the origin and
heritability of variation solved, Morgan’s main criticism of Darwin, the lack of an acceptable
explanation for the origin of variation in an organism, had disappeared.
Morgan’s second main concern with Darwinian evolution was that Darwin’s work was not
supported by experimentation. This changed in 1927 when Theodosius Dobzhansky, a Russian
immigrant, joined the fly group. Dobzhansky invented experimental methods that enabled
drosophilists to conduct experiments on the mechanics of variation and speciation in natural
populations. Dobzhansky’s work combined field practice with the theoretical biometrics of
J.B.S. Haldane, R.A. Fisher, and Sewall Wright (Kohler, 1984). With the success of
Dobzhansky’s work, Morgan’s second area of opposition to Darwin collapsed.
Morgan’s students supported Darwinian evolution and continually engaged him in discussions
about natural selection. Muller has claimed that Sturtevant had the most influence on Morgan’s
thinking (Allen, 1968). Morgan became a Darwinian..
Morgan’s and his student’s work in the Fly Room changed not only Morgan’s ideas, but also the
face of biology. Fruit fly research has illuminated transmission genetics and enabled discoveries
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in biochemistry, natural selection, and development. Along the way, many biologists, like
Morgan, have experienced conceptual change. Fly research continues to be an important part of
biology, through it biologists, now more than ever, are integrating genetics, cell biology,
evolution, and development.
Instructional Strategies
This paper reports on the value of incorporating the history of biology as an integral part of an
instructional strategy for teaching biology and the outcomes of that instruction for undergraduate
general biology students. As noted above, students often complete their biology education with a
poor understanding of the processes, mechanisms, and applications of genetics and evolution. In
addition to their poor understanding of these concepts, students often fail to comprehend the
links and interrelationships of their knowledge.
General Biology II taught at Edgewood College, Madison, Wisconsin, is an introductory biology
course taken by both biology majors and non-majors. The course includes instruction in both
genetics and evolution. Teaching tightly links lecture, discussions, and laboratory experiences
with the history of biology. Intended learning outcomes included:
• Development of content knowledge in the areas of transmission genetics, molecular
biology, evolution, natural selection, and population genetics;
• Development of content knowledge and understanding of the history of biology;
• Understanding that biology is an accumulated, yet impermanent body of knowledge.
Instructional strategies employed to achieve these goals are grounded in constructivist learning
theory implemented through a conceptual change framework (Hewson, Beeth, & Thorley, 1997).
The history of biology, student engagement with a variety of organismal phenomena,
opportunities for inquiry, and open student discussion are all incorporated into instruction
strategies designed to achieve these goals.
General Biology II
General Biology II is the second semester of a two-semester introductory biology sequence that
explores both the unity and diversity of life on Earth. The course is organized around the flow of
information in biological systems. General Biology II begins with transmission genetics (the
flow of information between generations), followed by molecular biology (the flow if
information within cells), and concludes with evolution (flow of information from species to
species). Evolution instruction focuses on common descent, the mechanisms and processes that
produce change in populations, and linking evolutionary theories and processes to genetics.
Multiple episodes from the history of biology (Mendel, Morgan, Darwin, Watson, & Crick) are
discussed during the course of instruction. Thomas Hunt Morgan’s work is highlighted. His
accomplishments and contributions to genetics and biology as well as his personal conceptual
change are an important element of instruction. Morgan’s accomplishments are used to open
discussion on the post 1900 acceptance of Mendel’s work by biologists, the discovery of X-
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linked inheritance, chromosome linkage groups, and chromosome mapping. The story of his
conversion to Mendelism and to Darwinism provide students the opportunity to discuss the
research conducted in the Fly Room and provide examples of three very important points: 1)
biologist do not easily give up their ideas, 2) biology content knowledge is an impermanent body
of information; and 3) the experimentalist tradition that characterizes biology today was built on
ideas developed in the German universities during the late 1800’s.
Students begin their introduction to chromosomal theory by reading “Thomas Hunt & the Whiteeyed Mutant” from Hagen, et al. book, Doing Biology (1996). They are then asked to identify
how Morgan’s pre-1910 beliefs compare with the genetics concepts found in their text. This
activity opens the discussion of why a biologist would change his or her mind concerning a given
theory or model. Moreover, the activity provides a platform to discuss the origin of the
experimentation ethic in biology, the value of experimentation, the construction of arguments
based on data, and the impermanent nature of scientific knowledge. Students are asked to
explain the factors that led Morgan to change his mind regarding Mendel’s theories and to
identify the ideas they hold that might keep them from embracing Mendel’s and or Darwin’s
ideas. Discussion includes direct dialogue of what constitutes belief, why a person believes as
she or he does, the importance of understanding a concept in order to incorporate it into one’s
belief system, and the concept of explanations being internally and externally consistent.
The following questions are posed for students to consider:
What is required to believe a concept or idea?
What “got in the way” for Morgan to embrace Mendel’s ideas?
What does it mean to be an experimentalist?
What counts as evidence in a scientific investigation?
How does one know when enough data/evidence has been collected to support a theory?
What does it mean for information to be internally consistent?
What does it mean for information to be externally consistent?
Student Discussions
Student discussions open with students reporting on Morgan’s identification of the white-eyed
mutant male fly. The significance of this event is not immediately apparent to students.
Students are then directed to four questions:
1.
2.
3.
4.
How did the new eye color originate?
Was it an accident of embryological development?
Could it be inherited?
Did it represent a trait distinguishing a new species?
These questions become the framework for student discussion.
Question one stimulates student discussion of issues associated with mutation: “need,” causes of
mutation, heritability, types of mutations as well as the concepts of proximate and ultimate
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causation. Students are quick to identify mutation as the cause of the white-eye variation, but
how the mutation occurred and the frequency of mutation in an organism, the preservation or
loss of mutations in a population all offer fertile areas for student discussion. Although students
readily suggest mutagen as the origin of white eyes, they give little thought to the complexity of
a mutation event. Many students indicate that mutations of a particular nature occur because an
animal “needs” to escape from a predator, reach a higher tree branch for supper, or enable it to
survive disease. This discussion leads to Payne’s work and Morgan’s fascination with mutation.
Question two introduces development, which is a new topic for most students. Students are
interested in development as the term ‘stem cell research” is well known to them. However the
processes associated with development and their links to genetics are not. Students are intrigued
that Morgan was an embryologist first, a geneticist second. Students are challenges to think
about the connections between genetics and development and how one could have an influence
upon the other.
The question, “Could it be inherited?” provides students an opportunity to ask questions about
chromosomes, meiosis, and the construction of Punnett squares. This is an area where students
are eager to talk about their confusion and/or misunderstandings. They share Morgan’s pre1910 state, they lack an understanding of genes, chromosomes, chromatids, the activities of
chromosomes during meiosis, and their role in inheritance, see Table 1. An often made student
comment is, “I always wondered where the letters on the side of the Punnett square came from.”
Discussion of the dominance and recessive nature of alleles is also discussed at this point as well
as how gender is determined.
The last question opens the discussion of speciation. What constitutes a species and the variety
of species definitions that currently exist (limited to those presented in their biology text) are
discussed as well as their applications for particular situations. For example Ernst Mayr’s
biological species definition is not useful when addressing bacteria or fossils.
Morgan’s skepticism regarding natural selection is addressed following the discussion of his
conversion to Mendelism. Morgan’s lack of support for Darwin’s work includes a discussion of
his support of Hugo de Vries’s Mutation Theory. Morgan’s motivation to use Drosophila as a
model system to study speciation is noted. It is important that Morgan’s dissatisfaction with
Mendel’s work be discussed before his criticisms of natural selection for two reasons. First, an
understanding of genetics is essential to discuss and understand the heritable traits required for
natural selection to operate. Second, wrestling with genetics misconceptions is not as
emotionally charged as wrestling with the misconceptions associated with biological evolution.
Students often misunderstand genetics concepts, but generally their misunderstandings are not
grounded in a challenge to their belief systems. This is not true for evolution. Students’ genetics
misconceptions most often arise from a misunderstanding of a particular concept or concepts;
student’s evolution misconceptions are also characterized as misunderstood concepts, but these
misunderstandings are often linked to deep belief system elements of student’s conceptual
ecologies in ways that genetics misconceptions are not. Here Morgan’s story can provide a
window on how deeply held ideas can color one’s thinking.
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Concept
T. H. Morgan
Biology Students
• Chromosomes are uniform in
• Lack an understanding of the character
Chromosomes
construction and chemistry
• Chromosomes before and after synapse
were the same.
• The number of chromosomes is small for
a given organism, while the number of
traits is large, therefore many trait origins
must be on the same chromosome. This
contradicted Mendel’s claim of
independent assortment.
Sex
Determination
Mendel’s theory of dominance and
recessive variations could not account for
the inheritance of sex in the observed oneto-one ratio.
Offspring often showed intermediate
conditions between the supposed dominant
and recessive characters claimed by
Mendel
Mendel’s “laws” might work for pea
plants, but they were not demonstrated in a
large variety of other organisms, especially
animals
No experimental evidence to support
Mendel’s postulated “factors,” had been
presented
of chromosomes (different sizes,
banding patterns, different genes, etc.)
• Lack an understanding of the
relationship between genes and
chromosomes
• Lack understanding of chromosome
activity during meiosis
• Little or no recognition of homologous
chromosomes and the role of synapse
(Stewart & Dale, 1989)
Mechanisms for determining gender
Continuous vs.
Discontinuous
Inheritance
Inheritance in
Plants vs.
Animals
Evidence for
Mendel’s Factors
Table 1. A comparison of T. H. Morgan’s reasons for doubting Mendelian theory and biology
students’ naïve conceptions/misconceptions conceptions.1
Morgan’s post-1910 support of natural selection was built on his acceptance of Mendelian
genetics (Allen, 1968). Students’ understandings of the processes and mechanisms of evolution,
like Morgan, benefit from a clear understanding of genetic phenomena (Rowe, 2001).
Discussion of the role of genes and chromosomes in cells prior to discussion of the origin of
variation in populations strengthens students’ understandings of natural selection and facilitates
their ability to discuss and to confront their own misconceptions.
I have found open discussions with students that build the link between genetics and
evolutionary phenomena to be useful in dismantling their misconceptions as well as in building
their understanding of both concepts. Too often evolution is taught with no overt link to
genetics. In these cases, students are expected to take on faith, as biologist in the late 1800’s
were also expected to do, that new variations are heritable and arise independent of natural
1
See page 1 for citations.
13
selection, that natural selection operates on heritable traits, and that natural selection has the
power to preserve favorable variations in a population. Such an approach did not work for 19th
century biologists (Allen, 1969) and it also appears to be wanting for 21st century biology
students.
Although students share with Morgan confusion regarding mutation, chromosomes and genes,
see Table 2, students do not share with Morgan a concern for the action of natural selection on
continuous versus discontinuous traits. I claim that this is because students received little or no
instruction in continuous variation at the high school level. As a result students arrive in their
college biology classrooms with little knowledge or baggage regarding quantitative inheritance.
Morgan’s position here provides an opportunity to bring discussion of continuous variation into
the discussion of natural selection.
Concept
T. H. Morgan
Origin of Variation
• Believed that Darwin had provided no
• New variations originate because of
acceptable explanation for the origin of
variation in organisms
• Morgan found Darwin’s reliance on
Lamarckian principles unacceptable
• Felt that Darwin was confused about
the types of variations (continuous vs.
discontinuous) on which natural
selection could act
• Supported de Vries’ Mutation Theory
as an appropriate explanation for
speciation
need
• New variation originate from natural
selection
Preservation of
Variation
Mutation
Chance
Natural Selection
Speciation
Experimental
Evidence
• The role of chance in the process of
natural selection was not clear to
Morgan
• Morgan felt natural selection could
only sort out the negative, not preserve
the positive or explain the origin of
new traits and variations
• He did not believe natural selection
could act on small adaptations to
improve the function of particular
organs, like the eye.
Morgan felt that species were a human
construction
Darwin’s work was not supported by
experiment.
Biology Students
• Credit mutation as the main
mechanism for the production of
variation
• Believe mutation is caused by natural
selection
• The role of chance in the process of
natural selection was not clear to
students
• See natural selection as both the
origin and preservation of variation
• Students see species as fixed and
usually come with the biological
species definition as there working
“fact”
• Students often claim there is no
evidence for evolution
Table 2. A comparison between T. H. Morgan’s reasons for doubting Darwinian theory and
biology students’ naïve/misconceptions conceptions.
14
Connecting Discipline Concepts
Highlighting the connections between genetics and evolution is important to facilitate students’
understandings of evolution, see Figure 1; it also serves as a platform to include discussion of
additional connections within biology. Moreover, the importance of theories being both
internally and externally consistent can be demonstrated and discussed. As noted above, a good
deal of Morgan’s resistance to Mendel’s work was grounded in his commitment to epigenesis.
The issue for Morgan was that of consistency. If he accepted the idea that chromosomes directed
cellular activity, then he was also accepting that morphological determination of development
was directed from within the nucleus. This he was unable to do. Morgan believed the cytoplasm
directed development, not the nucleus. He stated his position clearly in his 1897 work The
Frog’s Egg: An Introduction to Experimental Embryology,
. . . a defect in the protoplasm often brings about a modified cleavage and also a
defective embryo, and this takes place even though the whole of the nuclear
material of the unsegmented egg remains present. There seems, therefore no
escape from the conclusion that in the protoplasm and not in the nucleus lies the
differentiating power of the early stages of development. (Morgan, 1897)
This was experimental data, evidence from Morgan’s point of view to support epigenesis. To
maintain external consistency, Morgan rejected Mendel’s position. Not to do so in Morgan’s
mind would have supported preformation. As a staunch epigenesist, Morgan rejected Mendel’s
particulate theory of inheritance.
A second opportunity for students to discuss internally and externally consistent arguments is the
determination of gender. The dominant view in the 1890’s was that sex was determined by
environmental factors (Gilbert, 1978). In 1905, Nettie Stevens and Edmund B. Wilson
proclaimed that gender was determined by chromosome, not the environment. Morgan did not
budge. In fact, Morgan interpreted their work as support for his position that Mendel’s theory of
dominance and recessive variations could not account for the inheritance of sex if chromosomes
were involved. Sex was observed in a one-to-one ratio; Mendel’s Laws clearly established a 3:1
or 9:3:3:1 offspring ratio. In Morgan’s mind chromosomes could not carry the sex determining
factor. Discussion of these events with students provides students with a real example of the
weight of inconsistent evidence.
Biology instructors often teach biology as a “basket of facts” with little or no effort to link the
concepts of the discipline. Thomas Hunt Morgan’s story can provide an opportunity to go the
other direction. Understanding the mechanisms of inheritance played a significant role in
Morgan changing his point of view to support Darwinian evolution. Students, like Morgan, can
benefit by seeing these same relationships. The student concept map, Figure 1, demonstrates
how a student linked genetics and evolution in a fruitful way to enable her understanding of both
topics.
15
Figure 1. Student concept map illustrating her personal conceptual framework for the union of
genetics and evolutionary biology concepts.
The history of biology in the 19th century provides a wonderful opportunity for students to see
the intertwined nature of biology’s sub-disciplines. Morgan, E.B. Wilson, and Theodor Boveri
all began as embryologists. All three of these men brought the study of cells and genetics to
their studies of embryology. The 1920’s saw the unification of cell biology, development,
physiology and common descent around genetics and during the same period of time the
synthesis of natural history (ecology), Mendelian genetics, Darwinian evolution (natural
selection), biometry, and population genetics came together in the Modern Synthesis. However,
the union of the two subgroups had to await the 1950’s and the molecular biology revolution.
16
Biology: An Accumulated, Yet Impermanent Body Of Knowledge.
A significant change in biology occurred during the first sixty years of the twentieth century.
From 1900, the rediscovery of Mendel’s work, to the 1960s, the age of molecular biology,
genetics became a unifying force in the study of living organisms. Thomas Hunt Morgan was a
major player in that change. During the 20th century discoveries made in his laboratory changed
the way cytologists viewed cells and the way ecologists viewed evolution. Currently, genetics is
revolutionizing how we understand the development of living organisms from the first diploid
cell to the adult organism.
People like Thomas Hunt Morgan are worthy of our and our students’ attention as we seek to
build students’ understandings of biology concepts. In the words of Bernard Chartres (and later
Newton), we see more because we have had the opportunity to “stand on the shoulders of giants”
and as a result we can see more and greater things, not by virtue of any sharpness of sight on our
part, but because we have been raised by their contributions.”
Biology is not “hard,” rather it is a continually changing discipline grounded in the discovery of
new phenomena and deeper explanations for well known phenomena. Science knowledge grows
because of the blood, sweat, tears, and the Sunday afternoons scientists have willing given to the
search for understanding of the natural world. Biology students, be they majors or non-majors,
should be aware of their contributors as well as the changing face of biology.
Results and Conclusions
This paper speaks to the merit of linking discussions of the history of science with conceptual
change teaching as a strategy to enable students to build their understanding of genetics and
evolution. Intended learning outcomes of instruction included:
• Development of content knowledge in the areas of transmission genetics, molecular
biology, evolution, natural selection, and population genetics;
• Development of content knowledge and understanding of the history of biology;
• Understanding that biology is an accumulated, yet impermanent body of knowledge.
Biology students often experience confusion and sometimes outright frustration as they work to
understand the mechanisms and processes that produce organismal variation, inheritance
patterns, natural selection, and speciation. Classroom experience has shown that constructing an
appropriate understanding of these biological concepts requires identification and inspection of
ones ideas, along with the opportunity to access the merits and limitations of those ideas. The
conceptual change model offers a teaching methodology that encourages students to identify and
confront their misconceptions as they move on a path that will lead them to an appropriate
understanding of genetics and evolution. It unites students’ identification of their own ideas,
introduction of new ideas, explicit confrontation of new and old ideas, and requires students to
negotiate the status of both. The history of science can function as an important tool in the
learning process to facilitate metacognition and in so doing enable students to identify, evaluate,
and dismiss their naïve conceptions and misconceptions.
17
Thomas Hunt Morgan’s path to Mendelian genetics and Darwinian evolution is a useful model
for students as they struggle with the construction of their own knowledge. Through Morgan’s
struggle, students have an opportunity to observe his construction of biology knowledge and the
role inquiry and epistemology played in that construction. Morgan’s observations in the Fly
Room provided the authority for him to modify his conceptual ecology and to accept Mendelian
and Darwinian theories. Morgan’s case also gives students the opportunity to negotiate the
status of their ideas in a non-threatening learning environment.
The incorporation of the history of biology described here shows great promise for biology
classrooms. Following their instruction, students in General Biology II were able to use their
genetics knowledge, as it had become intelligible, plausible, and fruitful, to understand the
processes of evolution (natural selection and speciation). An additional learning outcome was
the development of knowledge and understanding of an important segment of the history of
biology and an appreciation for biology as an accumulated, yet impermanent body of knowledge.
18
Work Cited
Allen, G. (1968). Thomas Hunt Morgan and the Problem of Natural Selection. Journal of the
History of Biology 1(1): 113-139.
Allen, G. (1969). Hugo de Vries and the Reception of the “Mutation Theory.” Journal of the
History of Biology 2(1): 55-87.
Allen, Garland. (1972). Introduction to the Reprint Edition of the Mechanisms of Mendelian
Inheritance by T. H. Morgan. New York, NY: Johnson Reprint Collection.
Allen, Garland. (1978). Thomas Hunt Morgan: The Man and His Science. Princeton, NJ:
Princeton University Press.
Bishop, B.A. & Anderson, C.W. (1990). Student conceptions of natural selection and its role in
evolution. Journal of Research in Science Teaching, 27(5), 415-427.
Brumby, M. (1984). Misconceptions about the concept of natural selection by medical biology
students. Science Education, 68, 493-503.
Clough, E.E. & Wood-Robinson, C. (1985). Children's understanding of inheritance. Journal of
Biological Education, 19(4), 304-310.
Collins, A. & Stewart, J.H. (1989). The Knowledge Structure of Mendelian Genetics. The
American Biology Teacher, 53(3): 143-149.
Deadman, J.A. & Kelly, P.J. (1978). What do secondary school boys understand about evolution
and heredity before they are taught the topics? Journal of Biological Education, 12(1), p.
7-15.
Demastes, S., Good, R., Peebles, P. (1995), Students' conceptual ecologies and the process of
conceptual change in evolution. Science Education, 79(6), 637-666.
Demastes, S., Good, R., Peebles, P. (1996). Patterns of conceptual change in evolution. Journal
of Research in Science Teaching, 33(4), 407-431.
Demastes, S.S., Settlage, J., & Good, R. (1995). Students' conceptions of natural selection and
its role in evolution: Cases of replication and comparison. Journal of Research in
Science Teaching, 32(5), 535-550.
Gilbert, S.F. (1978). The Embryological Origins of the Gene Theory. Journal of the History of
Biology,11(2): 307-351.
Halldén, O. (1988). The evolution of the species: Pupils' perspectives and school perspectives.
International Journal of Science Education, 10, 541-552.
19
Hagen, J., Allchin, D. & Singer, F. (1996). Thomas Hunt & the White-eyed Mutant. In Doing
Biology. New York, NY: HarperCollins College Publishers.
Hewson, P.W. (1981). A conceptual change approach to learning science. European Journal of
Science Education, 3(4), 383–396.
Hewson, P. W., Beeth, M. E., & Thorley, N.R. (1997). Teaching for Conceptual Change.
Internatioinal Handbook of Science Education. Boston, MA: Klumer.
Kohler, R.E. (1984). Lords of the Fly: Drosophila Genetics and the Experimental Life.
Chicago, IL: The University of Chicago Press.
Jensen, M.S. & Finley, F. (1995). Teaching evolution using historical arguments in a conceptual
change strategy. Science Education, 79 (2), 147-166.
Jensen, M.S. & Finley, F. (1996). Changes in student's understanding of evolution resulting from different
curricular and instructional strategies. Journal of Research in Science Teaching, 33(8), 879-900.
Lederman, M. (1989). Research Note: Genes on Chromosomes: The Conversion of Thomas
Hunt Morgan. Journal of the History of Biology, 22(1): 163-176.
Morgan, T. H. (1897). The Development of The Frog’s Egg: An Introduction to Experimental
Embryology. New York, NY: MacMillan.
Morgan, T. H. (1903). Evolution and Adaptation. New York, NY: Macmillan.
Morgan, T.H. (1910). Chance or Purpose in the Origin and Evolution of Adaptations, Science,
31: 201-210.
Morgan, T.H. (1910). Chromosomes and Heredity. American Naturalists, 44: 163-176.
Morgan, T.H. (1910) Sex Limited Inheritance in Drosophila. Science, 32.
Morgan, T.H. (1911). An Attempt to Analyze the Constitution of the Chromosomes on the
Basis of Sex-Limited Inheritance in Drosophila, Journal of Experimental Zoology, 11:
365-414.
Morgan, T.H. (1927). Experimental Embryology. New York, NY: Columbia University Press.
Posner, G.J., Strike, K.A., Hewson, P.W., & Gertzog, W.A. (1982). Accommodation of a
Scientific Conception Toward a Theory of Conceptual Change. Science Education,
66(2): 211-217.
Rowe, M. F . (2001). The Role of Genetics in Students’ Understandings of Biological
Evolution. (Doctoral dissertation, University of Wisconsin-Madison, 2001).
20
Settlage, J. (1994). Conceptions of Natural Selection: A snapshot of the sense-making process.
Journal of Research in Science Teaching, 31 (5), 449-457.
Shine, I. & Wrobel, S. (1976). Thomas Hunt Morgan: Pioneer of Genetics. Lexington, KE:
University Press of Kentucky.
Stewart, J.H. & Dale, M. (1989). High School Students’ Understanding of Chromosome/Gene
Behavior During Meiosis. Science Education, 73(4): 501-521.
Sturtevant, A. H. (1959). Thomas Hunt Morgan: Biographical Memoirs. National Academy of
Sciences.
Sturtevant, A. H. (1965). A History of Genetics. New York, NY: Harper and Row.
.
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