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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 1 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). 2 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 3 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 4 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 5 • 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 6 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 7 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 8 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 9 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- 10 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 11 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. 12 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). 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