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Germ theory states that specific microscopic organisms are the cause of specific diseases. The theory was developed, proved, and popularized in Europe and North America between about 1850 and 1920. Because its implications were so different from the centuries-old humoral theory, germ theory revolutionized the theory and practice of medicine and the understanding of disease. It was, however, compatible with existing ideas about health, especially those associated with 19th-century hygiene and sanitation.

5:11 video Chat with a friend about an established scientific theory, and she might reply, "Well, that's just a theory." But a conversation about an established scientific law rarely ends with "Well, that's just a law." Why is that? What is the difference between a theory and a law... and is one "better"? Matt Anticole shows why science needs both laws and theories to understand the whole picture.

You will learn about "Dalton's Atomic Theory" in this video. John Dalton was an English chemist who is very well known today for his work in the development of atomic theory. Let us learn the postulates of Dalton's atomic theory. All matter, whether an element, a compound or a mixture are made up of very tiny particles called atoms. All atoms of a given element are identical in size, mass and chemical properties. Atoms of different elements differ in size, mass and chemical properties. Atoms are indivisible particles which can neither be created nor destroyed in a chemical reaction. Atoms combine together in fixed whole number ratios to form compounds. In a chemical reaction, atoms only get rearranged. An atom of an element does not change into the atom of another element.

Evolution is often challenged by those who oppose its teaching or do not understand it. One of these challenges is that evolution is just a theory. This activity addresses this misconception.

Promotes accurate understanding and comprehensive teaching of evolutionary theory for a wide audience Explores the practical applications of evolutionary principles in daily life and the impact of evolutionary theory on culture and society throughout history Offers teaching tools such as unit and lesson plans and classroom activities, as well as additional online content such as podcasts and powerpoint presentations

31min video  The epic voyages of Darwin and Wallace led each to independently discover the natural origin of species and to formulate the theory of evolution by natural selection.

This case study introduces students to the "Red Queen Hypothesis." The hypothesis states that when two species compete for a limited resource or exist in a predator-prey relationship, in order for the antagonists to remain in relative population equilibrium, there must be ongoing and reciprocal adaptation on each side. If one species fails to adapt, it may quickly face extinction. Students are introduced to the Red Queen Hypothesis through an excerpt from Lewis Carroll's novel, Through the Looking Glass, in which Alice meets the Red Queen. They then review experiments from the research literature to evaluate the validity of the hypothesis in extant populations.  Students culminate their learning by contrasting their knowledge of the Red Queen Hypothesis to Darwinian evolutionary theory based on a passage from On the Origin of Species.  Students then craft a letter to Darwin helping to explain his "wedge" theory, a notion he originally developed but removed from later iterations of his book due to a lack of experimental evidence. Originally developed for advanced high school biology students, the case also may be used in a college-level introductory course on evolutionary biology.

Buckle up for a prokaryote and eukaryote comparison before taking a ride into a cell to explore organelle structures and functions! This cell introduction also includes the modern cell theory.

Science is working tirelessly night and day to disprove its own theories about how the universe works (or at least, that's what science thinks it's doing). Hank tells us a quick history of how we came to create and adopt the scientific method and then gives us a vision of the future of science (hint: it involves a lot more computers and a lot less pipetting).

6 min video

How do we know what matter is made of? The quest for the atom has been a long one, beginning 2,400 years ago with the work of a Greek philosopher and later continued by a Quaker and a few Nobel Prize-winning scientists. Theresa Doud details the history of atomic theory.

This case is an account of the events that led Drs. Robin Warren and Barry Marshall to the bacterial theory of ulcers. The two physicians refused to accept the standard explanations for what they had observed and instead developed an alternative hypothesis, saving countless patients from unnecessary pain and suffering. Along the way they transformed the way the medical community viewed peptic ulcer disease. The case shows the importance of curiosity, serendipity, and tenacity in scientific inquiry; how science is built upon the work of others; how assumptions can cloud people's views, leaving them closed to new ideas; and how scientific "truth" can and does change when faced with new data and new interpretations. It is appropriate for use in any course at the high school or undergraduate level that teaches students about the scientific method and/or the sociology, nature, and history of science.

Darwin's theory of biological evolution helps us understand how simple life forms can give rise to complex lifeforms, but how did the first reproducing creatures come about? The origin of life needs its own explanation.

Origin of life - volcanic pools - ocean floor - outer space

In this case study students are provided with information for piecing together the story of how forest fragmentation and biodiversity loss can affect the risk of Lyme disease transmission to humans. The case introduces the dilution effect, a widely accepted theory-and one of the most important ideas in disease ecology-which suggests that disease risk for humans decreases as the diversity of species in an area increases. It also explains how landscape fragmentation, one of the most common threats to biodiversity, can influence the risk of Lyme disease for humans. Students interpret and discuss various figures to develop a concept map that connects all the individual results of the story. Students gain an appreciation for the complexity of species interactions in an ecosystem, the effects of forest fragmentation on these interactions and the possible consequences for human health. This activity was developed for an undergraduate introduction to environmental sciences course under the topic of biodiversity and conservation, but would also be suitable for interdisciplinary studies interested in examining the connections between conservation and public health.

The Ethics Primer provides engaging, interactive, and classroom-friendly lesson ideas for integrating ethical issues into a science classroom. It also provides basic background on ethics as a discipline, with straightforward descriptions of major ethical theories. Several decision-making frameworks are included to help students apply reasoned analysis to ethical issues. Although the Primer is designed for secondary school science classrooms, it has been used by teachers in a variety of classes and grade levels. It is particularly suited to social studies and integrated/interdisciplinary classrooms. It has also been used with adults. The Primer is not designed to be used cover to cover. Teachers should review materials and select lessons that fit their needs. The Primer is also intended to be used as a general resource, with a wide variety of topics.

Some of the anti-glutenists argue that we haven't eaten wheat for long enough to adapt to it as a species. Agriculture began just 12,000 years ago, not enough time for our bodies, which evolved over millions of years, primarily in Africa, to adjust. According to this theory, we're intrinsically hunter-gatherers, not bread-eaters. If exposed to gluten, some of us will develop celiac disease or gluten intolerance, or we'll simply feel lousy. Most of these assertions, however, are contradicted by significant evidence, and distract us from our actual problem: an immune system that has become overly sensitive.

When an elementary school teacher calls in sick to work, she finds out that she is not the only one who will be missing school that day.  Children from her fifth grade class have also become ill and parents are calling to report the absences. The concerned principal of the school finds out the fifth grade class had a field trip to one of the local parks the day before. The case shows the areas visited by the class and provides discussion between the principal and a physician that teaches the readers about water quality and EPA standards for bacteria levels in recreational water. The case has an optional lab component that provides the theory for three methods that can be used to conduct microbial water analyses.  Procedures are included for teachers to lead a lab activity to test water to identify which location in the park caused the illness. This case study and accompanying laboratory activity are recommended for an environmental engineering course or a lower level science course.

This activity supports the HHMI short film The Origin of Species: The Making of a Theory. Students are presented with a map of the Malay Archipelago and some field notebooks with observations of animals. By plotting which animals are found on which island, the students discover the Wallace Line-a sharp boundary that separates distinct Asian and Australian fauna. 

This animated short video tells the story of Alfred Russel Wallace, who independently formulated the theory of evolution by natural selection at the same time as Charles Darwin.Co-directed by Flora Lichtman and Sharon Shattuck of Sweet Fern Production, the video tracks A. R. Wallace's life from growing up in England to his voyages with Henry Walter Bates and later adventures in the Malay Archipelago. The 8-minute video ends with an examination of the naturalist's legacy. (The video is presented here with permission from Sweet Fern Productions.)