Teaching materials : 9-12 teachers' lounge :
Avoid common teaching pitfalls
Ever wonder if your students left class confused? Confusion can be a good thing if it causes students to think through ideas for themselves and find solutions. But sometimes confusion leads to persistent mistaken impressions or to major misconceptions. You may wish to learn more about these misconceptions. Meanwhile, here are a few things you can do to avoid unintentional confusion when it comes to teaching your students about concepts related to evolution:
- Choosing your words carefully
- Clarifying misconceptions
- Scientific terms that leave the wrong impression
- Handling bad press and sloppy science
- Correcting outdated information
- Slippery slopes in common classroom activities
Choosing your words carefully
Sometimes two terms are used interchangeably in the vernacular but have distinct meanings in science. Teachers need to be cautious of their own usage of these terms and encourage students to use this language correctly.
Function not purpose
The purpose of a hammer is to pound nails, and one's purpose in using the hammer might be to build a bench. However, it's not appropriate to say that one purpose of a hand is to hold a hammer. Instead, you can say that one function of a hand is to grasp or grip. Designed tools and independent agents have purposes. Structures of living things have functions. Since purpose implies design or intent, this is an important distinction in the science classroom.
Adaptation not design
Use of the word design may imply that living things are designed and there is a plan at work. It is more appropriate to use terms like structure and adaptation when referring to organisms. For example, "How is an aardvark designed to eat ants?" should be replaced by, "What adaptations do aardvarks have that allow them to eat ants?" or, "What structures and behaviors aid an aardvark in eating ants?"
Evolution not development
Colloquially, one might say that a callow youth evolved into a strong young woman or that the human lineage developed an upright stance over millions of years, but this is conflating the process of evolution with that of development. Development (ontogeny) occurs as a living thing grows up. Evolution occurs as the genetic makeup of a population changes over time.
Accept not believe
"Do you believe in evolution?" is a question often asked of biology teachers by their puzzled students. How one answers is important because the word belief is often associated with ideas about which we have strong convictions, regardless of the evidence for or against them. Science is not about belief; it is about making inferences based on evidence. It is more correct to say that a scientific idea is accepted, which means that the idea seems to be the most accurate available based on a critical evaluation of the evidence. An appropriate response to the student's question would be, "No, I accept the fact that the Earth is very old and life has evolved over billions of years because of the evidence supporting that idea." To be even more explicit, you could say "Belief often implies faith, and science is not about faith. It's about evidence. I agree with the ideas that make up evolutionary theory because of the strong evidence supporting them."
Evidence not proof
We often hear news stories that refer to a scientific idea having been proven. This is an example of confusing the terms proof and evidence. The term proof is used in mathematics and in courts of law, but does not belong in science because it implies absolute certainty. Scientists gather evidence which might help support or refute hypotheses and theories, but these ideas can never be absolutely proven, even when supported by many different lines of evidence.
In casual conversation we might hear, "My theory is that the Niners are gonna win the big game," when what is really meant is "I've got a hunch about who is going to win the football game this weekend." Socially, the first version is perfectly acceptable, but in scientific terms, a theory is much, much more than a casual hunch. This misuse reflects a persistent misconception about scientific theories. In the classroom, subtle differences in phrasing can reinforce or discourage such misconceptions. Here are a few cases in which watching your words can really pay off in terms of helping students build accurate conceptions. For more, visit our page on misconceptions regarding evolution.
In everyday language it's appropriate to say that we adapt to a new working environment or that a dog adapts to cold weather by growing a thicker coat. Unfortunately, students may apply this use of the term to evolution. This results in the erroneous impression that evolution consists of individuals adapting to changes in their environments within their own lifetimes. Evolutionary adaptations, on the other hand, occur through the action of natural selection working on populations of genetically varying individuals. Some of those genetic variations may have advantages over others in that environment and so will increase in frequency over the course of many generations.
Learn more about misconceptions regarding adaptation and natural selection.
Some students have the mistaken impression that evolution is random because some elements of the process are random. Hence, this term should be used with care. Mutations and the variation generated by them are "random" in the sense that the sort of mutation that occurs cannot generally be predicted based upon the needs of the organism. (Note that this does not imply that all mutations are equally likely to occur or that mutations happen without any physical cause.) While some aspects of mutation are random, natural selection is not. Natural selection favors mutations that confer a fitness benefit to the individuals that carry them. The process of selection winds up sorting through randomly generated mutations, weeding out some and favoring others. The evolution that occurs through natural selection is not random at all, even though the genetic variation, upon which natural selection acts, is generated by random mutations.
Ancestor versus relative
When we fail to distinguish between common ancestors and relatives, we set our students up for confusion. You and your cousin are related because you share a common ancestor. You did not evolve from your cousin. This seems obvious when we talk about family relationships, but it can be confusing when it comes to evolutionary relationships. For example, many students think that humans evolved from chimpanzees that they are our ancestors. However, this is incorrect. Humans and chimps are relatives and share a common ancestor, but humans did not evolve from chimps any more than chimps evolved from humans.
Neither primitive nor advanced
Some people might describe an opossum as more primitive than a cat because opossums retain some conspicuous features of the ancestral mammal, while many specializations for carnivorous lifestyles have evolved in cats. However, primitive and advanced are not appropriate ways to describe these differences. Opossums and cats are both successful, modern organisms that are well-adapted to their current lifestyles. While it is correct to refer to features that an organism shares with its ancestral lineage and new traits that have evolved since, describing any organism as primitive or advanced is a value judgment that has no place in science.
Confusing evidence with causation
It is tempting to use phrasing such as "Humans and chimpanzees are related because they share similar features and genetic sequences." However, this statement is misleading because it confuses evidence with cause. A better way to put this is to say, "Similarities shared by humans and chimpanzees support the idea that they are related." Having similar features does not cause relatedness. Relatedness is due to common ancestry.
Theory versus hypothesis
Much confusion surrounds these two terms because of common misconceptions and colloquial meanings that conflict with the terms' scientific meanings. In everyday language, we often use the word theory interchangeably with hunch. In science, however, a theory is much more than a hunch; it is a broad, natural explanation for a wide range of phenomena. Theories are concise, coherent, systematic, predictive, and broadly applicable, often integrating and generalizing many hypotheses. Theories accepted by the scientific community are generally strongly supported by many different lines of evidence, but even theories may be modified or overturned if warranted by new evidence and perspectives. Gravitational theory, for example, attempts to explain the nature of gravity. Cell theory explains the basic unit of life. Evolutionary theory explains the history of life on Earth, is supported by many lines of evidence, and is accepted by the scientific community. Describing evolution as "just a theory" conflates the scientific and everyday meanings of the word and is inappropriate. With students, it may be helpful to address the scientific and colloquial meanings of the terms directly by explaining the difference.
In everyday language, the word hypothesis usually refers to an educated guess or an idea that we are quite uncertain about. Scientific hypotheses, however, are much more informed than any guess. They are explanations for a fairly narrow set of phenomena and are usually based on prior experience, scientific background knowledge, preliminary observations, and logic. In addition, hypotheses are often supported by many different lines of evidence in which case, scientists are more confident in them than they would be in any mere "guess." To further complicate matters, science textbooks frequently misuse the term in a slightly different way. They may ask students to make a hypothesis about the outcome of an experiment (e.g., table salt will dissolve in water more quickly than rock salt will). This is simply a prediction, an expectation, or a guess (even if a well-informed one) about the outcome of an experiment. Scientific hypotheses, on the other hand, have explanatory power. A more scientific (i.e., more explanatory) hypothesis might be "The amount of surface area a substance has affects how quickly it can dissolve. More surface area means a faster rate of dissolution." This hypothesis gives us an idea of why a particular phenomenon occurs and it is testable because it generates expectations about what we should observe in different situations (e.g., table salt will dissolve more quickly than rock salt will). Textbooks and science labs can lead to confusions about the differences between a hypothesis and an expectation regarding the outcome of a scientific test.
To learn more about hypotheses and theories, visit the Understanding Science website.
Scientific terms that leave the wrong impression
Sometimes even scientists and textbooks end up using terms that can give students the wrong impression. These terms may require special clarification so that students understand the underlying concepts correctly.
Survival of the fittest
For many people, this phrase suggests that evolution only gives a pass to the best of the best. However, a better way of expressing how natural selection works is "survival of the fit enough." Portraying nature as a constant life-or-death struggle against competitors grossly oversimplifies what is really going on. Many life forms get by for eons by existing in niches for which other organisms are not suited or by simply being "good enough" to get their genes into the next generation. For example, brine shrimp live in water that is unsuitable for potential aquatic enemies, and they apparently have no significant competitors for food.
Also, it may be important to remind students that natural selection is not just about survival. To pass their genes on to the next generation, organisms must both survive and reproduce. By focusing on survival, the phrase "survival of the fittest" may encourage students to overlook sexual selection and the key role that reproduction plays in evolution by natural selection.
Learn more about misconceptions regarding adaptation and natural selection.
Transitional features not missing links
When we describe something as missing, it usually implies that the item is supposed to be present, but for unknown reasons, no longer is. In this way, the term missing link is misleading. Fossilization is a chancy process. Most organisms that have lived on Earth are not preserved as fossils, and of those that are, the majority have not yet been found. Thus, biologists expect that most intervening steps of an evolutionary transition (e.g., from ancestral arthropod to modern butterfly) will not be recorded as fossils. Because we know why so-called missing links do not always put in an appearance in the fossil record and because we expect this to be the case, missing links aren't technically missing at all, and having these "gaps" in the fossil record is perfectly consistent with evolutionary theory.
Instead of using the term missing link, it is preferable to discuss organisms with transitional features with your students. Different features of a modern organism may have evolved at different times in that lineage's history. The tetrapod transition from water to land, for instance, involved the evolution of many features for moving, sensing, breathing, and bearing young in this new dry environment. However, these changes didn't all occur at once. This means that the fossil organisms that represent the transition from fin to leg may not be the same organisms that most clarify the evolution of modern tetrapod ears. Hence, in many cases, it's more accurate to focus attention on a specific transitional feature than on a "missing link."
For more on transitional features, read our article on a fossil that helps illuminate the origin of tetrapods.
The Cambrian explosion
The Cambrian "explosion" is an unfortunate misnomer since it suggests life forms springing forth fully formed, virtually in an instant. Of course, this is not at all what happened during this time in Earth's history. The fossil record of the early Cambrian contains a large number of animals (with rather well-preserved hard parts) that happen to be obviously related to organisms alive today. This is the first record we have of these life forms, but the organisms themselves evolved over millions of years prior to being fossilized in formations like the Burgess Shale. While these forms appear "suddenly" in geological terms, this so-called explosion was lit by a long biological fuse.
Read more about the Cambrian Explosion in our article on the evolution of the arthropods.
Handling bad press and sloppy science
Science is a human endeavor. The process leverages human strengths (e.g., our creativity and problem-solving abilities) and has safeguards in place to account for human weaknesses (e.g., bias). Furthermore, science changes with human culture. Shifting values and perspectives may influence what is considered rigorous science. When such cases intersect with evolutionary research, as in the following examples, it is important to be straightforward with students to explain the whole story so that students are less likely to be misled by antievolution propaganda.
Ernst Haeckel, a 19th century German biologist, noticed remarkable similarities between the embryos of different vertebrate species during their development. This is now known to be an important observation because these similarities reflect the common ancestry of vertebrates. Unfortunately, in his enthusiasm to make his point, Haeckel modified the drawings of these embryos to make them appear more alike than they actually are. These fudged sketches (or versions derived from them) have appeared in many biology textbooks since then (although most current textbooks use accurate representations of embryos) and have recently provided much fodder for antievolutionists. Haeckel's indiscretion takes nothing away from the fact that embryonic development provides a great deal of information about the evolution of vertebrates.
Learn more about how evolutionary relationship may be reflected in development.
The peppered moth
Many biology and life science textbooks use industrial melanism in the peppered moth as an example of natural selection. The frequency of dark forms of the moth increased dramatically as coal burning in England blackened the surrounding landscape and allowed dark moths to survive better than light forms. Correspondingly, with recent increased pollution restrictions, the light moths are once again favored by natural selection and have become more common. Textbooks that use this example often include a photograph of preserved moth specimens stuck to tree bark. (If your textbook contains these photos, note that the wings of the moths may be in unnatural mounted positions.) Unfortunately, antievolutionists have seized upon these "faked" photos and the now outdated methods used in original study of these moths as evidence that evolutionary biologists have trumped up this whole example of evolution in action. However, these criticisms are misleading. Modern studies confirm that industrial melanism is a genuine phenomenon and that the case of the peppered moth holds up well as an example of it. The frequency of dark moths did rise and fall in synch with industrial pollution, and this was most striking in regions of the countryside with high rates of pollution just as we would expect if the population were evolving with the shifting selection regimes of the environment.
Learn more about the peppered moths on the Miller and Levine website.
Correcting outdated information
With new research and new perspectives, science advances and helps us understand the world around us more clearly. The fact that all scientific knowledge is fundamentally tentative and may be modified over time is one of science's great strengths, but it also means that the information that is in your textbook or that was in your college biology course can rapidly become outdated. Here are a few updates.
Kingdoms or domains
Two hundred years ago all living things were classified as either plants or animals. This required shoehorning fungi into the plant kingdom and classifying ciliated protists as animals. By 1969, this had evolved into a five-kingdom scheme, partly as the result of improved microscopy techniques which allowed biologists to better study unicellular organisms. However, as biologists switched to the view that our classification scheme should reflect evolutionary history, focus on the kingdoms has fallen by the wayside. Genetic studies and phylogenetic analyses reveal that the tree of life has three main branches. These are known as domains, and include the Bacteria, Eukaryotes, and Archaea.
View and review the three domains.
Presenting Linnaean classification as phylogenetic
Linnaean classification is still a fixture in biology classes and biology itself; however, most biologists now prefer to classify organisms according to their evolutionary history and sometimes adapt Linnaeus' classification scheme to suit this modern view. Linnaeus' classification scheme preceded the idea of evolution, and he had little information to work with other than gross anatomy. In the past century, scientists have learned much about the relationships of living things through lines of evidence that were unimaginable to scientists of the 18th century and have changed their views of the foundations of our classification system. So, although in many cases, Linnaean classification reflects actual phylogenetic relationships, in many other cases, the original groupings posed by Linnaeus do not represent evolutionary lineages.
Learn more about phylogenetic classification.
Defining a reptile
In grade school, many students learn that reptiles are cold-blooded, land-dwelling vertebrates with scales. However, in modern biological classification (which is based on evolutionary history) birds are a part of the clade Reptilia because, on the tree of life, the birds are a small twig on the reptile's branch. (In fact, according to modern biological classification, birds are also considered to be dinosaurs because they evolved from this particular group within Reptilia.) So, strictly speaking, reptiles are not just cold-blooded, scaly creatures; they are also warm-bodied, feathered creatures. This can prove to be a challenge in the classroom since the word reptile is used colloquially in one way and scientifically in another. Furthermore, it's useful to have a word that refers to animals that meet our traditional definition of a reptile. (For comparison's sake, the living amphibians do form a distinct clade and so this term can be used both colloquially and scientifically in the same way.) What's a teacher to do? One solution is simply to keep this detail in mind as a teacher of younger students and ensure that the issue of the phylogenetic classification of reptiles is explicitly addressed when students are older. Another possible approach is to use the word reptile only in the strict, scientific sense, and to teach students that snakes, lizards, turtles, and crocodiles share certain key features like cold-bloodedness, scales, and egg-laying.
Learn more about the phylogenetic definition of a reptile.
Great apes without humans
It is common to use the words great ape to refer to chimpanzees, bonobos, gorillas, and orangutans. However, this reflects an outdated view of classification. Humans are the closest living relative of chimpanzees and bonobos, and their branch on the tree of life is nested in among gorillas and orangutans. There is no unique set of traits that set chimpanzees, bonobos, gorillas, and orangutans apart from humans. Technically speaking, humans are not just closely related to great apes; we are great apes. You can reinforce the correct conception with phrasing such as "The great apes, including humans, are omnivorous."
Slippery slopes in common classroom activities
Evolution is more difficult to observe than, for example, Newtonian physics. Thus, we often conduct activities in the classroom that are intended to be analogous to evolution and to the scientific enterprise. However, some of these activities can lead to misconceptions unless they are carefully planned.
Having students hypothesize (guess) before they know anything
Hypotheses are explanations for natural phenomena that are based on prior experience, scientific background knowledge, preliminary observations, and logic. There are two main ways in which activities misuse the idea of having students form hypotheses. First, they may ask students to formulate a hypothesis without any prior knowledge or experience. This does not reflect the way science actually works and so should be avoided. If you want students to make a guess, that's fine just be sure to call it a guess, not a hypothesis! Second, many activities ask students to form a hypothesis about what will happen in a particular situation or in a particular experiment. This conflates the idea of a hypothesis with that of an expectation. Hypotheses are explanations. There is nothing explanatory about the prediction that plants that get more water will grow taller or that students wielding tweezer-"beaks" will pick up the most lentils off the carpet so these are not hypotheses. Rather they are expectations generated by the hypotheses that water is a critical component of plants' growth process and that a small "beak" is most efficient at picking up small food items. Be sure to help students distinguish between ideas that are true explanatory hypotheses and those that are merely what they think is going to happen. You can help students formulated real hypotheses by probing them about why they expect one outcome and not another to occur.
Debating creationism vs. evolution
There are some topics that students should be encouraged to think about, but not debate in school. Conspicuous examples include evolution and abortion. Young people tend to have limited experience and knowledge of such topics, making it hard for them to have authentic debates in these areas. Instead students tend to take whatever position the most influential adults in their lives take and may simply end up parroting arguments and propaganda promoted by various advocacy groups. The role of teachers is to provide information and opportunities to build conceptual understanding; such debates do neither.
Classifying designed objects
Students learn early how to classify objects by grouping them according to key characteristics and then assigning a name to each group based on the criteria. This is a valuable activity, especially for young students. However, many instructors of older students use a similar activity to introduce the classification system by which organisms are assigned scientific names. When teaching biological taxonomy, many teachers incorporate an activity in which students develop a classification scheme for hardware, tools, furniture, candy, or other human-designed objects, in order to draw a parallel to biological classification. Unless handled carefully, such activities can be misleading because in modern taxonomy, organisms are classified according to their evolutionary history, not according to key characteristics
Lessons that have students group artifacts and then draw parallels to biological taxonomy are misleading on several specific counts. First, hardware, tools, and other artificial objects are designed by humans, while organisms have no designer and have taken on their current forms through evolutionary processes. Second, classification schemes for designed objects may be based on any number of criteria (purpose, appearance, taste, etc.), while the biological classification of organisms is now based solely on evolutionary relationships. (Biologists may also use other criteria to group organisms e.g., according to eating habits, mode of locomotion, position in the food web, etc. but these are a bit different than the system of phylogenetic classification that is currently used to assign names to organisms.) Third, many different classification schemes for designed objects may be useful and or correct for different purposes; however, biologists strive for their classification of organisms to reflect the true evolutionary history of those organisms. Because of such fundamental differences, it can be easy to leave students with the wrong impression when they are asked to illustrate key concepts in biological classification with designed objects.
To see a lesson that tackles the issue head on, check out this lesson from the Evolution and the Nature of Science Institutes.