DEPARTMENT OF BIOLOGY
Complaints About How We Teach Evolution
The following is a short list of issues that are raised by non-scientists, often in the form of arguments against evolution. I have re-cast them here as "complaints," because my Creationist friends have presented them to me in this manner. I offer my thoughts about how we might teach evolution differently.
Evolution is taught as "fact"
To those who have been raised to believe that creation is Absolute Truth, it seems impossible for evolution to be true. Therefore, to have evolution taught as "fact" is distressing, and easily results in closing the mind to the information.
Whether or not we teach evolution as "fact," many of our students and their parents perceive our teaching as such. This probably is a result of the general belief that teachers are Authorities, whose job is to pour knowledge into students' heads. It also results from the general belief that science is memorizing facts. Thus, science teachers are seen as Authorities who dispense facts for everyone to memorize--regardless of the actual words or pedagogical methods that we use.
With this view of what a science teacher is supposed to be, it is very easy to conclude that science is a form of Received Wisdom that is handed down to us by Science Teachers. To some extent, this puts it on a par with Received Wisdom that is handed down to us by religious authorities and religious texts.
In order to dispel this notion, it is essential that we provide students with actual data to analyze, so they can develop an understanding of the Theory of Evolution as an explanatory framework for a large number of findings.
Evolution is all speculation
A moderately common approach to teaching evolution begins with a description of "evidence for evolution," and follows this with descriptions of disruptive selection, directional selection, etc. This makes perfectly good sense for those of us who know the data that is summarized in this approach. To someone who does not know the data, and has not made the mental connections between different pieces of information, it can easily come across as unsupported dogma.
At the level of presentation of many public school textbooks, the discussions and the diagrams have been simplified to make them more accessible to students. Unfortunately, the simplification removes some of the details that are necessary to move beyond a superficial understanding. Add to this the lack of enthusiasm with which many students approach their science classes, and we end up with many students knowing very little about the evidence upon which evolutionary theory is based. To them, it seems as if we offer them a bit of information (such as "there are fossils") and then jump to the statement that all of life is derived from a single ancestral species of ancient bacterium. This is a large jump, which feels like speculation.
To counter this perception, we should provide students with actual data to analyze, so they can develop an understanding of the Theory of Evolution as an explanatory framework for a large number of findings.
We teach supporting evidence, but not contradictory evidence
To those who know the data, it is clear that the evidence supports evolution. Contradictory evidence seems not to exist, except, perhaps, at the level of the fine details of the mechanisms. As a result, we describe the more important findings and their interpretations. Given the time constraints in the classroom, we usually aim toward presenting sufficient supporting evidence to make it clear what the theory of evolution actually explains. We don’t belabor incomplete studies that were once offered as contradictory evidence, but that, with additional examination, have proven not to be contradictory at all.
Yet, the popular conception is that there are studies that contradict evolutionary theory, and that this information is not presented in the science classroom.
It is somewhat tricky to deal with this directly, inasmuch as an accurate presentation of the "contradictory" evidence would demonstrate deficiencies in the evidence, leading to a loss of credibility for the person or organization that offered the "contradictory evidence." To a large extent, this is the problem with the Critical Analysis of Evolution lesson plan that has been mandated in Ohio--an accurate critical analysis supports evolution very well, while illustrating the deficiencies of the "challenges."
Again, it seems that an appropriate solution would be to present evidence, and allow the students to develop their own interpretations.
1. Begin with non-controversial information that everyone would recognize as fact
I recommend beginning with the underpinnings of evolutionary mechanism: genetic inheritance, and mutation. It is not clear that a deep understanding is necessary of meiosis or of dominance, recessiveness, co-dominance, incomplete dominance, expressivity, penetration, or any of the other "tricky bits" of inheritance. Rather, what is necessary is the understanding that:
- genes determine much of developmental biology
- genes are passed from parents to offspring
- genes are subject to mutation
- mutations can alter metabolic or morphological characteristics.
It may not be necessary to understand the flow of information from DNA to RNA to protein, and from protein to the regulation of embryological development, although it certainly helps make the connection between genotype and phenotype.
It is also useful to begin with a discussion of crop breeding. We know this has occurred, and we know how it works. It therefore serves as an explicit example of evolutionary mechanisms--particularly the occurrence of mutations, followed by selection and the change in allele frequencies in the population.
For some issues, it is important to give students an understanding--and practice using the thought processes--of real science. We are not simply memorizing facts. We are searching for understanding of our world. This requires obtaining data and interpreting it. Sometimes, there are alternate interpretations for the same data. In such a case, we must identify criteria by which to determine which interpretation is better. This is a natural part of the process of scientific inquiry.
However, it is not a natural part of our students' world views. As shown by Perry, most students enter college with a "dualistic" world view--every question has one single right answer, and other answers are wrong. Students with this world view expect the teacher to be the source of The Right Answer. In science, there may not be a right answer. Or, we suspect that there really is one right answer, but we can never say for certain that our current interpretation of the data is that final answer. New data may require that we re-evaluate our understanding.
To move students toward a better understanding of science in general, and evolution in particular, we should give them data to interpret. It may need to be relatively limited data in some cases, depending on the complexity of the interpretation, but data interpretation is essential. For example:
The Fossil Record
Fossils of a given age tend to be of particular types. When paleontologists collect fossils, they know, within a certain range, the age of the sediments in which they are collecting. Therefore, we begin with an understanding of the relative ages of different types of fossils. We would not reconstruct a lineage based solely on appearance.
This suggests several interesting student investigations.
1. At a particular time in the earth's history, what was life like? For example, what kinds of fossils have been found from the Devonian? Construct an image of the world at that time, based on the data provided by the fossils.
2. A collection of fossils of various kinds of animals, but from different ages, gives us a sequence of images of what life was like at those different times. What kinds of scenarios can we develop that would explain what happened over time to produce this sequence?
3. For a specific lineage of animals, fossils of different ages are not identical. If we have the fossils (or images of them), and information on the ages of the rocks in which the fossils were found, as well as information from rock types and other, associated fossils about the environments in which the animals lived, can we develop a reasonable explanation of what happened to produce this data? An excellent lesson plan that provides students with exactly this experience involving whale evolution can be found on the website of the Evolution and the Nature of Science website (which has a number of other lesson plans as well).
Genetics is the essential, underlying mechanism that makes evolution possible. Every individual inherits its genes from its parent(s)--but sometimes Mutations Happen. There is always genetic diversity, because mutations cannot be prevented. Sometimes, mutations are selected against; sometimes they are selected for; sometimes simple genetic drift changes the frequency of different genetic variations in a population. Given this basic, and rather simple information, students should be able to predict that separated populations will become different. Students should be able to develop their own explanations for observations that have been made in the world around us. For example:
Historically, we know that limes were introduced to the Americas only after 1492. Some lime trees were abandoned by the Spanish in the Florida Keys in the early 1500's. The descendents of these limes are the Key Lime, which is distinctly different from the "traditional" lime. How can we account for this difference, knowing what we know about genetic inheritance and mutation?
On the island of Hawaii, there are many species of fruit flies that are found nowhere else in the world. Several lines of evidence suggest that a particular species of fly colonized "the big island" on its western coast. There are now two different species of flies on the eastern coast. To the north of the central volcanoes of the island we find D. heteroneura. To the south, we find D. sylvestris. These species of flies are similar to each other, and to a third species that lives on the west coast, except that the heads of D. heteroneura flies are strangely shaped--very broad, with the eyes out at the sides (somewhat reminiscent of a hammerhead shark). Knowing what we know about genetic inheritance and mutation, how can we explain what happened during the migration of fly populations from the west coast to the east coast, separated by the central volcanoes of the island? That is, what scenario can we develop to describe a plausible series of events that would lead to what we see today?
We can also re-use one of the questions noted above: A collection of fossils of similar kinds of animals, but from different ages, gives us a sequence of images of what life was like at those different times. What kinds of scenarios can we develop that would explain what happened over time to produce this sequence? In addition to the whale series mentioned above, one can also use hominid skulls, and the series ranging from the lobe-finned fish to the earliest amphibians (see http://tolweb.org/tree?group=terrestrial_vertebrates, http://hometown.aol.com/darwinpage/tetrapods.htm, http://www.sciencemag.org/cgi/content/full/304/5667/57 ). The transition from lobe-finned fish to tetrapods is particularly inviting, since we know from the fossil assemblages that these animals lived in near-shore swamps reminiscent of current mangrove swamps, in which pushing against underwater roots would be a more successful mode of locomotion than traditional swimming. We know the molecular biology of limb development, so we can easily understand how mutations in certain genes would result in changes in limb morphology. Coupled with information about these animals' environment, we have a good idea of the selective pressures that would select for mutations that make the fins more robust--bone fide limbs.
What will happen if a population of organisms splits into two smaller groups? Over time, will they necessarily remain the same? Why or why not?
We have a cross section of a geological region, and we take samples of appropriate strata from different locations (eg the bottom of this segment of the geological column, part-way up the segment, farther up the segment, and at the top of the segment). We send these samples to a laboratory with the appropriate instruments to analyze the ratios of potassium and argon. Radioactive potassium decays to argon, with a known half-life. We present our students with the raw data (and have them do some math to figure out the ages of the samples) or with the data and the inferred ages of the samples (if we want to side-step the math). How do we interpret the data?
A well-documented series can be found in the Bearpaw Formation of Saskatchewan, discussed in detail in the TalkOrigins Archive.
We determine the DNA sequences, for a particular region of DNA, for myself, my sister, and the guy across the street. We do the same for a chimpanzee, it's sister, and a chimp from the other side of the forest. We do the same for two sibling cows, and a cow from elsewhere in the country. We do the same for two sibling sunflowers, and a sunflower from a different state. We count how many DNA base differences there are for each comparison of sequences. What do we find when we make a tree diagram of the data? This diagram is nothing more than a pictorial representation of the information in the table, and as such, is "fact" (presuming that we know how to count, and that our sequences were accurate). How do we interpret the data? We are not surprised to find that siblings have similar DNA sequences, and that less-closely related individuals have less similar DNA sequences. We know the genetic relationships, so this makes sense. But, what is the significance of the connections between species -- very similar DNA sequences from very different species? The tree would look something like this:
The point here is that there are a number of types of information that go into developing a Theory of Evolution. Our students cannot work with all of the information in the short time available, but they can work with some of it. If they work with some of the data, and develop interpretations themselves, they will have a much better sense of where evolutionary theory comes from. It will not have been given to them "as fact" by an "Authority," but will be an understanding that they have built themselves.