DEPARTMENT OF BIOLOGY
Evolution: A Recommended Sequence of Presentation
Present molecular biology first, so students will know what we're talking about when we do genetics. It's easier for many students to follow if they can visualize physical entities going through meiosis. This is where we need to make the link between gene and trait, which is critical. Interestingly, dominance, recessiveness, and Mendel's Laws are not particularly important here (and this being said by a geneticist!).
Present genetics next. Dominance, recessiveness, strict Mendelism are actually not the important parts. They become obvious when you know how dominance comes about. I use pigments for this, because it's easy to imagine. Indeed, it is possible to use pigments -- specifically hair color -- as the foundation of a complete unit on genetics that strives to overcome the common student-learning bottlenecks.
Revisit how we describe how genes influence traits. We've just done it with pigments, now we have to use morphology. I suggest limbs because we know a lot about them, and because they are a valuable example for evolution.
Enter evolution via crop breeding. This is non-threatening microevolution, which everyone will see as "facts" because we deal with documented events. Use this to illustrate the role of ongoing mutation (needed to get new varieties that are unlike the originals, like corn vs teosinte, or bell peppers vs little hot peppers). Use this to illustrate the role of selection, in this case by humans choosing which plants' seeds to use next season. I like to start with the question, "Columbus took small, hot peppers to Europe. How did Europeans turn them into bell peppers?"
It is also helpful to use the food supply for another important feature of evolution: separation of populations. I use The Great Chile Poster to illustrate that there are lot of varieties. Why? Each village saves their seed. They don't trade often. So, each village maintains its own population. As mutations occur and are fixed (probably by drift, though selection probably plays some role), each population becomes different.
Move to natural, isolated populations. I like the Key Lime. It's become distinctly different from standard limes in the 500 years since the Spanish abandoned the trees on the Keys. Why do I like this better than the Galapagos? Partly because we can bring Key Limes into class. They can see the difference, and handle the objects, so it's less abstract. In addition, it's documented history. We know when the Spanish abandoned Florida. Well, what happened afterward? Mutation, drift, selection, and change. It's all of the processes we examined with crop breeding, except this time, the only human intervention was the planting of lime trees on the keys. The rest was natural.
From this, work out the Rules for change. We need mutation as a source of genetic variation. We need inheritance. We need some mechanism that will result in some individuals having more offspring than other individuals. With breeding programs, this last bit is easy: the breeder chooses who the parents of the next generation will be. With peppers in different villages, or with Key Limes alone on their island, all of the individuals have an equal chance of having offspring, but some don't do so well, and some do better. That's all you need--a slight difference in reproduction.
From this, work out the process of change. Does the entire population go *pop* and everyone changes at once? No. If the breeder chooses big peppers, then big-pepper plants will be more common in the next generation, and small-pepper plants will be less common. It will take many generations for the big peppers to out-compete the small ones completely. At no time does any plant change as it is growing.
Now, we can look at the real world, and wonder what its history is. What are the different kinds of living things? We could start with butterflies, and see that there are groups of species (genera), and groups of genera (families), etc. We'd find that there are skippers, and moths, which are much like butterflies, but have some differences. We'd find that there are other kinds of insects, some of which are somewhat like the Lepidoptera, and others of which are quite different--but for each group, there are families and genera and species. This is a nested hierarchy that students can work out for themselves. And, of course, within each species, there are many varieties--which gets us back to the Great Chile Poster. What would students predict if we let our different chiles go on, separated from each other, for many, many more generations? They can predict that the peppers would become different enough that they'd qualify as different species.
This builds a sense of evolution-in-action, without ever straying from "obvious facts" and inferences that the students can derive for themselves. We can now make a Hypothesis: that the nested hierarchy that we see reflects the process we discussed for the chiles, except over longer and longer time scales. Varieties: populations were separated not so long ago. Species: populations were separated longer ago. Genera: populations were separated rather long ago. Families: populations were separated quite long ago. Classes, Orders, Phyla, etc: populations were separated longer and longer and longer ago. What predictions does this hypothesis make? 1. There must have been time for all of this. 2. Things we classify as "closely related" should have DNA sequences that are similar, while things we classify as "distantly related" should have DNA sequences that are less similar. But, there should be some level of similarity for everything that is related at any level, if there is genetic continuity from parents to offspring at every generation. 3. More-similar things should be found in relatively near-by regions. That is, we expect to find "clusters" of species within some geographic region, especially for plants that can't run around. That is, peppers should be native to one part of the world (Mexico and tropical America), and not be found elsewhere (which is true for 1492, prior to Columbus' taking pepper seeds back to Europe, from which they were carried all over the world. The greatest variety still is in Mexico). We see the same things for corn, potatoes, tomatoes, rice, citrus, "Mediterranean" spices like rosemary and thyme…in fact, for lots of things.
We can test these predictions. #3 has the answer above, which students can discover. #2 can be investigated on the Web. #1 requires moving to geology and the interpretation of fossils. It would be worthwhile talking about radiometric dating, and presenting students with some data to interpret. Does it make sense that different rocks are the ages we find them to be? Are there patterns in sedimentary strata that confirm the relative ages? What about fossils in those strata? Strata of different ages contain fossils that are different. What kind of world do we imagine if we build an image based on the fossils of a certain age? The Devonian is good, because there were no land animals, so the reconstruction produces a world very unlike the one we know.
But how can we account for major morphological transitions, like fish to land animals? Let's give our students information about the fossils that have been found for lobe-finned fish, and for the many intermediates between them and the earliest amphibians. Let's give them information about the other fossils with which these are associated--revealing an environment much like our current mangrove swamps. The students could even act it out: a lobe fin, trying to catch smaller fish in the tangled roots of the swamp, must push against the roots with its fins. Mutations that make the fins more robust would be an advantage. After about 10 million years of this kind of selection, we have the change from fins to feet.
If we have talked briefly about limb development as part of genes-to-phenotypes, we can refer back to those genes. Slight changes in the limb-development genes are sufficient to change the relative proportions of the limbs, from lobe-fins to amphibians, to dinosaurs and birds, to mammals, including humans and whales. The discussion of limb development, of the environment of the lobe-fins and the selective pressures they faced, allows us to see more easily how to visualize other morphological transitions.
What about human evolution? Again, give students the data, not just the conclusions derived from the data. There are many websites that provide the information from various viewpoints. This one presents brief descriptions of what is known, and some interesting interpretations at the end. Here and here there are good charts of the rather bushy hominid tree, with links to images of fossil skulls representing the different species. And, here is an all-about us site with links to everywhere.
The creationist complaint is that we present supposition as if it were fact. This reflects our failure to illustrate the nature of science, and train students to look at data first, then draw their inferences from the data. In the scenario I've imagined here, we present relatively non-controversial facts as facts, and build much of the evolutionary theory by giving the students the data and having them derive the inferences themselves.
last updated:Dec. 22, 2008