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NUTS & BOLTS
Based on a Presentation by
Adapted for ENSIweb by
|Students working in teams classify furniture, share their categories and rationales, then note how their different schemes are perfectly logical and useful, but they vary and are completely arbitrary. They then see how living organisms are classified, and note how these natural groupings always reflect about the same ancestral relationships in the same nested hierarchies, regardless of the different criteria used. Such patterns are revealed with a look at several phylogenetic trees of primates. Finally, teachers are encouraged to give their students lab experience collecting data from a variety of primate characteristics (skulls, chromosomes, DNA and hemoglobin), to see for themselves the congruency of those data sets.|
|1. Science deals only with natural
patterns and mechanisms
2. The groups-within-groups hierarchical pattern of Linnaean classification is a result of both extinction and branching from common descent.
3. Classification of Non-biological objects is purely arbitrary; biological classification is not.
|Darwin's Phylogenetic Tree (from his Origin
Order Carnivora: working layout
Order Carnivora key
Phylogenetic Trees of Primates, based on various molecular criteria
Primate Morphology: Primate Skull and Dental Attributions (matrix)
Primate Morphology: Glossary of Terms (to be added later)
Primate Cladogram: based on Primate Morphology (pre-molecular)
Phylogenetic Tree based on Primate Chromosome Banding Patterns
The following will provide useful background, and is adapted from the Session Abstract by Martin Nickels, with his permission:
As Darwin noted in 1859, taxonomic classification in biology differs fundamentally from the classification of other objects. The difference is that organisms naturally cluster into discrete, non-arbitrary, hierarchically-nested groupings only because of their mutually-shared evolutionary ancestry. This nonarbitrary grouping of organisms is evident from-and only from- congruence across different sets of biological data. We will provide and demonstrate a teaching unit that illustrates this congruence using molecular and morphological data sets from humans, apes and other primates.
An all too common-but erroneous-approach in teaching biological classification is teaching that the classification of human artifacts (hardware, furniture, etc.) accurately mirrors biological classification. Another flawed approach entails using drawings or pictures of organisms (or even imaginary organisms) without providing information about their characters that makes clear to students the basis on which the organisms naturally cluster. Both approaches make it seem that all classifications humans devise are just a matter of choosing arbitrarily which of several superficial features one can use in constructing the classification. Such an approach clearly results in the conclusion that any coherent classification of a group is as meaningful and valid as any other classification because there is no non-arbitrary, intrinsically superior scheme.
It is proposed, therefore, that teachers seriously reconsider how they introduce and teach classification. Rather than having their students classify some non-biological materials (e.g. furniture, vehicles, hardware, etc.), spend some time comparing the patterns of similarities actually found in some major group of organisms, looking at their morphology, cytology, and molecular aspects, finding that their degrees of similarities generally conform to the same pattern, suggesting fundamental biological relationships between the members of the group. For pedagogical reasons, and the fact that excellent data sets are readily available, it is suggested that primates be the group of choice, comparing humans, apes, monkeys, and prosimians.
Excellent lessons are available which will facilitate this exploration, and they are on the ENSI web site. A recent addition "Molecular Sequences & Primate Evolution" compares the amino acid sequences of beta hemoglobin in a cross section of primates. In the "Hominoid Cranial Comparison" lesson, resin replicas of the skulls of hominins (formerly "hominids") and apes are carefully compared, revealing a pattern of gradual mosaic change through time. And with the "Comparison of Hominoid Chromosomes", the chromosome banding patterns from apes and humans are critically compared, suggesting degrees of relationship very similar to the patterns found independently in the other two lessons. See links below to access those lessons.
IF you insist on using the classification of non-biological items to introduce the general concept of classification, BE SURE to give your students some experience with the content of these lessons, and, most importantly, help them to discover how such classification DIFFERS from biological classification (into NON-arbitrary hierarchically-nested groups). If they don't see the difference, be sure to lead them to that realization with some hints and coaching.
(loosely adapted from the NABT session):
2. As teams are arriving at their particular versions, hand out an overhead acetate and pen to each team (or to a few selected teams, based on different formats and early completions noticed during a walk-about). Ask those teams to put their arrangement (groupings) on the acetate. An alternative could be to do this on large sheets of blank newsprint or butcher paper, for subsequent display around the room.
3. When all or most teams are essentially finished, call in the acetates, and display each one in turn to the class, asking the team spokesperson to explain the team rationale for their classification.
4. Point out the different versions, based on different criteria, resulting in certain furniture pieces being placed into different groups.
5. On the overhead, show the only illustration Darwin used
(in chapter IV) in his best known publication, The Origin
of Species: a generalized branching tree showing descent
with modification over several generations, and with most branches
going extinct sooner or later. In particular, point out:
6. On the overhead, show the array of the order Carnivora. Draw a box surrounding the genus Canis and the three species below it (jackal, wolf, and coyote). Do likewise for each of the other four genera, then around the two families and their contained genera and species, and finally around the entire order. Point out how species today naturaly cluster into hierarchically nested groups, based on their shared traits. These are non-arbitrary groups, since they sort consistently the same way, even though based on different cirteria.
7. Draw converging lines down from each species in a genus, to a common branching point (common ancestor), then likewise from each adjacent common ancestor to an earlier common ancestor, etc. (see completed Carnivora diagram).
8. On the overhead, show a series of phylogenetic trees of a particular group of organisms, but based on different criteria, and note how similar each one is to the other. Good examples to use would be primate trees; several can be found in Evolution by M. Strickberger, 1996, and Science and Earth History by A. Strahler, 1987. Another tree is based on comparisons of chromosome banding patterns, from The Cambridge Encyclopedia of Human Evolution, 1992, ed. by S. Jones et al. Adaptations of some of those diagrams are available here (see Materials). In addition, students can build a tree using the morphological features found in selected primates and presented here in the matrix: "Distribution of Primate Skull and Dental Attributes". Such a tree is also included.
9. These Multiple Independent Lines of Evidence (MILEs) constitute a congruence of data sets, in which the pattern of one can be used to predict the pattern of another. This doesn't happen with non-biological groups, e.g. furniture, vehicles, hardware, etc. One might see a continuity of the development of various forms of technology and art, but that is only history, and historical development lacks intrinsic biological relationships, so it doesn't necessarily retain any "ancestral traits", which are natural and mandatory in biological systems. BE SURE YOUR STUDENTS CLEARLY UNDERSTAND THESE POINTS, and the distinctions between biological and non-biological classifications, and why these distinctions exist. Students should be reminded that biological classification is naturally non-arbitrary, while classifications of any non-biological collections are clearly arbitrary.
10. In conclusion, you can point out that the result of biological classification itself, as a consistent nested hierarchy, can be considered as another MILE in the support of evolution. A very helpful graphic presentation showing this can be found in the Macroevolution / Classification diagram (with instructions). At the same time, it's intersting to show Darwin's only diagram (of a phylogenetic tree) in his Origin of Species - with few branches way back in time, branching and rebranching upwards over many generations of geological time, with most branches shortlived (going extinct) and the surviving branches continuing on and most branching again. The many branches going extinct brings a more realistic picture of descent with modification than the more simplistic diagram used for the Macroevolution / Classification connection, which purposely omits most of the branches that went extinct, so as to focus on those that form classifiable groupings in the present - and to more clearly show their origins. READ Darwin's explanation of his only diagram, excerpted from Ch. 4 of his Origin of Species.
11. In order for students to fully appreciate these things,
they should experience a few of the lessons on this site which
produce these kinds of data sets. These can be done in sequence,
one after the other, or they can be done at different times in
their appropriate context, in which case the students should
be reminded then of this introduction to biological classification,
and how and why it is different from non-biological classification.
The suggested lessons are:
1. There is a somewhat simplified, self-contained one-page activity which nicely illustrates both the nested hierarchy and the evolutionary basis for that hierarchy in primate classification. See the Primate Classification Lesson (with its worksheet and key). This has worked quite well by showing a boxes-within-boxes visual to illustrate the nested hierarchy of biological taxonomy. It also provides a convenient reference sheet for students throughout the course for checking primate groups.
2. See Gendron, Robert F. 2000. "The Classification
& Evolution of Caminalcules." The American Biology Teacher, October
2000, pp. 570-576. Includes sharp diagrams of "living"
and "fossil" caminalcules and a Caminalcule Evolutionary
Tree that those caminalcule "specimens," when properly
arranged, would likely display. Excellent vehicle for questions
probing and analyzing fossils and phylogeny.
3. The UCMP (University of California Museum of Paleontology) has an excellent presentation of cladistics, phylogenies, and modern systematics (what, why, when, and how). Take a look at it.
4. See the excellent online tutorial by the UCMP: "What did T. rex taste like?" It makes an excellent introduction to classification, phylogenetic trees, and cladistics. This could be given as a homework assignment (online).
5. For an excellent tutorial to introduce phylogenetic
(evolutionary) trees, see our review of an article
in The American Biology Teacher: Phylogenies & Tree Thinking by David
Baum and Susan Offner, The ABT, April 2008, pp. 222-229.
This is an excellent introduction to phylogenetic trees and their
several versions. The suggested presentations would be perfect
for high school biology, filling a gap in biology textbooks
and providing a deeper understanding of biology. Evolutionary
trees provide an excellent visual aid to understanding evolution
and its relationship to classification, but there are
several conventions in their use that should be explained in
order to prevent or remove misconceptions..
This article by Baum and Offner is probably a teaching application based on the paper in Science by Baum et al: EVOLUTION: The Tree-Thinking Challenge, 11 November 2005, 310 (979) at: [requires subscription to Science], but you can get the excellent 22-page supplement part: that gives various trees for students to interpret; makes a good pre-post quiz tool to determine misconceptions and assess the effeciveness of your approach. See References and Resources (below) for additional materials.
An example of a classic problem for "classification" like those discussed in Baum's articles can also be found in an excellent book by Carol Yoon: Naming Nature. It presents a diagram of a salmon, a lungfish, and a cow, and asks how should we group them? Are they all separate? Do the salmon and lungfish form one branch, and the cow the other? Do the cow and salmon go on the same branch with the lungfish on the other? Or, do the lungfish and the cow form a branch with the salmon on the other? Turns out, that the last answer is the correct one WHEN we use modern evolutionary theory as the basis for drawing the tree. Cows have a more recent common ancestor with lungfish than either does with salmon. It is clear that overall similarity is not the most important aspect of the organism for organizing the relationships among living things.
6. CAUTION: A word about DICHOTOMOUS KEYS - making them and/or using them. Many teachers traditionally do this as part of their classification unit. HOWEVER, unless one of your goals is to introduce your students to field identification of organisms in their area, using identification keys, I would strongly discourage doing this.
If you DO do this, be sure to clearly demonstrate that ID keys are seldom based on features used for classifying them! The features used are typically features that are easy-to-see in the field. It IS helpful for students to build a key from a particular collection of items, so they understand 1) how a key is created, and 2) that it's useful only for a particular group of organisms in a particular habitat and/or region.
Most importantly, emphasize that making or using a KEY is NOT CLASSIFYING ORGANISMS! Keys are for people who want to IDENTIFY (find the name of) an organism (or its group) in a particular habitat/region that has already been discovered, named and CLASSIFIED (placed into its proper biological category based on certain diagnostic features). Unfortunately, classifying organisms is often confused with identifying them. They are not the same thing. Be sure that your students recognize the difference.
See The American Biology Teacher (Vol. 67 #5, p.283-289,
May 2005) entitled Beware of Nuts and Bolts: Putting Evolution
into the Teaching of Biological Classification, by ENSI Co-Directors
Martin Nickels and Craig E. Nelson.
For an excellent tutorial to introduce phylogenetic (evolutionary) trees, see our review of an article in The American Biology Teacher: Phylogenies & Tree Thinking by David Baum and Susan Offner, The ABT, April 2008, pp. 222-229.
NEW 2009: Excellent articles on "Transforming Our Thinking About Transitional Forms" and "A Name by Any Other Tree" in the June 2009 Evolution: Education & Outreach Online (CLICK HERE for reviews and links to the article; scroll down to the brief reviews near the bottom; further down is link to the free online journal where you can download these excellent articles in PDF format).
Darwin, Charles. 1859. The Origin of Species
Some of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks.
1. Original Source: Martin Nickels 11/8/2001 Presentation at NABT convention in Montreal.
2. Edited / Revised for website by L. Flammer 12/2001