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Sample Discussion Thread
This discussion was selected from the AMNH online course The Link Between Dinosaurs and Birds. This is an excerpt from an actual course discussion, but learner names have been changed. The Link Between Dinosaurs and Birds is part of Seminars on Science, a
program of online graduate-level professional development courses for
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Week 1 Discussion: Classification|
Both cladistics and the traditional Linnaean classification method rely on characteristics to classify organisms. What do you think are some of the similarities and differences between the two methods of classification?
Cheryl (initial post) 26 Sep 6:32 PM
Humans have a need to sort and classify and we do this in our everyday lives. The classification of living and extinct taxa however, provides important information on evolutionary relationships and the history of life. Cladistics uses shared derived characters (synapomorphies) to unite groups to reflect a shared evolutionary history. The Linnaean system used (uses) mostly external features - for example, feathers and warm-bloodedness to group birds. Cladistics unites birds with dinosaurs based on shared derived characteristics such as the S-shaped neck and breast bone characters. These are not primitive shared characters, but features that evolved in a species ancestral to both dinosaur and bird species. Characters can also be "presence or absence". By using derived characters that are not weighted, we have a better chance of grouping taxa based on true ancestral relationships -- our eyes may see something as more significant than it should be and lead us to group incorrectly. The characters used in cladistics indicate shared ancestry rather than simply shared features.
(response to Cheryl) 29 Sep 11:59 AM
I agree that humans seem to have a need to sort. Linnaean classification can be useful on some levels, because it tends to be simpler to understand for the layperson. My students have trouble with cladistics because often the differences are not obvious. As I discuss in my paleo class, the more we know about organisms and their characteristics, the more the "simple" classification becomes blurred. We use archeopteryx as an example. That is where cladistics becomes very handy. I have not attempted to teach cladistics as of yet. Perhaps this will be the year.
(response to Shaunti) 29 Sep 1:35 PM
Hi Shaunti, I use the class Reptilia as an example of how Linnaean classification doesn't always get it right - but students are reluctant to group birds with dinosaurs with crocodiles on a separate branch. It doesn't seem right to them. Our human eyes can mislead us!
(response to Shaunti) 29 Sep 6:44 PM
How might you go about introducing your students to cladistics, Shaunti? How big a part of your paleo class is classification?
Shaunti (response to Patrick) 5 Oct 3:56 PM
With any luck, the students coming into the class have some background in biology. They all should have passed that as a prerequisite to the class, but sometimes I get students who are in bio now. I review evolution and classification. I have tried to have them classify organisms in the Linnaean method, and then demonstrate that dinosaurs don't "fit." I have not taught cladistics, but I am game to try. I like the candy method. Food always gets them! We have mostly discussed the different groups of dinosaurs and their characteristics, starting at ornithischian and saurischian and the groups within. I have 9-12 graders, so I also cannot get as complicated as the information in the textbook. I do have to generalize. We do try to compare what we know about dinos and modern animals and come to inferences about behavior based on fossil evidence. I try to eliminate the "dinosaur stereotype" of big and stupid.
Of course, I point to the pigeons outside and say "dinosaur," and they all laugh. By the end of the course, I have at least some of them convinced.
(response to Shaunti's first post) 29 Sep 9:50 PM
Hi Shaunti, I find that teaching cladistics is a lot easier if you use familiar objects at first and then move on to organisms. I have a lab that uses candy to get students familiar with the concept of shared characters. I can post it into the doc sharing folder tomorrow if you like. As you and many others have said, humans have the urge to sort, and your students already know how to sort things. It's just doing it in a methodical way that seems weird at first. Also, trying to sort animals can be intimidating because it seems so science-y and official. Using everyday objects is a good way to ease into it.
(response to Jonathan) 30 Sep 6:08 AM
I like the coin cladistic exercise. You can make a short cladogram using pennies, nickels, dimes and quarters based on their shared and "advanced" characteristics:
1) group all the coins that are round (all of them)
2) now branch the coins that are silver and round (dimes, quarters branch off after pennies)
3) the next branching point is for the coins that are round and silver, but with smooth edges (nickels only)
On this cladogram, pennies represent ancestral species because they exhibit "primitive traits". Nickels represent the most "advanced" group. A tad simple, but to the point
(response to Amar) 30 Sep 6:13 AM
Keep in mind that both Jonathan & Amar have suggested using inanimate objects for their lessons. I usually use vehicles (bike, car, plane, space shuttle... you can add in motorcycle to test for understanding) And these are great ways to introduce the ideas of how to mark presence/absence of characters, but don't forget that none of the features of candy or coins or vehicles are derived, they are all determined by their makers. Also, primitive means earlier, while advanced means later. As we see in the biodiversity of our planet, many primitive traits are still allowing success. Advanced is not necessarily "better". Anthony
Agnes (response to Anthony) 1 Oct 6:29 PM
In my biology class I actually use a cladogram when we learn about the evolution of homo sapiens. I map out the fossils that have been found in order to show the link between apes and humans. The kids seem to pick up on it fairly well.
(response to Jonathan) 30 Sep 10:12 AM
I would love to try a new activity with them. Thanks! I never really taught it because until this year, the entire course was one semester. It has now been spit into invert for 1st semester and vert for 2nd semester. I will be teaching the 2nd semester, so this year I should have time. I have them list characteristics of birds and reptiles. Then I introduce a reproduction of archeopteryx and demonstrate how its characteristics do not fit into "traditional" Linnaean classification. It works well toward getting them to understand the relationship between dinos and birds, but they still don't quite accept that today's birds are really dinos. I try to get them to classify archaeopteryx as reptile or bird, and of course it doesn't work well for them. That is where I can introduce clasdistics.
(response to Jonathan) 6 Oct 10:40 AM
HI Jonathan - I have very fun activities that I just did yesterday with my kids - sorting and recognizing differences between organisms. When I go trekking with my older students each year, one of the things we do is a beach litter shell count on the beaches we document. We've been doing this since l989, so we have lots of sets and several different beaches. When I do cladistics with my younger 9th graders, to get the kids to begin observe different characteristics, I put a big pile of shells in the center of each lab table and have them "sort" them by what they thick are species. It takes them quite a while to separate them, but when they are done, some tables have lots of species and a few of each species, some groups have a few species and lots of examples. I then have them think of reasons why their collections might be different. This introduces all kinds of interesting discussion about what might be happening on the beaches. Painlessly they figure out how to make careful observations and then begin to see why this might be an important study. We then use this to begin the discussion of how paleontologists don't have living animals to use, and what are the limitations of making assumptions based on fossil remains, etc. It's very fun. I should have taken a digital pic before they cleaned up, but just didn't think of it. Christina
Christina (response to self) 6 Oct 11:13 AM
Ok, here's another collecting activity with a twist. I have the kids go and collect leaves to make their own key, but to initially get them to observe the differences, I have them make rubbings of each leaf after they have dried in their press foe a couple of days. This focuses them on characters like veining and leaf border and overall shape. It's also fun for them, and fixes in the minds the idea of careful observations.