In this EvC Forum's first column, Mammuthus touches on a key issue of the Creation/Evolution debate. We hope to bring you more intriguing columns in the future. The column appears below, or you can click here to view in a separate window. If the subject interests you, please feel free to begin a discussion in this thread.
This new forum revealed two bugs. This is the first forum with an apostrophe in the name, and it revealed the first bug, which has been fixed. The second bug is that members can't post to new forums until they logout and login again, which has not yet been fixed. If you can't post replies to this forum then logout and login again. I'm expecting to fix this problem very soon.
quote:Does all the data contradict or support the conclusions, i.e., morphologically and molecularly, do they say the same thing? If not, is there something wrong with one methodology? Both? And what should be done next?
This seems to be the crux of the matter. It seems that convergent evolution may limit the construction of phylogenies by morphology alone. It would seem to me that molecular phyolgenies based on DNA studies would more accurately measure divergence times relative to different lineages. I would assumed that convergent evolution of morphology is much more common than two species developing the same exact mutations. Being that the two species were in different geographic areas, they may have evolved into similar niches through the same changes in morphology. However, speciation in the separate geographic areas should not involve the same mutations (but the same genes could have been affected).
My hypothesis is that ground sloths adapted to arboreal life styles among two different groups. This is a tough hypothesis to test, as I am guessing that the numerous genera mentioned in the column represent several fossil species. Further fossil finds may not differentiate Mylodon and Nothrotheriops, but the biogeographic isolation of the two groups may help to elucidate the problem somewhat. Nonetheless, a very muddled picture indeed.
I'm not sure that it would be unreasonable to accept multiple independant development of behavior such as an aboreal life. After all there are numerous examples of multiple independant inventions nearly everwhere we look. Aslan is not a Tame Lion
quote:I'm not sure that it would be unreasonable to accept multiple independant development of behavior such as an aboreal life. After all there are numerous examples of multiple independant inventions nearly everwhere we look.
Totally agree (which I hope is apparent in my post). However, convergent morphology shouldn't be due to convergent mutations, which is why I think the DNA sequences are more accurate than phylogenies constructed from morphology.
Just to pop in briefly..before going on the last leg of my travels..What are the creationist "answers" or suggestions to approach the subject of sloth diversity. One often hears the concept of "kinds" brought up as a supposed valid alternative to systematics and taxonomy. How could creationism further this particular science? Is there a "special divine creation" testable hypothesis that could be proposed to answer the question of the origin of arboreality in specific sloth groups?
Note: As an apology to all, not all of the references are accessible for free. The abstracts can be accessed without a subscription but except for PNAS, all charge for the articles. However, most of the journals should be availabe in most university libraries.
Hi Mr.H, Glad that you are also still around. I should be a bit more active this coming week...though I fear the pile of work i.e. lab disasters that await me I also see the stunning creationist response to my article...they must be thinking hard or testing their "hypothesis" in the lab
Cheers, weak from French cheese, sunshine, and strange Gaudi art Mam
However, mtDNA may not yield an accurate picture either because the length of time separating some of the lineages is so great that you get multiple substitutions occuring at many sites which totally obscure the phylogenetic signal. In the Greenwood, Castresana et al. article, they did an analysis to compare the "saturation" levels of mutations for 12S rDNA and cytb...and both show that mutliple substitutions are more apparent for 12S though a problem for both.For extremely long branches, morphology may in fact be a better indicator of phylogeny than DNA. It will be interesting to see what the nuclear genes say when Mylodon is added in eventually, but then one may run into the problem of too slow evolution of the sequences leading to no phylogenetic resolution....as you mentioned..a muddled situtation...
so, any creationists out there going to help unmuddle the poor scientists?
Thank you for the inquiry about writing a column. Columnists are recruited by board administration. I hope to ramp the column forum up to a fair amount of activity by year end, but wanted to start slow in order to develop a feel for how best to approach it, and what level to ask columnists to target their columns to. Plus this is a bit of a busy period for me.
Mammuthus provided a wonderful column that can be the benchmark for future columns. This isn't the place for a dialogue, but I'll take this opportunity to mention to Mammuthus that I was hoping his columns could be a permanent feature here, perhaps monthly or semi-monthly, and I hope he has another column idea (and another and another!).
Writing periodic columns is what a columnist does, say monthly or semi-monthly, but not less often than that. I think a monthly column focused on the Islamic side of the debate is a great idea. If you're willing to commit to a regular column, then send me your column or a column proposal to Admin. I can't promise to accept it as a column or you as a columnist, but we can begin exploring it through email. Maybe Mammuthus can make some suggestions, since he's been involved in getting the bugs out of these early stages.
Hi Percy, Given the initial rather limited response to the first column, I can only suggest that columnists perhaps try to be less technical than I was and keep the references down to a minimum. While I tried to distill the issues down to a more generally understandable level, I doubt very many people are familiar with sloths or their evolution. I also introduced way to many separate topics from ancient DNA, to phylogenetics, to a bunch of really strangely named animals. It also does not help that most of the references are not open access and require either purchase of the articles or an institute registration.
I will be glad to contribute monthly or semi-monthly columns and I do have ideas for more...I will try to make them a bit more controversial and accessible in the future.
And I hope Andya becomes a columnist as well! cheers, M
Mammathus, thanks for the article. You're a prime genius! Thanks, too, for the PubMed links to all this great info.
Since we can't all be Svante Paabo, examining ancient mtDNA or even the occasional nuclear DNA, can you give us the morphological scoop on any of these fossils, particularly regarding the cervical vertebrae? When I read your article, I recalled one of the few interesting things I previously learned about sloths - they don't have the standard 7 cervical vertebrae that mammals typically have (manatees are the other exception I recall - they have 6). I went digging for more info, and one site said that the 2-toed sloths generally have 6, while the 3-toed guys have 9 or 10.
Basically, I'm curious as to whether the molecular phylogenetics correlates with any relationships of fossil and modern sloths based on the cervical vertebrae. Please excuse my ignorance on the topic, and sorry if it sounds like I'm asking you to do all the research on my question, I just thought you might have already tracked down the info on the cervical vertebrae in the fossil sloths, and have a ready answer.
Besides, it gives us something to do while waiting for all those creationist responses that are sure to be rolling in any day now.
Thanks again for the interesting info. I'm learning more about sloths and coprolites than I have in many years. (My only other experience with a coprolite is a dino coprolite I own - I called it the "Barney Stone").
Re: Mammuthus, no genius...but I can drink a lot of beer
Hi Ediacaran, Though I am one of the authors on three of the cited papers (will try to preserve some anonymity by not saying which one), I am not Svante
While I will probably disappoint you as I am a molecular biologist and certainly no expert morphologist, what I can say is that because of convergent evolution, sloth (and all Xenarthran) phylogenetics based on morphology is a mess. It is not really clear what they are. This is not really unique problem as it is still not entirely clear what the primates are most closely related to either. Another problem with sloths is that they appear to have diversified rapidly over a relatively short time period while filling similar ecological niches in different places i.e. Antillian sloths, South American sloths etc. which makes finding phylogenetically informative characters difficult ...this is also a problem for DNA based analysis.
For two morphology based analyses of sloth phylogeny there are McKenna, M.C. and Bell, S.K. (1997) "Classification of Mammals above the Species Level", Columbia University Press, New York
and Gaudin, T.J. (1995) The ear region of edentates and the pyhlogeny of the tardigrada (mammalia, xenarthra). J. Vertebr. Peleo. 15: 672-705.
One more general reference is
Engelmann, G.F. (1985) The phylogeny of Xenarthra. in "The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas" (G.G. Montgomery, Ed.) pp. 51-64, Smithsonian Institution, Washington, D.C.