|
Author
|
Topic: Endosymbiosis and science
|
Mike Gene
Member
Member # 149
|
posted 27. November 2002 13:39
To prevent the minimal genome thread from being frayed, let me start a new thread concerning the endosymbiont hypothesis.
Yersinia: For example, you didn't mention the fact that the similarities aren't just any old similarities, but some very particular ones that are only expected if mitochondria did indeed descend from bacteria..... C'mon, the mitochondrial sequences could have been virtually anything, and yet they just happened to nest within those of a particular bacterial subgroup? Or perhaps your hypothesized designer was trying to fool us?
This, in a nutshell, is the essence of the argument for the endosymbiotic origin of mitochondria - mitochondria look like they share a common ancestor with bacteria. And I happen to be convinced by it. Nevertheless, there are some important lessons to be learned from this example. I think they are important because some have a tendency to engage in a mild form of revisionism when arguing for the neo-Darwinian model of evolution, where they portray the model as having provided more focus and output that it really has. The history behind the endosymbiotic theory is actually more complicated.
The first thing to note is that the neo-Darwinian model of evolution did not predict the origin of these organelles by endosymbiosis. When the endosymbiosis hypothesis was first proposed by Wallin in the 1920s, it was essentially dismissed as something as too "fantastic"to be discussed in "polite biological society." When Margulis resurrected this hypothesis, some traditional evolutionary biologists (Uzzell and Spolsky) actually labeled it as something akin to a revival of "special creation." Just as Darwinists originally resisted lateral transfer, so too did they resist endosymbiosis.
Nevertheless, what was Wallin's evidence. According to Margulis, his evidence "was based on the size, shape, staining properties and general cytological behavior or the organelles which he claimed were comparable to bacteria." In other words, Wallin drew up a list of similarities which seems awfully much like some form of argument from analogy.
Yet it is noteworthy that Wallin did not (AFAIK) predict that mitochondria should thus possess bacterial genetic factors (at this time, no one knew that DNA was the genetic material). When Margulis revived Wallin's hypothesis, she pointed to the newly discovered facts that indicated mitochondria had their own DNA and the mitochondria replicated rather than formed de novo. This extended the bacteria-like appearance of mitochondria, making it look like bacteria and mitochondria are related. Yet I cannot find where Margulis actually predicted this DNA sequence would nest with bacteria, let alone some specific bacterial subgroup.
The history behind the endosymbiotic hypothesis is quite complicated. First of all, Margulis did not revive the hypothesis to account for two eukaryotic organelles, but for the origin of eukarya itself. Her first paper, "Origin of Mitosing Cells" makes it clear that the lion's share of her attention was devoted to the centrioles, where she even went so far to argue that centromeres were of bacterial origin. Suffice it to say that the main thrust of her hypothesis has been rejected by mainstream science, where endosymbiosis currently accounts for the origin of mitochondria and chloroplasts, not eukarya (although some now favor a symbiosis these between archaea and eubacteria). Furthermore, classical evidence, cited by Wallin, is also suspect. For example, while mitochondria of higher organisms may appear similar in shape and size to bacteria, this is often not true among protozoa.
Furthermore, Margulis' offered a mismash of predictions early on, some validated, some not. For example, Margulis predicted corroboration of the main part of her thesis - the centrioles/MTOC were of bacterial origin. She also erroneously predicted that we would be able to culture mitochondria and that tubulin sequence would be homologous to the flagellar sequence of bacteria. On the other hand, she successfully predicted that we would better demonstrate gene transfer from organelles to nuclei.
I mention all this not in any attempt to discredit Margulis and her hypothesis, but to illustrate that some have a tendency to forget the failed predictions and remember the successful ones. In other words, in terms of predictive abilities alone, both the Darwinian model and endosymbiotic theory have not been very successful. The simple observations of similarity have done most of the work, where with increased resolution, the similarities not only remain, but become more impressive. For example, when Nic quotes, " In R. americana and J. libera, gene clusters are found that resemble, to an unprecedented degree, the contiguous ribosomal protein operons str, S10, spc, and alpha of eubacteria. In addition, their mtDNAs code for an RNase P RNA that displays all the elements of a bacterial minimum consensus structure.," we need to remember that this is not some type of verified prediction.
Finally, there is something odd about the whole thesis. Nic quotes:
quote:
Gene repertoire and gene order comparisons as well as multiple-gene phylogenies support the view of a single endosymbiotic origin of mitochondria, whose closest extant relatives are Rickettsia-type alpha-Proteobacteria.
Yet as Nic notes, "As if this weren't enough, there are cases where the mutualistic association of bacteria inside the cells of eukaryotes has been observed to occur and persist in the lab."
Questions
1. Since endosymbiosis is a common phenomena, how does this square with a "single endosymbiotic origin of mitochondria?" It would seem to me that a polyphyletic origin follows more naturally from the basic tenets of the serial endosymbiosis thesis.
2. Why do mitochondrial genes nest with a "Rickettsia-type alpha-Proteobacteria", since such bacteria are crown groups?
IP: Logged
|
|
Cre8ionist
Member
Member # 140
|
posted 03. December 2002 08:11
I submit to Mike's view on the likelihood of the bacterial origin of mitochondria, while still maintaining my original position.
Through my admittedly creationist lens, I can see a designer wiring the two together easier than I can see nature doing the wiring. There does seem to be some ID research lurking here though. However, I'm not the most qualified to do it, perhaps someone else already knows the answers to the following questions?
Does the wiring involved, e.g., the host's partial control, rise to the level of being considered new specified complexity? How much information does the host control? Do we know from whence the information sprang? After all, if we find a Ford engine in a Chevy truck, we could conclude a certain amount of intelligence was required to connect the two, even though we could trace the engine to another source.
In the end, I would have to say that if there is new specified complexity associated with the connection of the two then you should invoke design, if not, then it's more of a lateral transfer, and we know that's not evidence against ID, for horizontal transfer certainly does not meet the criterion set by Dembski.
Hope that's not too short to be clear, but no time to elaborate.....
Mike, I enjoy reading your work.............................Cre8
[ 03. December 2002, 17:16: Message edited by: Cre8ionist ]
IP: Logged
|
|
|
Printer-friendly view of this topic