Now that inheritance of acquired genomes seems to be corroborated by a number of studies, at least in tiny critters, the question opens up, is it time to un-taint the Lamarckian heresy?
After all, acquiring new genomes happens in adult organisms. And the heritable event is something acquired in the lifetime of a particular organism, then passed on to offspring...
I am going off Acquiring Genomes by Margulis and Sagan.
Margulis says: "Microbes are champions at passing their DNA to others in the form of entire functional genes. These machinations underlie the story of darwinian evolution. Microbes living on their own, under conditions of stress and deprivation, tend to merge with other forms of life, Some of these associations last for a season or less, but occasionally microbes and larger life forms fuse permanently. Lamarck was correct: Acquired traits can be inherited not as traits but as genomes." She then adds: "But Lamarck, and Darwin with his pangenesis idea, were both wrong when they suggested acquisition might be the fate of any characteristic."
Ernst Mayr wrote the intro to her book and he thinks that she overstates the case. (Huh? He just published another book! Isn't he like 100 years old by now? Maybe he stole, um, acquired some of the bacteria's immortal genes.... )
Well, as for Lamarck, I mean it in the more, uh, generous sense of saying that somehow it must be possible to inherit some of what we go thru as individuals. After all, let's give him some credit for the hunch, even if the specs he envisioned were wrong!
This is not restricted to microbes. If you are infected with a virus (usually retroviruses), and your germline is infected, you can have the viral genome integrate into your own as a provirus. You will then pass this provirus on to your children, i.e. you have acquired a trait (or several since most retroviruses have a gag, pol, and env gene and some like HIV have several others as well) you were not born with and pass it on to your offspring. 8% of the human genome is composed of the results of such infection events though each one is not due to a novel infection from an exogenous retrovirus. Defective proviruses can retrotranspose themselves around the genome without forming exogenous,infectious retroviruses...they are so called human endogenous retroviruses or HERVs.
The actual Lamarckism had two forces: 1. An intrinsic force which tended to increase the complexity of body plans "upward", and 2. The diversification of these increasingly complex body plans by the inheritance of aquired characteristics and the principle of use and disuse.
The first force isn't really discussed much in regular biology for some reason. But just going by the second force, I still don't think it's time to apologize to Larmack. The large majority of his explanations stressed the principle of use and disuse.
If we are just to go by "the inheritance of aquired characteristics", then Lamarck was right in the sense that this happens. As Mammuthus said, basically anything that affects your germ line will be considered the inheritance of aquired characteristics (if the genome is included). But I don't think it can be considered the major, or even a major, driving force of evolution as Lamarck had in mind.
[This message has been edited by JustinCy, 03-01-2004]
There are three ways in which bacteria can gain genetic material that are not related to mutation: transformation, tranduction and conjugation. Transformation – when bits of dead bacterial DNA are taken up by others directly from the environment; transduction – when a bateriophage “accidently” packages a donor cell’s DNA (rather than its own) and transfers it to a new cell; and conjugation, which is direct transfer of genetic material (usually on plasmids) from one bacteria to another.
Bacteria and other single celled beasties can be pretty promiscuous. Both transformation and especially conjugation can literally create a new clonal lineage with one or more completely novel characteristics in one go. In essence, it IS the “inheritance of acquired characteristics” a la Lamarck. It’s sort of like if a seagull was able to pass genetic material directly to a mouse, and create a flying, feathered rodent in one generation.
Where Margulis is overstating the case is her claim that:
a) this falsifies or calls into question neodarwinism (i.e., role of mutation and ns in evolution): It doesn’t – bacteria and their ilk are something of a special case in a lot of ways, not least of which is the difficulty in identifying “species” because of the lack of reproductive barriers and the fact that they are clonal. It has been recognized for quite a while that bacteria and the biological species concept don’t mix. This tends to be one of Margulis’ favorite strawman arguments. OTOH, it does have substantial implications for the very roots of the “mangrove of life” and may render moot the search for the elusive LUCA – not because this hypothetical organism doesn’t exist, but rather its trail may be impossible to untangle from the web of bacterial interactions.
b) there is sufficient evidence that genomic fusion between single-celled critters and multicellular critters has had a significant impact on evolution. This is an area where there is outstanding room for future research. The role of retroviral insertions (as Mammuthus mentioned) for instance and their effect in altering the genotype or phenotype of a complex organisms is relatively undeveloped – although there’s a good bit of work on-going. I think it’s too early to state what role they play – Mammuthus probably has a zillion papers under his thumb at this very moment… OTOH, even if there is a role, IMO it would be merely in supplementing random mutation in creating genetic novelty, and would not refute or replace the critical role of natural selection.
A related area that I find particularly interesting – and with more empirical support that what Margulis is proposing, but still a very new area of research - is the role of parasitism in driving evolution. I know you like well-written popsci books, and if you’re interested in a contrast to Margulis, I think you’d find Carl Zimmer’s “Parasite Rex” (Touchstone/Simon & Schuster, 2001) fascinating. (*Q thus pays Tamara back for forcing him to buy “Acquiring Genomes”).
Are you trying to demonstrate that bacteria are not fully concerned by evolutionary principles debated by both Darwin and Lamark because of asexual & clone reproductive behaviors? I know there are numerous problems concerning the systematic classification of bacteria. I wonder if easier to elaborate the phylogenetic resolution (designing common descent) more safe should be our investigation of a "potential unified mechanism". Rules seem to be different for bacteria and metazoan. If researchers finally solve the bacteria systematic, would this have an important impact on metazoan evolutionary theories?
Are you trying to demonstrate that bacteria are not fully concerned by evolutionary principles debated by both Darwin and Lamark because of asexual & clone reproductive behaviors?
I'm not a microbiologist, so I may be a bit off here. However, I think the point I was trying to get across is that the part of Lamarck's theories concerning "inheritance of acquired characteristics" is - at least by analogy - more or less what can happen in clonal (whether unicellular or metazoan) lineages. I don't consider it a violation of neodarwinism. It's still natural selection and descent with modification. It messes up the biological species concept is all, and most microbiologists I've talked to simply state that the BSE doesn't apply at the unicellular level (i.e., begging the question in a way ). Since the BSE isn't really much more than a very handy "rule of thumb" anyway, I don't think clonal lineages pose any problem for evolutionary theory. Clones simply have some different reproduction options.
I know there are numerous problems concerning the systematic classification of bacteria. I wonder if easier to elaborate the phylogenetic resolution (designing common descent) more safe should be our investigation of a "potential unified mechanism".
My understanding is that bacterial lineages for instance are classified by percent relatedness. If DNA analysis shows X% difference (I can't remember what the percent is, but I want to say <75% similarity which is probably wrong), then the organisms are classified as different "species", although lineage is probably a better term for it.
As to "potential unified mechanism", if I understand what you're suggesting, I would think this would be the "holy grail" of biology. It would be an incredible breakthrough - along the lines of Einstein's relativity theory - if such a unified theory could be developed. Way more brilliant minds than mine (not saying much here) have pondered the problem, promulgated fascinating theories, and come up short when the details were examined. I think the problem derives from the "nature of the beast". Organisms simply don't lend themselves to simple rules. I mean, in physics, an electron is an electron is an electron whether it's part of a hydrogen atom or a nobelium atom. It's one example of an identical class of components whose properties are identical. Compare that to an individual biological organism - each one is unique and different from all others, even if it's a member of the same population or species. Generalizing on something like that is, hmmm, problematic in my opinion.
Rules seem to be different for bacteria and metazoan. If researchers finally solve the bacteria systematic, would this have an important impact on metazoan evolutionary theories?
I don't really think the rules are different. I think it would be better said that bacteria have some additional rules and complications that need to be taken into consideration. As to your question, I think the significant impact would be on the very root of the tree of life, not on the course of evolution per se. Unless Margulis for instance is shown to be substantially correct, and symbiosis is shown to be a more important force than natural selection and competition in the evolution of the majority of organisms - which would cause us some major re-thinking of how diversity and evolution played out.
Lucky I am, you still are standing on a general (comprehensible) knowledge layer for your answering. I'm seeking for some Margulis's PDF papers.
Your arguments gave me an idea.
Briefly, perhaps the first metazoan was a viable hybridation of two different prokaryote species, formerly symbiotic and each specialized in two complementary inorganic recycling. For an unknown reason, genetic material was mixed (viruses, solar radiance?) and so the genetic discrepancy of this artificial hybrid was both (1) so important that not allowed any more further separate division and (2) rather than disintegrate (should have been expected) the duo multiplied itself with the same genetic alteration.
Of course this is speculation. The main assumption is that both organisms had a very simple genetic material and each of them was together in a lucky period.
However, I think that it will be quite interesting to dig more deeply in the "association" field. These phenomena probably played important (crucial) roles in evolution. Still much to learn before receiving a "Prix Nobel". Humour.
This is the elusive "first organism" - not first replicator - from which all cellular organisms on the planet descend. Our most ancient ancestor. Here's a nice on-line article which reviews the topic: My Name Is LUCA. BTW, you might enjoy that website a lot. Actionbioscience.org is a cross between pop sci and peer-reviewed, and contains articles and essays by many of the "big names" in biology including Eldredge, Simberloff, Wilson, etc.
You're probably asking the wrong guy. If you want my opinion - then yes, I think the ultimate root is more like a bush than a tree. Or, if you want to extend the tree analogy, it's like a tree with a whole root system.
Here are a couple of articles that might help explain why I think that:
Gogarten JP, Doolittle WF, Lawrence JG, 2002, "Prokaryotic Evolution in Light of Gene Transfer", MolBioEvo 19:2226-2238
quote:Accumulating prokaryotic gene and genome sequences reveal that the exchange of genetic information through both homology-dependent recombination and horizontal (lateral) gene transfer (HGT) is far more important, in quantity and quality, than hitherto imagined. The traditional view, that prokaryotic evolution can be understood primarily in terms of clonal divergence and periodic selection, must be augmented to embrace gene exchange as a creative force, itself responsible for much of the pattern of similarities and differences we see between prokaryotic microbes. Rather than replacing periodic selection on genetic diversity, gene loss, and other chromosomal alterations as important players in adaptive evolution, gene exchange acts in concert with these processes to provide a rich explanatory paradigm—some of whose implications we explore here. In particular, we discuss (1) the role of recombination and HGT in giving phenotypic "coherence" to prokaryotic taxa at all levels of inclusiveness, (2) the implications of these processes for the reconstruction and meaning of "phylogeny," and (3) new views of prokaryotic adaptation and diversification based on gene acquisition and exchange.
And of course Carl Woese's work (who's one of the first proponents of the theory, along with Gould), like Woese C, 2000, "Interpreting the universal phylogenetic tree", PNAS 97:8392-8396.
quote:The universal phylogenetic tree not only spans all extant life, but its root and earliest branchings represent stages in the evolutionary process before modern cell types had come into being. The evolution of the cell is an interplay between vertically derived and horizontally acquired variation. Primitive cellular entities were necessarily simpler and more modular in design than are modern cells. Consequently, horizontal gene transfer early on was pervasive, dominating the evolutionary dynamic. The root of the universal phylogenetic tree represents the first stage in cellular evolution when the evolving cell became sufficiently integrated and stable to the erosive effects of horizontal gene transfer that true organismal lineages could exist.
It probably looks more like this:
Or maybe, this:
That's my opinion, for what it's worth. Drat those pesky prokaryotes and their failure to develop cam corders.