Why Do Bacteria Stay Bacteria in Evolution?

[D H]

Bacteria have two advantages over eukaryotes. They reproduce quickly. Quickly as in lightning fast: Bacteria populations can double in ten minutes under the right circumstances. The other advantage is sloppy inheritance. Sloppy as in where the heck did that gene come from? While bacteria reproduce asexually, they suffer (and benefit from) a much higher mutation rate than do eukaryotes. There is also a catch, as in catching genes from completely unrelated bacteria. This lateral gene transfer means drug resistance developed in one kind of bacteria can spread to an unrelated kind of bacteria.

[jimmydasaint]

Thanks for the reply RE. My question came from thoughts on the sheer power of mutation in bacteria and a multicellular organism like a fly and I wondered why they have stayed as they are for so long. You have partially answered ny question by saying that the original community have also survived after natural selection. I am aware of allopatric and sympatric speciation and it makes sense. However, something else comes to mind. If wolves are ancestors of dogs, does it mean that dogs are unable to breed with wolves? Because if the can breed then this is not classical speciation is it?

[ecoli]

Thanks for the reply RE. My question came from thoughts on the sheer power of mutation in bacteria and a multicellular organism like a fly and I wondered why they have stayed as they are for so long. You have partially answered ny question by saying that the original community have also survived after natural selection. I am aware of allopatric and sympatric speciation and it makes sense. However, something else comes to mind. If wolves are ancestors of dogs, does it mean that dogs are unable to breed with wolves? Because if the can breed then this is not classical speciation is it?

Humans can force breed and control the selection pressures (and thus the evolution of) organisms.

 

Actually, dogs aren't a great example of what you're talking about, since dogs, taxonomically, are a subspecies of wolves. http://en.wikipedia.org/wiki/Wolfdog

 

However, just because two animals can interbreed doesn't necessarily mean they're the same species. They have to be able to produce viable offspring as a result of that reproduction. And, they would have to willingly mate with each other. So, human intervention wouldn't really count here.

[D H]

this argument is implicitly of the form "if humans evolved from apes, then why are there still apes?"

I agree. Evolution is inherently a local phenomenon, localized in time and in environment. Evolution is localized in time: Evolution has no long-term goals. Evolution occurs when an organism with some mutation has offspring and passes that mutation along. Evolution doesn't foresee that humans are a "better" lifeform than plain old vanilla kinds of ape and adjust the ape genome accordingly. Anthropomorphizing evolution in this way endows evolution with exactly the characteristics claimed by cdesign propentists. Evolution does not have a plan.

 

Evolution is localized in environment: Just because a biological change does occur and take hold does not mean it spreads like a contagion everywhere. Suppose a beneficial mutation does occur in one particular place in a species that occupies a wide range. That change does not magically teleport to all members of the species throughout its range. That change might not even be beneficial everywhere. While the change has created a new species, that does not mean the old species has to die.

 

In closing, "It is said most people in the Americas are descended from Europeans. Why are there still Europeans?"

 

==============================

 

Now, back to the original question: why are there still bacteria? The answer is that the have an ecological niche in which they thrive. When something happened a long time ago that first made the some bacteria become eukaryotes, the whole bacterial world did not follow suit. That would have been magical! Evolution is not magic. The bacterial world remained largely bacterial. This new primitive eukaryotic life form thrived because it had a niche that bacteria didn't occupy. What would happen if this same thing happened today? The descendants of that original primitive eukaryotic life exist and have armed themselves to the hilt. The original eukaryotic life thrived because it found an empty niche. That niche is occupied today. A bacteria that repeated the original feat would have a much worse go at survival than did our original eukaryotic ancestor.

 

Bottom line: Bacteria still exist today because they have a niche in which they reign supreme. OTOH, the evolutionary pathway from bacteria to eukaryote has been closed off precisely because following that path worked so well a long, long time ago.

[halogirl]

If I understand you correctly, are you saying that the level of mutations are causing new species to arise on human bodies? Is there any evidence for this process? Also, if we look far enough back in the evolutionary tree, I think we may find a common ancestor cell but not necessarily a bacterium. Interesting points though...

 

why shouldn't there be any evidence. with all the gunk people put in their bodies, of course the bacteria inside us are changing. they need to adapt to survive, and for bacteria, that means growing, and multiplying, but only those that are built correctly survive, thus it may eventually change into an entirely new bacterium. why shouldn't it,? especially if it is compatible with our bodies.

also as far as evolution goes, if you look back far enough, you will always find a common ancestor of some sort.

 

to: ecoli

im pretty sure Pioneer was just trying to make a point. you can't really complain about someone's veiws on something if you don't know them. i also didn't see anything in their post that suggested they thought evolution was morally wrong, they just didn't consider that we are in symbiosis with bacteria inside us, they don't just serve us, we'd die without them and they without us. i don't really see what your complaining about though.

[ecoli]

to: ecoli

im pretty sure Pioneer was just trying to make a point. you can't really complain about someone's veiws on something if you don't know them. i also didn't see anything in their post that suggested they thought evolution was morally wrong, they just didn't consider that we are in symbiosis with bacteria inside us, they don't just serve us, we'd die without them and they without us. i don't really see what your complaining about though.

 

I can complain about someone's views if these views are based off of fallacious logic, and simple ignorance about the subject.

 

He was being pejorative and sarcastic about humans preventing bacteria from evolving. Even if was being sarcastic in a humorous way, it still served to demonstrate the fact that he was attacking evolution, while being naive about how it actually works.

 

I can attack that condescending approach all day... and will do so.

[Radical Edward]

If I understand you correctly, are you saying that the level of mutations are causing new species to arise on human bodies? Is there any evidence for this process?

 

not bacteria, but pubic lice and head live have certainly speciated. Looking at their genes, we even know roughly when.

[CharonY]

Well, mitochondria are a good example of bacteria that have evolved (in case someone mentioned it already and I overlooked it). Moreover, there are bacteria that have multicellular part of their life-cycle like e.g. Myxococcus xanthus. The problem with long-time bacterial evolution is that they simply do not leave us fossils to recognize changes over long time scales. In theory it should be possible to see the transition from one bacterial species, however it would require the sequencing of single cells, which, while in theory possible, is quite complicated to do.

[halogirl]

I can complain about someone's views if these views are based off of fallacious logic, and simple ignorance about the subject.

 

He was being pejorative and sarcastic about humans preventing bacteria from evolving. Even if was being sarcastic in a humorous way, it still served to demonstrate the fact that he was attacking evolution, while being naive about how it actually works.

 

I can attack that condescending approach all day... and will do so.

 

if you say so, though honestly i didn't pick that up from his reply, even if it was a sucky analogy.

[jimmydasaint]

Well, mitochondria are a good example of bacteria that have evolved (in case someone mentioned it already and I overlooked it). Moreover, there are bacteria that have multicellular part of their life-cycle like e.g. Myxococcus xanthus. The problem with long-time bacterial evolution is that they simply do not leave us fossils to recognize changes over long time scales. In theory it should be possible to see the transition from one bacterial species, however it would require the sequencing of single cells, which, while in theory possible, is quite complicated to do.

 

I find mitochondria fascinating due to their close match to bacteria in terms of protein modifications and loose DNA structure (not packed in chromosomes like human DNA) and plasmids etc... Plants also have chloroplasts which seem bacterial-like in several respects. However, I don't know if I would classify that as an evolutionary event - I would rather treat it as a case of symbiosis with subsequent accommodation.

[lucaspa]

I disagree with this. Doesn't natural selection depend on the choice of the correct set of genes with correct mutations to survive in a certain place? For example, a bacterium living in hot springs would be expected to have genes mutated to help them survive in these hot spots. Evolution is entirely driven by the quality of mutations available.

 

Mutations provide the raw material for selection. What gets selected depends on the environment the population is facing. Bacteria in hot springs do have alleles (forms of genes) that are different from bacteria living elsewhere. But that doesn't mean all those bacteria have to give up those genes. The other alleles do better in cooler environments.

 

Remember, every mutation has a cost as well as a benefit. It depends on the environment if the benefit outweighs the cost. When the cost is higher than the benefit, the older form does better.

 

Most speciation comes about by splitting a population in 2. The "old" population is well-adapted to its niche. It doesn't change. The "new" population faces a different environment with different selection pressures. Those selection presssures dictate change.

 

Read the thread "Three forms of natural selection". You are thinking of natural selection only in terms of directional selection. You are forgetting purifying selection.

 

Long ago one species of bacteria evolved to first a eukaryotic organism. Then one species of eukayote evolved to a multicellular oraganism. That species then kept splitting to give us all the multicellular species we see today. Meanwhile, the niches occupied by bacteria and single celled eukaryotes (like algae or amoeba) were still good ways to earn a living. Natural selection kept those species adapted to those niches -- including being single celled.

 

As someone pointed out, tho, many bacterial species have multiple interactions with other bacteria and often act almost like a multicellular organism. It's an intermediate form on the way to multicellularity. However, that intermediate is still a good way to earn a living.

 

I find mitochondria fascinating due to their close match to bacteria in terms of protein modifications and loose DNA structure (not packed in chromosomes like human DNA) and plasmids etc... Plants also have chloroplasts which seem bacterial-like in several respects. However, I don't know if I would classify that as an evolutionary event - I would rather treat it as a case of symbiosis with subsequent accommodation.

 

It's still evolution -- descent with modification. Mitochondria and chloroplasts are much modified from their bacterial ancestors. One modification is that many of their genes are either in the nucleus or the mitochondria and chloroplasts use the proteins from genes that were originally in the nucleus. As I said -- cost/benefit. The cost of making 2 copies of the genes outweighed the benefit, so modern eukaryotic cells only make one copy that is used in both the cytoplasm and the mitochondria.

 

BTW, there is a thread on "Endosymbiosis" that talks about the fact that mitochondria and chloroplasts are the result of bacteria becoming parasites, then symbionts, and then endosymbionts.

[Radical Edward]

I find mitochondria fascinating due to their close match to bacteria in terms of protein modifications and loose DNA structure (not packed in chromosomes like human DNA) and plasmids etc... Plants also have chloroplasts which seem bacterial-like in several respects. However, I don't know if I would classify that as an evolutionary event - I would rather treat it as a case of symbiosis with subsequent accommodation.

 

well they have evolved to live in the cell.

[CharonY]

However, I don't know if I would classify that as an evolutionary event

 

well they have evolved to live in the cell.

 

Technically just living in a different habitat does not constitute evolution, however the mitochondria have undergone significant genomic changes (basically reducing almost all of its genome, partially including eukaryotic genes).

And this is of course, evolution. But of course, evolution in bacteria is even more complicated to trace than in higher eukaryotes.

[lucaspa]

Technically just living in a different habitat does not constitute evolution, however the mitochondria have undergone significant genomic changes (basically reducing almost all of its genome, partially including eukaryotic genes).

 

Right. Lots of modifications in that "descent" from the original bacteria. And, of course, modifications in the eukaryotic cell.

 

But of course, evolution in bacteria is even more complicated to trace than in higher eukaryotes.

 

Due to lateral gene transfer. Bacteria get genes not only from inheritance but via plasmids from quite unrelated unicellular organisms. Even across Kingdom lines: Archea to bacteria, for instance.

[CharonY]

Yes, that too. Although I think you mean Domain instead of Kingdom? Of course transfers (not only of plasmids) is more often within a domain. What I also meant is the fact that especially in prokaryotes it is problematic to trace evolutionary units also because there is no real species definition (besides arbitrary parameters). Genetic changes and thus evolution within a clonal strain can be (more or less) easily monitored under lab conditions, but trying to try the same in nature is extremely problematic.

[jimmydasaint]

Just to add to the points raised here, this information suggests:

that the origin of the mitochondrion was coincident with, and contributed substantially to, the origin of the nuclear genome of the eukaryotic cell. Defining more precisely the alpha-proteobacterial ancestry of the mitochondrial genome, and the contribution of the endosymbiotic event to the nuclear genome, will be essential for a full understanding of the origin and evolution of the eukaryotic cell as a whole.

http://www.ncbi.nlm.nih.gov/pubmed/10690412?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&linkpos=4&log$=relatedarticles&logdbfrom=pubmed

 

In other words, mitochondria helped in the evolution of the host cell - Science is wierd!

[lucaspa]

Yes, that too. Although I think you mean Domain instead of Kingdom?

 

Yes. Archaea and Bacteria are 2 Domains.

 

What I also meant is the fact that especially in prokaryotes it is problematic to trace evolutionary units also because there is no real species definition (besides arbitrary parameters).

 

That's not so much a problem. Remember, there is no precise definition of species for any group because one species transforms to another. IOW, because evolution is true any definition of species (ability to interbreed, morphology, etc.) is always going to have populations that are "in-between". The morphological species definition is no more arbitrary than genetic. As long as you have lineal descent of genes from ancestor to descendent, then tracing a phylogenetic tree is possible.

 

But when genes are introduced from other distinct lineages thru lateral gene transfer, then you can't trace ancestor-descendent. Because the new gene is not a modification from the ancestor; it appears within a single generation as it is transferred from a completely different evolutionary lineage. Poof! the gene is just there! And has no relationship to any of the existing genes within the organism.

 

So, as phylogenetic analysis gets back closer to the last common ancestor of all life, the analyses tend to break down and show many possible candidates for the LCA.

 

Genetic changes and thus evolution within a clonal strain can be (more or less) easily monitored under lab conditions,

 

Right, but that is because your clonal strain doesn't have any contact with any other unicellular organism to get plasmids from. If the culture dish gets contaminated with another microoganism, you throw it out!

[CharonY]

That lack of distinct species concept makes it so hard to trace horizontal gene transfer. If you can assume a core genome of a species then one can trace what might be acquired horizontally. However if the whole genome is not really defined it makes phylogenetic analyses rather tricky. For instance, if an individual of a bacterial species acquire a megaplasmid, increasing the whole genome by, say, a third. How would one classify it in relation to other members of the species, without it? If we simply disregard that there are from the same species, whole genome analysis could place it quite differently. However if we assume that there is a certain core, belonging to a given species (which 16s analyses simply assume) then it would be classified quite differently.