…and this is a 
How do we distinguish bacterial species? Cohan shows us some nice diagrams of phenotypic and molecular clusters, and they show groups separated by gaps — therefore, species. Unfortunately the species defined thereby are big and contain considerable diversity within them. Darwin defined species as divergent forms between which one finds morphological gaps. Mayr: cohesive set of organisms whose divergence is constrained by genetic exchange. Speciation requires a breakdown of that exchange.
Mallet has developed a version of Darwin's species definition that includes molecular characters. Under Mayr, speciation is tough, under Mallet, speciation is easy. The two models differ in the frequency of cladogenesis.
How do bacterial species maintain cohesion? Periodic selection purges divergent populations. Diversity within ecotypes is maintained by selective purges, but ecotypes that found new populations in new environments will not face the same selective effects.
Why doesn't the free(er) exchange of genetic material between bacterial species lead to a convergence or fusion of species? One reason is the rarity of genetic exchange. If two ecotypes have a suite of niche-specifying genes, low frequency of interchange will not be sufficient to prevent divergence.This does not prevent free exchange of niche-transcending genes, genes that are useful in different environments.
Lots of details from Cohan's work followed, and I confess to being a bit lost in places. He's looking at different soil bacteria that are found in different ecotypes—for instance, having different characteristic fatty acid content depending on whether they are found on a north-facing or south-facing slope. He argues that speciation is easy because genetic exchange doesn't prevent speciation. They're working on finding and confirming ecotypes with whole genome sequencing.
There is cohesion with local populations in one niche, but there is also niche-specifying divergence that is in defiance of cohesion. In animals and plants, niche-transcending genes are only shared between close relatives; in bacteria, they can be shared by the most distant relatives. This sharing doesn't interfere with divergence.
Life Science
Comments
Posted by: Cujo359 | October 30, 2009 3:36 PM
Is "Mayr" Ernst Mayr? I hadn't realized he'd contributed to our understanding of what a species represents, although I suppose I shouldn't be surprised.
Posted by: John S. Wilkins
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October 30, 2009 3:58 PM
My own views on prokaryotic species is here, FWIW.
Posted by: msw13 | October 30, 2009 4:15 PM
Bacterial speciation is a fascinating topic...and it may be that the term 'species' DOESN'T apply to microorganisms. Even if we use a genetic species definition along the lines of 'a group of organisms sharing homologous genes that are constrained from divergence' the usefulness of that definition presumes that microorganisms are in fact constrained from diverging. It may well be that the vaguely coherent groups we observe now are due to limited sample sizes...remember that soils contain a billion cells a gram, the ocean a million cells a ml...better sampling might reveal a more continuous range without any apparent gaps.
And if microorganisms ARE in fact constrained from diverging, it may have more to do with continual genetic exchange and periodic selection than anything to do with niche theory.
You macro-biologists fall easily into niche theory viewpoints, but.....if most microorganisms in nature are dormant and possess enormous phenotypic plasticity, then what niche are they occupying while they are persisting off saved resources until some exploitable resource in their broad range of possible C and energy sources come along?
The issue is genuine and we don't have a clear answer...half the genes in a pathogenic E. coli weren't even in our lab rat E. coli K12...but no one has suggested that they aren't both E.coli (b/c they are Gram negative, lactose-fermenting to gas and acid, etc.) Contrast this with humans and the essentially identical chimps (not even in the same genus to you macro types).
Perhaps not the place for this, but I'd like to lament the NSF decision to lump together microbial biology programs with non-microbial programs. While we have a lot to learn from each other, I really think that most macro types have difficulty evaluating microbial proposals given their macro mindset and the very real differences between the biology of these groups.
Posted by: msw13 | October 30, 2009 4:39 PM
The Wilkins article linked to in #3 is very good - thanks.
Posted by: David Marjanović, OM | October 30, 2009 9:13 PM
Yes.
He formalized and pushed one of the two Biological Species Concepts, the one where organisms belong to different species if they can't interbreed.
This concept is not applicable to asexual organisms. Mayr realized this, and said explicitly that such organisms "do not form species". Unfortunately, the codes of nomenclature require us to pretend they do anyway.
Posted by: Necubi | October 30, 2009 9:36 PM
It's exciting to read Pharyngula and see a talk given by one of my professors! In response to those that argue that the species concept does not apply to asexual, horizontal gene-transferring organisms like bacteria, I will attempt to summarize Professor Cohan's thoughts on the topic (which perhaps is what he presented here). Essentially, he argues that a species of bacteria is distinct from another when a periodic selection event (when one particularly fit bacterium reproduces and outcompetes everything else) occurs in a species of bacteria, if another population is unaffected (i.e., their genetic diversity remains intact) then it is a separate species.
Posted by: David Marjanović, OM | October 31, 2009 9:24 AM
As of February 2009, there are 147 species concepts out there. I hope Cohan's is already counted among these :-)