A mechanism for Punctuated Equilibrium? (re: evolution)

Great Debates
1

First, let me explain to those who may not be aware of the term, what Punctuated Equilibrium means. It is a theory of evolution that was put forth by Niles Eldredge and Stephen Jay Gould in 1972. In essence, the theory was put forth in contrast to Darwin’s characterization of evolution as a slow, continuous process. From the web site linked to above, punctuated equilibrium describes evolution as being characterized as “long periods of virtual standstill (equilibrium), punctuated by episodes of very fast development of new forms.” “Instead of a slow, continuous progression, the evolution of life on Earth seems more like the life of a soldier: long periods of boredom interrupted by rare moments of terror.”

Here is some more information about paleontology’s “dirty little trade secret”, namely that a "few supposed examples of gradual evolution were featured in the journals and textbooks, but paleontologists had long been mum about [the fact that] most species appear suddenly in the fossil record and show no appreciable change for millions of years until their extinction.

Now, from Science News (June 22, 2002. Vol. 161. pp 394-396), I read about something that could perhaps be the underlying mechanism for punctuated equilibrium. [note that while I can link to the Science News site, a link directly to the article I’m referring to will not work because it requires the user to be a subscriber.]

The gist of the article is that they’ve “discovered” a molecule called Hsp90 (heat shock protein 90) that seems to mitigate the impact of mutations. Essentially concealing the mutation. However, when stressed during an organism’s development, Hsp90 does not function properly and the mutation expresses itself. And the mutation is passed on to future generations. The researchers claim that it is “the first evidence for an explicit molecular mechanism that assists the process of evolutionary change in response to the environment”.

I became excited when I understood the possible connection to punctuated equilibrium.
Equilibrium occurs when the environment remains relatively stable, and Hsp90 does its thing.
Punctuation occurs when the environment becomes stressed, Hsp90 becomes ineffective, allowing the expression of mutations.

Sounds pretty cool to me. Opinions?

Rather than try to fully restate the article, here are just a few excerpted paragraphs from the Science News article.

"Two provocative studies, one published in 1998 on fruit flies and one this year on plants, suggest that organisms developing under stressful conditions can unleash novel mutant forms. In doing so, some biologists speculate, some offspring may quickly adapt to new environmental conditions and improve their species’ odds of survival. "

"This isn’t comic book science. The studies have thrown the spotlight on a molecule called heat shock protein 90 (Hsp90) that’s central to the stress-induced effects. Susan Lindquist of the Whitehead Institute of Biomedical Research in Cambridge, Mass., who coauthored the two studies, contends that when Hsp90 activity is disrupted during development, genetic variation that’s normally concealed can emerge and generate diverse physical forms of a species. "

"Evolutionary plant biologist David Baum of the University of Wisconsin–Madison suspects that Hsp90 doesn’t stand alone in this surprising story. He says the Hsp90 findings are “profoundly important” because they show that hidden genetic diversity exists within species and can erupt when conditions change. "

"The two biologists considered several explanations for their observations. In the scenario they ultimately favored, Hsp90’s role as a chaperone normally suppresses the disruptive influence of mutations in genes crucial to an animal’s development. Hsp90 could do this by folding slightly mutated versions of its clients into their normal shapes, for example. "
“By breeding the Hsp90-deficient flies, the researchers showed that individual abnormalities are inherited consistently from generation to generation. That, they said, supports the theory that the defects are caused by mutations freed of some Hsp90 control, rather than by a random mishap triggered by reduced Hsp90 activity at some point during a fly’s development.”

"At the end of their 1998 paper on the fly research, Rutherford and Lindquist offered a provocative conclusion. “We have provided what is, to our knowledge, the first evidence for an explicit molecular mechanism that assists the process of evolutionary change in response to the environment,” they said. "
“The biologists raised eyebrows even more by suggesting that this Hsp90-based evolution mechanism could help explain the rapid bursts of body-plan changes that occasionally punctuate the fossil record. Most evolutionary theory predicts small, gradual changes in a species over long periods and is hard-pressed to explain such morphological explosions.”

2

It’s interesting, to say the least. I’m gonna wait and see what those more educated in biology have to say.

3

Punctuated equilibrium is a theory whose central premise is that species arise, not from anagenic change (Darwin’s slow, inexorable progress of change), but from splitting. Prior to Eldredge and Gould’s formulation, it was widely thought that it was the anagenic change, rather than the cladogenic (the formation of species by splitting) changes, which resulted in the formation of trends and novelties; speciation in general was thought to be little more than a means of increasing overall diversity, but was not granted any “creativity”.

It is important to realize that punctuated equilibrium does not propse any novel mechanism for speciation (such as that propsed by Darwin in Origin), only that speciation occurs by splitting in the vast majority of cases, and that this is what results in the appearance of the fossil record: rather than representing a deficiency of forms (as must be the case if one accepts gradualism), it represents discrete instances of speciation. Such speciation events occur very rapidly, in a geological sense.

Since punctuated equilibrium still draws upon natural selection as the ultimate mechanism, it becomes evident that the “heat shock” genes do not, in fact, propose a mechanism for punctuated equilibrium. What they do represent, perhaps, is a mechanism for increasing variation, the pool upon which natural selection acts.

4

Hmmm… I think the above might make a bit more sense with the following alterations:

…[S]peciation in general was thought to be little more than a means of increasing overall diversity, and was, consequently, not granted any “creativity”.
[…]
Such speciation events occur very rapidly, in a geological sense, thus creating the illusion of a “sudden” appearence (and, in most cases, an equally sudden disappearance).

5

Algernon wrote:

This sounds eerily similar to a discovery made with E. Coli bacteria about a decade ago.

Microbiologists found that when the E. Coli were subjected to environmental stresses (e.g. being only fed a form of sugar that they couldn’t digest well), the number of mutations that occurred during mitosis (or at least the number that the researchers counted) went way up. This resulted in subsequent generations of E. Coli with much more diverse genomes, a few of whom were lucky enough to be able to digest their new-and-only food source better and thus thrive in what was otherwise a hostile environment.

If Hsp90 is present in single-celled bacteria such as E. Coli, it would lend a new possible twist to the above E. Coli stress experiment: It may have been that the E. Coli weren’t actually mutating more when exposed to environmental stress, it may have been that their mutations were simply expressed more of the time.

Is Hsp90 found in bacteria, and if so, does it serve the same mutation-suppressing function in them that it does in multicellular creatures?

6

I have worked extensively both with so-called adaptive mutation in E. coli and with fruit flies. They are fundamentally different.

This property of HsP90 is only masking genetic variability already present in a population. It is not causing a population to necessarily “evolve” faster. Sure, it can let a population expand into a new niche quickly. Let’s say a subpopulation of fruit flies moves into a warmer area. The Hsp90 thing allows heat-resistance polymorphisms to become unmasked, and the subpopulation is now more viable in a new niche. The subpopulation has no new genetic changes from the old population, though. I suppose on some levels, one could argue that this is evolution because it is a significant variation of allelic frequencies – at some level you are partially isogenizing a population by quickly eliminating all of the unfit alleles. But where evolution is many small steps, this unmasking property allows only one step until natural mutation can build up the number of silent polymorphisms in the population again. Again, evolution is not my strong point.

Adaptive mutation goes by a number of names in E. coli – stressful lifestyle associated mutation, stationary phase mutation, etc. It is very important to remember that it has never been noticed outside of bacteria. It is a specific adaptive mechanism by which the cell becomes mutator[sup]+[/sup] – the number of mutations go way up – while the cell is non-mitotic in stationary phase. DNA is constantly being damaged, which the cell repairs with a set of repair tools. Most DNA mutations normally happen when DNA is being replicated quickly during mitosis. Stationary phase mutation happens when the cell can’t grow anymore, when it is in stationary phase. The normal DNA repair enzymes are actually turned down, and this in turns allows the spontaneously occuring mutations in the genome to pile up.

Stationary phase mutations allow a cell in a non-optimal situation to generate a bunch of mutations. Some of these may turn out to be useful. Drosophila Hsp90 mutations mask the genetic variability inherent in a population, and only allow these to be unmasked when a population is stressed. The number of mutations is never increased.

7

Lemme just state that other, IMHO more plausible mechanisms for very rapid body shape changes have been proposed.

Ronshaugen M, McGinnis N, McGinnis W. “Hox protein mutation and macroevolution of the insect body plan.” Nature 415:914-7.

They basically showed that a specific loss of function of one domain of one homeotic protein between brine shrimp and fruit flies is responsible for the many legged vs. six legged body plan. This change can happen in one generation.

8

Darwin’s Finch (I so want to refer to you as Darwin, but on principle I’m against artificially shortening one’s username, no matter how appropriate), could you explain in more detail what you mean when you say “splitting” (or more specifically, “cladogenic”)? And if I correctly understand how Hsp90 works, it does not increase variation per se, but only the expression of underlying genetic variation. Isn’t this kind of like a long history of mutations being “collected” during periods of equilibrium in a Pandora-like box, and when Hsp90 stops inhibiting the expression of these mutations (in environments of stress), then Pandora’s box is opened and then natural selection “acts” upon the resultant organisms? Couldn’t this at least partially be responsible for the pattern of evolutionary “punctuation”?

This generates in me the same question I posed to Darwin’s Finch, namely, isn’t this the whole point? That genetic variation is accumulating in all organisms, but is being suppressed by Hsp90. Then one day, the suppression stops and the result is “massive” variety in the resultant organisms. Most of these are not useful mutations of course, but the potential for substantially “better” organisms for the specific environment arising would be greatly enhanced.

It is probably intuitively obvious to all the erudite posters in this thread that while I may have a deep interest, I don’t have deep knowledge. As a consequence I voraciously read your thoughts, and am appreciative of the time you’ve spent outlining your perspectives.

9

I’m unclear how this molecule can supress mutations which have already occurred in any general sense. I’m not doubting anything, just very unclear on this behavior.

10

erislover, here are some additional paragraphs from the Science News article. I don’t know if this explains it to your satisfaction, but it’s all I have.

11

The main thing I have against calling this some kind of accelerated evolution is that it doesn’t accelerate evolution over the long run. The individual steps suppressed by Hsp90 are relatively small – large mutations, neomorphic change, genome rearrangement are all out of the realm of the small point mutation polymorphisms being suppressed.

What this IMHO ensures is that instead of evolution taking millions of tiny steps, Hsp90 allows hundreds of thousands of slightly bigger steps. Or something along that line. Over the long run, I can’t imagine a one-step or two-step adaptive process being all that significant to large evolutionary change.

Unlike adaptive mutation, Hsp90 does nothing to increase the mutation rate, so the amount of new mutations generated is staying constant. Therefore, I can’t see that there would be any significant acceleration of evolution. But again, I don’t deal well with population genetics and evolution.

Correct me if I’m wrong.

12

Certainly: speciation by “splitting” occurs when one population (often termed a “daughter population”) becomes reproductively isolated from its parent population. The two populations then begin to diverge, eventually resulting in the creation (so to speak) of two distinct forms. In contrast, anagenic speciation is the result of a lineage slowly transforming over time, until the “end point” of the lineage is markedly different from the early form.

Cladogenesis, then, is the formation of clades, or groups of organisms descended from a common ancestor. Clades are formed by branching lineages, rather than slow, steady transformation.

Punctuated equilibrium contends that most species form via cladogenesis, and that the process is rapid, in a geological sense. Thus, species appear suddenly in the fossil record. In the meanwhile, between such speciation events, natural selection acts more gradually.

The gradualistic alternative is that species slowly transform from one to another, with occasional splitting to increase the diversity pool. The gaps in the record, then, are attributed to a loss of the multitude of intermediates in the continuum from species A to species B. A sudden appearance is simply the result of not having the immediate precursors present.

**

Increasing genetic expression is not going to result in speciation all by itself. As the Science News article you quoted above indicates, the removal of the heat shock proteins allows variation to increase dramatically, but the net effect is that there are now more options for natural selection to choose from. In the end, some of these new mutations will pass muster, and some will not – exactly as at any other time. The only real difference is the amount of variation, which, if anything, might only allow a greater probability of having sufficient individuals live through whatever event invoked the stress response.

I will grant that the sudden expression of multiple mutations may well result in what some might term an “evolutionary leap”; however, such instances are likely to be very rare indeed, since there is no guarantee that the previously stored mutations will suit the organism any better now than when they first arose (thereby requiring their suppression in the first place), nor is there any guanatee that the accumulated mutations will be synergistic in any fashion.

Because of the extreme rarity of evolutionary novelties being the product of variation itself, their possibility (or actuality) does not overrule the more typical mechanism of natural selection.

13

Thank you, Algernon.

14

I was hesitant to place this OP in Great Debates because, well, there just isn’t a lot to debate. After all, this isn’t about Israel or religion or race or politics. Heck, just look at the view count to see how little interest there is in this.

And I find I tend to agree with most of the points made about the minimal speciation impact Hsp90 may have relative to other mechanisms. It appears that I was jumping to an unsupported conclusion about Hsp90 being the underlying mechanism for punctuated equilibrium.

To me however, the subtle consequences of this mechanism is fascinating. Metaphorically, Life is sitting back saying “OK Mother Nature, I’m ready for whatever you want to throw at me. Bring it on. But I warn you, if you do, I’ll have a dozen different versions of me in an instant, and no matter what you do at least one of me is going to thrive.” Of course, a cataclysmic asteroid collision is a radical challenge, but who knows, perhaps Hsp90 allowed some organisms to survive even that environmental disaster.

I really don’t have much else to say, other than to express my thanks and appreciation to Darwin’s Finch, tracer, edwino, erislover, and ultrafilter for taking the time to post your thoughts on this somewhat obscure topic.

Oops. I lied. I do have a couple of other things to say.

With respect to total change over time, I’d have to agree with you. My thought however is that with this Hsp90 mechanism the “change curve” over time is not a smooth line, but rather it stays flat for awhile and then jumps up in a step-wise fashion. The end points of both “curves” may end up at the same place, but the way they got there is much different. To me, this seems to mirror a punctuated equilibrium process.

Thanks for the clarification. I didn’t connect the term splitting with reproductive isolation.

Well said.