Populations survive despite many deleterious mutations: Scientists investigate evolutionary model of Muller's ratchet

Aug 10, 2012
Equilibrium of mutation and selection processes: A population can be divided into groups of individuals that carry different numbers of deleterious mutations. Groups with few mutations are amplified by selection but lose members to other groups by mutation. Groups with many mutations don't reproduce as much, but gain members by mutation. Credit: Richard Neher/MPI for Developmental Biology

From protozoans to mammals, evolution has created more and more complex structures and better-adapted organisms. This is all the more astonishing as most genetic mutations are deleterious. Especially in small asexual populations that do not recombine their genes, unfavourable mutations can accumulate. This process is known as Muller's ratchet in evolutionary biology. The ratchet, proposed by the American geneticist Hermann Joseph Muller, predicts that the genome deteriorates irreversibly, leaving populations on a one-way street to extinction. In collaboration with colleagues from the US, Richard Neher from the Max Planck Institute for Developmental Biology has shown mathematically how Muller's ratchet operates and he has investigated why populations are not inevitably doomed to extinction despite the continuous influx of deleterious mutations.

The great majority of mutations are deleterious. "Due to selection individuals with more favourable genes reproduce more successfully and deleterious mutations disappear again," explains the population geneticist Richard Neher, leader of an independent Max Planck research group at the Max Planck Institute for in Tübingen, Germany. However, in small populations such as an asexually reproducing virus early during infection, the situation is not so clear-cut. "It can then happen by chance, by stochastic processes alone, that deleterious mutations in the viruses accumulate and the mutation-free group of individuals goes extinct," says Richard Neher. This is known as a click of Muller's ratchet, which is irreversible – at least in Muller's model.

Muller published his model on the evolutionary significance of deleterious mutations in 1964. Yet to date a quantitative understanding of the ratchet's processes was lacking. Richard Neher and Boris Shraiman from the University of California in Santa Barbara have now published a new theoretical study on Muller's ratchet. They chose a comparably simple model with only deleterious mutations all having the same effect on fitness. The scientists assumed selection against those mutations and analysed how fluctuations in the group of the fittest individuals affected the less fit ones and the whole population. Richard Neher and Boris Shraiman discovered that the key to the understanding of Muller's ratchet lies in a slow response: If the number of the fittest individuals is reduced, the mean fitness decreases only after a delay. "This delayed feedback accelerates Muller's ratchet," Richard Neher comments on the results. It clicks more and more frequently.

"Our results are valid for a broad range of conditions and parameter values – for a population of viruses as well as a population of tigers." However, he does not expect to find the model's conditions one-to-one in nature. "Models are made to understand the essential aspects, to identify the critical processes," he explains.

In a second study Richard Neher, Boris Shraiman and several other US-scientists from the University of California in Santa Barbara and Harvard University in Cambridge investigated how a small asexual population could escape Muller's ratchet. "Such a population can only stay in a steady state for a long time when beneficial mutations continually compensate for the negative ones that accumulate via Muller's ratchet," says Richard Neher. For their model the scientists assumed a steady environment and suggest that there can be a mutation-selection balance in every population. They have calculated the rate of favourable mutations required to maintain the balance. The result was surprising: Even under unfavourable conditions, a comparably small proportion in the range of several percent of positive mutations is sufficient to sustain a population.

These findings could explain the long-term maintenance of mitochondria, the so-called power plants of the cell that have their own and divide asexually. By and large, evolution is driven by random events or as Richard Neher says: "Evolutionary dynamics are very stochastic."

Explore further: Testing shows billfish demonstrate bone remodeling without osteocytes

More information: Richard A. Neher, Boris I. Shraiman: Fluctuations of fitness distributions and the rate of Muller's ratchet. Genetics, Vol. 191, pp. 1283-1293, August 2012. doi:10.1534/genetics.112.141325

Sidhartha Goyal, Daniel J. Balick, Elizabeth R. Jerison, Richard A. Neher, Boris I. Shraiman and Michael M. Desai: Dynamic Mutation-Selection Balance as an Evolutionary Attractor. Genetics, Vol. 191, August 2012. doi: 10.1534/genetics.112.141291

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kevinrtrs
2.6 / 5 (30) Aug 10, 2012
From protozoans to mammals, evolution has created more and more complex structures and better-adapted organisms.

Here we have a pertinent reality check. The above is the axiomatic assumption of the evolutionary theory which need to be carefully separated from the actual observation below.

This is all the more astonishing as most genetic mutations are deleterious.

What is boils down to is that the genetic material is deteriorating far faster than the supposed beneficial and complexing mutations are thought to have happened. In fact research shows that such so-called beneficial mutations are so far outpaced as to be useless in supplying the needed building blocks to transform a single ancestor into the currrent complexity we observe today.
Biologists/geneticists know this. That's why the research behind this article attempts to mitigate that glaring gap.
It is well known that the rate of deterioration is such that the human race should already have been wiped out.
kevinrtrs
2.6 / 5 (28) Aug 10, 2012
The result was surprising: Even under unfavourable conditions, a comparably small proportion in the range of several percent of positive mutations is sufficient to sustain a population.


So don't be misled by this statement.

Whilst the population is sustained because of EXISTING genetic material being intermixed randomly, the so-called beneficial mutations are observed to happen at such a slow rate that they could not supply the required building blocks in time to form even a mildly complex protozoan - nevermind a highly complex human being. Even over supposed millions of years.

This is the unhappy conundrum for evolutionary genetics: in a million years, the deleterious effects should have killed us already but in the same time not enough beneficial mutations could be build up and selected.
Ionian
1.8 / 5 (5) Aug 10, 2012
This is just more proof that the Genome Operating Device is still mysterious.
barakn
1.5 / 5 (16) Aug 10, 2012
Well, that idiot Kevin is of course ignorant as usual, but I am kind of surprised by Neher. He's glossing over some things, probably because they're too difficult to model. What both are assuming is that a genome is of a relatively fixed size and that the only changes that occur do so in a pointwise fashion. However, more dramatic events occasionally occur which result in duplication of anywhere from a part of a gene to thousands of genes. And that completely changes the ballgame. A deleterious mutation in one copy of a gene becomes far less harmful, and in fact this copy could then be free to mutate into something else with a completely different function, or rapidly accumulate changes and become a pseudogene. By focusing on genetic change only at the smallest scale, these people are ignoring the tool evolution uses the most.
210
2.7 / 5 (14) Aug 10, 2012
@kevinrtrs...you are beginning to see...I know plenty of people here want to hate on you...and I have taken exception to the way you have sometimes applied biblical text, BUT, much of what you say here...is correct...and logical. I am saying I still disagree with elements of your dogma BUT you are beginning to see...and demand proofs -keep going.
For one, to supplement your observation, we have known for a while that the genetic material we thought we had found in the genome, provided all the answers to genetically based functionality , we erred!Point, the stuff we thought was 'junk' is NOT junk, mutations are NOT the only way to introduce speciation, etc, etc, etc Speaking strictly with regard to genes, we now know there was NOT enough gene-stuff there to make all that we see everywhere! And evolution is not the master or creator-force of life...it is in fact as much subject to reality as any other being or entity in life seeing as it-evolution, is life adapting to a live earth.
word
Deathclock
2.7 / 5 (19) Aug 10, 2012
This is all the more astonishing as most genetic mutations are deleterious.

What is boils down to is that the genetic material is deteriorating far faster than the supposed beneficial and complexing mutations are thought to have happened.


kevin for fucks sake... I've personally explained this to you more than once now... YES, the majority of mutations are detrimental, OBVIOUSLY... but it's irrelevant because detrimental mutations are not selected for.

Beneficial mutations, while rare, ARE SELECTED FOR AND BECOME A PERMANENT FEATURE OF THE SPECIES.

That's the difference, it doesn't matter what gap there is between beneficial and detrimental mutations, because detrimental ones don't affect the fitness of the species, while beneficial ones increase it.

research shows that such so-called beneficial mutations are so far outpaced as to be useless in supplying the needed building blocks...


No research shows this, this is your INCORRECT interpretation.
TheGhostofOtto1923
2.5 / 5 (42) Aug 10, 2012
This is the unhappy conundrum for evolutionary genetics: in a million years, the deleterious effects should have killed us already but in the same time not enough beneficial mutations could be build up and selected.
You know kev if you actually understood this stuff you wouldn't think this way. FYI
BUT, much of what you say here...is correct...and logical.
If Kevin says something about science which appears to be true it is only by chance. It would be like seeing faces in cloud formations. Or Jesus on a moldy piece of toast I suppose.
Deathclock
2.8 / 5 (13) Aug 10, 2012
This is the unhappy conundrum for evolutionary genetics: in a million years, the deleterious effects should have killed us already


This is just stupid and represents a complete misunderstanding of evolution, even though it's been explained to you ad nauseum. You are WILLFULLY ignorant, which is a thousand times more disgraceful than being incidentally ignorant.
Phil DePayne
3.2 / 5 (6) Aug 10, 2012
Perhaps kevin should take a dogmatic approach, and see that the genetic code is obviously a message from the Creator, and accept that evolution is really part of intentional design. My born-again aunt is a molecular biologist and I can only assume she takes her work seriously by thinking in this way.
Torbjorn_Larsson_OM
2.7 / 5 (12) Aug 10, 2012
@ barakn:

I think the simplicity was intended. They identified the least mechanism that would stop Muller's ratchet and at the same time they could predict why mitochondria with their small genomes are maintained.

@ Deathclock:

It is called Lying For Zombie Jesus, and is officially sanctioned in their "moral" texts. Because nothing and no one else has a value in their bigoted eyes.

************
Creationists shouldn't comment on science, it is hilarious to see.

Especially here, since all the "arguments" were already run to the ground in the text.

As we all comprehend, this was yet another test for evolution, as science most well observed fact, that it passed with flying colors.
Deathclock
2.8 / 5 (16) Aug 10, 2012
I also love that everyone giving me 1 ratings is on a shill account that has never posted a single comment... what pathetic losers you guys must be... as if anyone with a modicum of intelligence links the comment rating with it's quality on this site anymore, it's been a joke for all the years I've been posting here.
TheGhostofOtto1923
2.3 / 5 (40) Aug 11, 2012
Perhaps kevin should take a dogmatic approach, and see that the genetic code is obviously a message from the Creator, and accept that evolution is really part of intentional design. My born-again aunt is a molecular biologist and I can only assume she takes her work seriously by thinking in this way.
So sorry for your aunts profound schism. But such is the allure of the promise of immortality. And wish-granting.

"Nobody should ever have to fear worshipping in public in the united states." -b Obama

-Fear, no. But they should feel embarrassed and ashamed for propagating such dangerous nonsense.
I also love that everyone giving me 1 ratings is on a shill account that has never posted a single comment... what pathetic losers you guys must be...
These are the same pouty people who vandalize public toilets. Right estevan?
Estevan57
2.6 / 5 (42) Aug 11, 2012
That's right, GhostofOtto1923, GhostofOttofartsonya, lite, diaperdogdick, TheAntiphilo, and others.
malapropism
2.5 / 5 (11) Aug 12, 2012
From protozoans to mammals, evolution has created more and more complex structures and better-adapted organisms.

Here we have a pertinent reality check. The above is the axiomatic assumption of the evolutionary theory which need to be carefully separated from the actual observation below.

Ah, Kevin, epic fail. You didn't read the actual paper, did you? (I did: it's an Open Access Article at the Genetics Society of America's web site if you follow the link.) If you had, you'd have found that this was never written in the paper but is instead just part of the article write-up by the PhysOrg journalist.

You'd also possibly have understood that Muller's Ratchet is only of real significance in non-recombining populations, and especially in small such populations. In other situations recombination is normally a much more potent force which tends to mask the accumulation of otherwise mildly-deleterious mutations that Muller's Ratchet impacts upon.
malapropism
2 / 5 (8) Aug 12, 2012
What both are assuming is that a genome is of a relatively fixed size and that the only changes that occur do so in a pointwise fashion. However, more dramatic events occasionally occur which result in duplication of anywhere from a part of a gene to thousands of genes. And that completely changes the ballgame. A deleterious mutation in one copy of a gene becomes far less harmful ...

You seem to have a misunderstanding of what the Muller's Ratchet model is about. It is not fundamentally concerned with the amplification of a (one or more) deleterious gene into multiple copies but is a model of the accumulation of usually different (that is, a variety of) deleterious genes in population genetics. It is not normally scientifically valid to try to translate a population genetics model to the level of the individual, which is what you seem to be suggesting.
malapropism
2.1 / 5 (7) Aug 12, 2012
What this model does have fascinating implications for however, and that the journalist very rightly notes, is in the continued retention of the Y chromosome.

Despite it's obvious involvement in sexual reproduction, the Y chromosome can be regarded as being part of an asexually reproducing (that is, non-recombinant) population because it appears only in males - it is therefore excluded from sexual recombination (females have 2 X chromosomes which cannot participate in recombining with a Y).

It would therefore be expected that the Y would gradually either accumulate a large set of mutations or disappear or both: what this new model shows however is that even a small positive selection pressure can reverse this situation, and the Y chromosome has a very significant positive selection pressure because in a sexually reproducing species no males equals species extinction.
TheGhostofOtto1923
2.6 / 5 (34) Aug 13, 2012
Ah, Kevin, epic fail. You didn't read the actual paper, did you? (I did: it's an Open Access Article at the Genetics Society of America's web site if you follow the link.) If you had, you'd have found that this was never written in the paper but is instead just part of the article write-up by the PhysOrg journalist.
Kevin would only read the paper like he reads the bible. His faith would tell him what it was supposed to say rather than what it actually says.
Deathclock
2.5 / 5 (8) Aug 13, 2012
Kevin would only read the paper like he reads the bible. His faith would tell him what it was supposed to say rather than what it actually says.


Reminds me of an argument I was in recently with a creationist who brought up Leviticus 17:11 as evidence that the bible has divine scientific knowledge... in this case about blood being essential for life... unfortunately that verse goes on to talk about spilling the blood of an animal on an altar as a sacrifice to God. When I asked if they really wanted to bring up ritualistic blood sacrifices of animals on sacrificial altars to support their assertion that the bible is scientific they told me that it wasn't meant to be taken literally. So I told them that what it said about blood being required for life must also not have been meant to be taken literally either... I'm sure you can imagine their reaction.

Idiots, the lot of them.