Featured Research

from universities, journals, and other organizations

Populations survive despite many deleterious mutations: Evolutionary model of Muller's ratchet explored

Date:
August 10, 2012
Source:
Max-Planck-Gesellschaft
Summary:
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.

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 loose 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.

Related Articles


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 Developmental Biology in Tόbingen, 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 genome and divide asexually. By and large, evolution is driven by random events or as Richard Neher says: "Evolutionary dynamics are very stochastic."


Story Source:

The above story is based on materials provided by Max-Planck-Gesellschaft. Note: Materials may be edited for content and length.


Journal References:

  1. R. A. Neher, B. I. Shraiman. Fluctuations of Fitness Distributions and the Rate of Muller's Ratchet. Genetics, 2012; 191 (4): 1283 DOI: 10.1534/genetics.112.141325
  2. S. Goyal, D. J. Balick, E. R. Jerison, R. A. Neher, B. I. Shraiman, M. M. Desai. Dynamic Mutation-Selection Balance as an Evolutionary Attractor. Genetics, 2012; 191 (4): 1309 DOI: 10.1534/genetics.112.141291

Cite This Page:

Max-Planck-Gesellschaft. "Populations survive despite many deleterious mutations: Evolutionary model of Muller's ratchet explored." ScienceDaily. ScienceDaily, 10 August 2012. <www.sciencedaily.com/releases/2012/08/120810083613.htm>.
Max-Planck-Gesellschaft. (2012, August 10). Populations survive despite many deleterious mutations: Evolutionary model of Muller's ratchet explored. ScienceDaily. Retrieved October 25, 2014 from www.sciencedaily.com/releases/2012/08/120810083613.htm
Max-Planck-Gesellschaft. "Populations survive despite many deleterious mutations: Evolutionary model of Muller's ratchet explored." ScienceDaily. www.sciencedaily.com/releases/2012/08/120810083613.htm (accessed October 25, 2014).

Share This



More Fossils & Ruins News

Saturday, October 25, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Deep Sea 'mushroom' Could Be Early Branch on Tree of Life

Deep Sea 'mushroom' Could Be Early Branch on Tree of Life

Reuters - Innovations Video Online (Oct. 24, 2014) — Miniature deep sea animals discovered off the Australian coast almost three decades ago are puzzling scientists, who say the organisms have proved impossible to categorise. Academics at the Natural History of Denmark have appealed to the world scientific community for help, saying that further information on Dendrogramma enigmatica and Dendrogramma discoides could answer key evolutionary questions. Jim Drury has more. Video provided by Reuters
Powered by NewsLook.com
Fossil Treasures at Risk in Morocco Desert Town

Fossil Treasures at Risk in Morocco Desert Town

AFP (Oct. 23, 2014) — Hundreds of archeological jewels in and around the town of 30,000 people prompt geologists and archeologists to call the Erfoud area "the largest open air fossil museum in the world". Duration: 02:17 Video provided by AFP
Powered by NewsLook.com
Oldest Bone Ever Sequenced Shows Human/Neanderthal Mating

Oldest Bone Ever Sequenced Shows Human/Neanderthal Mating

Newsy (Oct. 23, 2014) — A 45,000-year-old thighbone is showing when humans and neanderthals may have first interbred and revealing details about our origins. Video provided by Newsy
Powered by NewsLook.com
Weird-Looking Dinosaur Solves 50-Year-Old Mystery

Weird-Looking Dinosaur Solves 50-Year-Old Mystery

Newsy (Oct. 23, 2014) — You've probably seen some weird-looking dinosaurs, but have you ever seen one this weird? It's worth a look. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:

Strange & Offbeat Stories

 

Plants & Animals

Earth & Climate

Fossils & Ruins

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins