Why Muller’s Ratchet?

In 1931, geneticist Hermann Joseph Muller gave a speech at a scientific conference in New Orleans, Louisiana. In his remarks, Dr. Muller proposed a theory to answer a long-standing and controversial question: why do so many organisms use sexual reproduction to produce offspring? At the time, biologists were puzzled by the relatively high prevalence of sexual reproduction among many forms of life. The only alternative mechanism for reproduction, the appropriately-named asexual reproduction, is relatively easy to ponder and (in principle) execute: an individual divides or buds off an offspring that is genetically identical to its parent. Lots of organisms, especially bacteria, reproduce asexually. Sexual reproduction is more complex and risky, with less of a guarantee that offspring will have the right combination of genetic instructions to grow, survive, and reproduce. Thus, Dr. Muller and his colleagues were puzzled by the widespread use of sexual reproduction. Countless organisms, from single-celled protozoa to humans, choose sexual reproduction. Why?

To his audience, Dr. Muller proposed an intriguing theory to explain why sexual reproduction was so widespread. He hypothesized that sexual reproduction carries with it a unique opportunity to correct mutations that pop up in our genes from time to time. Mutations are changes that occur in the genetic instructions of all living beings. They play a critical role in the evolution of species, but can also sometimes have negative consequences. For example, they can cause diseases such as cancer. But how does sexual reproduction provide an opportunity to correct potentially detrimental mutations?

In organisms that undergo sexual reproduction, there’s a specific process called recombination, which occurs during the production of egg and sperm cells. Recombination is a complex (but elegant) process of “shuffling” or “mixing” our genetic instructions; Dr. Muller hypothesized that this process could allow parents to correct potentially deleterious mutations that had occurred in their gene sequences before passing along those genetic instructions to their offspring. Asexual organisms, which do not undergo that complex (but elegant) process of recombination, would lack this opportunity to correct bad mutations; thus, Dr. Muller proposed that in asexual species, deleterious mutations could slowly and irreversibly accumulate, leading to the unpalatable consequence of extinction. Decades later, Dr. Muller called this irreversible accumulation of mutations a “ratchet,” after the mechanical device that permits movement in one direction, but not the opposite direction. By the 1970s, other biologists simply described this theory as Muller’s Ratchet, the steady and irreversible accumulation of mutations. The idea is still discussed today because, obviously, asexual organisms are still with us; they haven’t succumbed to the gruesome fate of accumulation of mutations and extinction predicted by Muller’s Ratchet. So, even though Dr. Muller proposed a sound theory, life has found a way to bypass the potentially destructive phenomenon of his ratchet.

Basic ratchet mechanism

Ratchet mechanism: there’s no going back now.

If you’ve made it this far, give yourself a pat on the back. You’ve just absorbed an earful about a theory that I couldn’t even wrap my head around until my third year of graduate school. I didn’t choose Muller’s Ratchet as the title of this blog because I wanted to explain complex genetic theories to a general audience in my inaugural post. In fact, as someone interested in communicating science to the public, starting off this blog with several paragraphs on Hermann Joseph Muller and his ratchet was probably an unwise move. For the uninitiated, thinking about such concepts as sexual versus asexual reproduction is probably more likely to turn readers off of science forever rather than get them excited about the atmosphere of Titan, the latest research into earthquake prediction, or the natural circadian rhythms of life on Earth. But these are exactly the types of research and discoveries that I want to bring to you. Through reading journal articles, attending conferences, speaking with colleagues, and conducting my own research, I daily encounter research and concepts that you, the general public, should know about. I don’t just want you to know about it because these discoveries and research endeavors are fascinating; you should know them because crafting and maintaining a scientifically literate society is of paramount importance to our economic vitality, competitiveness, and global prestige.

It is also a task we as a society have sadly neglected. Large majorities of American citizens cannot correctly answer even basic questions of scientific literacy. We cut programs in education, research, and outreach in our schools and universities. Our students rank among the bottom of their peer group in science knowledge, while many companies bring in skilled foreign graduates to work in their firms. Many Americans value science and scientific research. But, science does not appear to be their priority when they vote. Time and again, American voters have elected representatives who take great pride in flaunting scientific discoveries, attacking scientific theories, and misrepresenting the value of scientific research to their constituents.

This sad catalog is a symptom of a far more sinister syndrome of scientific illiteracy in our society. Evidence suggests that, by and large, many Americans like the idea of science, and certainly benefit from scientific discoveries. But, in general, they have little idea what scientists do, what they study, why it is important, and how we all benefit from their dedication. This ignorance can be self-perpetuating: confusion and disinterest about science leads to policy decisions which erode standards in scientific education and communication, which leads to yet more confusion and disinterest about science… In the end, this process dampens economic prosperity, fuels ignorance and resentment, and widens an already large gap between scientists and the public they serve.

To me, this self-perpetuating scenario sounds a lot like Muller’s Ratchet. Dr. Muller theorized that, in asexual organisms that lack the reparative benefits of recombination, deleterious mutations would accumulate steadily and irreversibly. Unless asexual organisms found a way to circumvent the ratchet, extinction would certainly follow. In the case of scientific literacy, a gap between scientists and the general public fuels a steady increase in scientific illiteracy among the latter. This increase is surely detrimental, but is it irreversible? For Muller’s Ratchet in asexual organisms, the ratchet can be reversed: there are countless examples of ways in which life has avoided the detrimental effects of the ratchet, and gone on to live another day.

Is the same true for our society? Can the Muller’s Ratchet taking hold here be stopped, or even turned back? I’m not sure. But, here’s a starting point. After all, I’m here, and you’re here. I love talking about science, especially to non-scientists. So, let’s see if we can reverse this ratchet…

FURTHER READING: where possible, I have included open-access sources

Muller’s Ratchet

  • Muller HJ. 1932. “Some Genetic Aspects of Sex.” The American Naturalist. Volume 66 (Number 703): 118-138. [This text contains Dr. Muller’s remarks at the 1931 meeting in New Orleans, Louisiana.]
  • Muller HJ. 1958. “Evolution by Mutation.” Bulletin of the American Methematical Society. Volume 64 (Number 4): 137-160.
  • Muller HJ. 1964. “The Relation of Recombination to Mutational Advance.” Mutation Research. Volume 1: 2-9. [This text contains Dr. Muller’s first reference of the “ratchet” metaphor for his hypothesized mechanism.]
  • Felsenstein J. 1974. “The Evolutionary Advantage of Recombination.” Genetics. Volume 78: 737-756. [I believe this scientific paper contains the first reference of the term Muller’s Ratchet.]

Scientific literacy among U.S. students and citizens

Perceptions of science and scientists

Attitudes of U.S. elected officials toward science

Image credit: from Brockhaus Konversations-Lexikon (1894, 14th edition, volume 10, page 420), adapted by Georg Wiora.

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About James Urton

I went to school to become a molecular biologist.  At some point in this long education, I discovered that I love communicating science to the general public: talks, writing, at a pub, on the street corner...  Whatever venue will let me hold your attention for a few moments.  Unfortunately, I can't do this for a living, since no one will pay me.  So, I have a job as a molecular biologist at the University of Washington, where I get to work with great scientists on some really awesome projects, and I'll blog about science here at Muller's Ratchet in my spare time. Why should the general public want to know anything about science? Here's my explanation (which also explains why I chose the name Muller's Ratchet for this site). Briefly as a graduate student (before I had to devote all of my time to graduating), I blogged at Adaptive Radiation.
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One Response to Why Muller’s Ratchet?

  1. Pingback: The Absence | Muller's Ratchet

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