Wednesday, August 22, 2012

Tomoko Ohta and Nearly Neutral Theory

There's an interview with Tomoko Ohta in the August 21, 2012 issue of Current Biology: Tomoko Ohta.

You should know who she is but in case you don't, here's part of the brief bio ...
In 1973, she presented her first major paper entitled ‘Slightly deleterious mutant substitutions in evolution’. This theory was an expansion of Kimura's ‘neutral theory’, which Ohta called the ‘nearly neutral theory’ of molecular evolution. Her theory emphasizes the importance of interaction of drift and weak selection, and hence the role of slightly deleterious mutations in molecular evolution. With the accumulation of genome data, some of the predictions of the nearly neutral theory have been verified. The theory also provides a mechanism for the evolution of complex systems. Her other subject is to clarify the mechanisms of evolution and variation of multigene families. She has received several honors, including the foreign membership of the National Academy of Sciences, USA and Person of Cultural Merit, Japan.
It's very important to understand the essence of Nearly Neutral Theory since it explains the relationship between fitness and population size. Everyone needs to understand that Ohta demonstrated how slightly deleterious alleles can be fixed in a population. Her work showed that an allele can become effectively neutral in small populations even though it may actually lower the fitness of an individual. It's a way of explaining the limits of natural selection and of extending the Neutral Theory of Kimura.

She describes what happened when she joined Kimura's group at Tokyo.
At that time, Kimura was thinking of combining the theory of stochastic population genetics, the field he had been working on, with biochemical data on the nature of the genetic material. He proposed his now famous ‘neutral theory of molecular evolution’ in 1968. The ‘neutral theory’ proposed that most evolutionary changes at the molecular level were caused by random genetic drift rather than by natural selection. Note that the neutral theory classifies new mutations as deleterious, neutral, and advantageous. Under this classification, the rate of mutant substitutions in evolution can be formulated by the stochastic theory of population genetics. Kimura's theory was simple and elegant, yet I was not quite satisfied with it, because I thought that natural selection was not as simple as the mutant classification the neutral theory indicated, and that there would be border-line mutations with very small effects between the classes. I thus went ahead and proposed the nearly neutral theory of molecular evolution in 1973. The theory was not simple, and much more complicated, but to me, more realistic, and I have been working on this problem ever since.
This has nothing to do with Darwinism even though it's a fundamental part of modern evolutionary theory. You can't have an intelligent discussion about genome evolution, adaptationism, molecular evolution, or junk DNA without a firm grasp of Nearly Neutral Theory.

It's a shame there's no Nobel Prize for evolution.


6 comments :

  1. It should be perhaps also noted that the nearly neutral theory has some very serious theological and philosophical implications and for some odd reason, I never really see that discussed. And that's actually true for both sides. Now, the theologians are too ignorant of evolutionary biology to understand such subtleties, that's understandable, but that adaptationism dominates so much what comes out of scientists is very unfortunate IMO. I recall this paper by Koonin:

    http://www.landesbioscience.com/journals/cc/kooninCC3-3.pdf

    but not much else.

    ReplyDelete
  2. This really is the basis of our current understanding of evolution. What's truly bizarre is that it is not central to the education of most evolutionary biologists.

    ReplyDelete
    Replies
    1. Adding to Konrad,

      ...or molecular biologists, bioinformaticians, genomicists, geneticists, etc, etc, etc...

      The "if it's not adaptive, why would it be there?" thinking lives on, despite her work.

      Delete
  3. A relatively recent (2002) review by her that's not behind a paywall

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC138577/?tool=pmcentrez

    "Near-neutrality in evolution of genes and gene regulation"

    ReplyDelete
  4. Tomoko Ohta is not without honors. She was honored a few years ago by being elected a Foreign Member of the U.S. National Academy of Sciences. She apparently is the first female scientist from Japan to be so honored. She had already received Oxford University's Weldon Memorial Prize and Medal, back in 1986. More recently she was honored as a "Person of Cultural Merit" by the Japanese government, an award presented by the Emperor of Japan.

    I am intrigued by the statement that nearly neutral theory has deep theological and philosophical implications. I imagine that if it is, say 0.1, a selection coefficient is just another goldanged selection coefficient. But as we consider weaker and weaker selection, down around ± 1/(4N), something strange and wondrous happens ...

    ReplyDelete
    Replies
    1. I am intrigued by the statement that nearly neutral theory has deep theological and philosophical implications. I imagine that if it is, say 0.1, a selection coefficient is just another goldanged selection coefficient. But as we consider weaker and weaker selection, down around ± 1/(4N), something strange and wondrous happens ...

      ... provided the population was mixed efficiently during the historic progression, and the allele's environment is sufficiently constant over that history to render a fixed exponent a good model, of course!

      Delete