Molecular Evolution Activities

The Nearly Neutral Theory Transcript

The Nearly Neutral Theory of Molecular Evolution

Tomoko STEEN: I have given more than 30 talks on the neutral and near neutral theory and several things come up over and over. Basically, they have a kind of ambiguous definition of the near neutral theory, and there are several points about it.
First of all, as a basic observation, people generally think of the neutral theory and near neutral theory as one theory, instead of it being different from the strictly neutral theory, which is what I call it. She (Tomoko Ohta) independently invented the near neutral theory which is mostly focusing on nearly neutral mutations as opposed to neutral mutations.

The second issue is from the 1973 paper. Well there is on important paper in 1971 as well for explaining the near neutral class of mutations. She [Tomoko Ohta] had a definition of nearly neutral mutation itself that has changed compared to 1990 and 1991 papers she published with Tachida. That is something that people do not know about, because the original one mostly focused on the standard negative mutation while the present one focuses on both negative and positive.

The third issue is the main reason she brought up the near neutral theory independently from the neutral theory is generation time and population size issue. It is interesting that when I give a talk people bring up the reciprocal relationship with population size as a kind of the neutral theory that is the older theory. But, this is the kind of thing that they are not very clear about. More recently, John Gillespie and Dr. Ohta also wrote a paper. His definition of the nearly neutral and hers are actually different.

Those are things that is Dr. Ohta could clarify and you could put up on your website would help a lot more. …

James CROW: Well I remember when I first became interested in this kind of question that the most impressive thing was the rapid evolution of long lifed organisms, which presumably have small population sizes and therefore are more likely to evolve. We [Crow and Ohta] talked about this.

Tomoko OHTA: Yes.

James CROW: I remember several times. Is that what led you to the nearly neutral theory, this consideration?

Tomoko OHTA: In the early 1970s, I think that was the start of the nearly neutral model. Of course, another reason is that I could not give is that the neutral and selected deleterious class were so separated. They must be continuous somehow. That is a basic point, but another reason from the generation time problem and the population size problem. If I assume that most of the new mutations with small effect are slightly deleterious, at least on the average, then we can expect that the evolutionary rate and the population size may not be able to correlate. If large organisms with small population size and a long generation time, then population size and generation time may not give a correlation effect and would produce nearly equivalent evolutionary rates between large and small organisms. I was thinking mostly of higher organisms at the start. Actually the model of nearly neutral mutations …

John BEATTY: Can I Interrupt this? Just a question of clarification. Maybe I’m pedantic, but one could think of generation time effects as supporting the neutral theory. But, then you talk about the generation effect problem. So I’m just wondering what generation effect exactly it was. When I think of generation effects that seem to support the neutral theory, maybe I’m wrong about this or just naïve, it would be that rodents evolved much faster than humans. That’s a generation time effect that would seem to support the neutral theory. So what is the generation effect problem?

James CROW: Well one thing is that what we know now and didn’t know then is how much of mutation is driven by the male mutation rate and how much that is dependent on age at reproduction.

William PROVINE: At the time that was not the issue. Generation time effect played zero role in Kimura’s theory. That was what was so neat about it. It took the population size thing and got rid of it entirely. But people then were using only the protein data. That’s all there was. They didn’t have DNA sequence data just protein data, and protein data had real problems in it that seemed to have a generation time problem connected with them. That is where Ohta’s theory was devoted to try and get a robust version of the neutral theory. She thought of it as a version of the neutral theory, as you said in the papers. But Kimura saw it as a different theory. I think more than a variant of his theory and that is a very curious kind of issue because what we need to have is Tomoko Steen sitting at the table where you and Kimura sat together for lunch everyday for many, many years hashing out these problems together. This is a very difficult thing for historians to understand. You are our last hope for understanding your collaboration with Kimura.

Tomoko OHTA: Well I think that Kimura was not sure which of the models was correct. I mean under the strictly neutral theory one has to assume that mutation rate is more or less independent of generation time.

James CROW: Right.

Tomoko OHTA: That I did not believe, but Kimura he thought it might be true. He was not sure, I think. For example, Nei was more strongly toward year dependent mutation rate. So strictly neutral theory is better to Nei at that time.





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