Over the last few decades, the
term “nearly neutral mutations” has been used
in various ways. However, it was Tomoko Ohta who first developed
an independent theory on the fate of nearly neutral mutations,
and named it the "nearly neutral theory" in 1973.
At that time, Ohta primarily focused on mutations that are
"slightly deleterious” instead of both “slightly
deleterious” and “slightly advantageous”
mutations because of the wide belief of the "status quo"
argument among molecular evolutionists. The "status quo"
meant that gene function was already close to its optimum
in general, and advantageous mutations were extremely rare.
Ohta incorporated this “status quo” argument to
her nearly neutral theory in the early 1970s; therefore, Ohta
did not see any need to discuss slightly advantageous mutations
Toward the end of the 1980s, however, increasing amounts of
data showed the existence of slightly advantageous mutations
and the effects of positive selection on them. Therefore,
Ohta modified her original nearly neutral theory by incorporating
the fate of slightly advantageous mutations, and proposed
this modified theory as “nearly neutral theory of molecular
evolution” in a series of two papers published in 1990
There are several other differences between the earlier
model of the nearly neutral theory and the later one. To differentiate,
Ohta also calls the earlier model, the “shift model”(1973)
and the later one the “fixed model” (1990, 1991).
Please see more details on these models in the Discussion
Transcript from the 2002 Dibner Workshop.
There is also much confusion for the differences between
Kimura’s neutral theory and Ohta’s nearly neutral
theory. The main difference between Ohta’s nearly neutral
theory and Kimura’s neutral theory is while Kimura put
his focus on so-called “strictly neutral mutations”
(this phrase is specified by Ohta), Ohta was more interested
in the evolutionary mechanisms of the “border line mutations”
or the “nearly neutral mutations,” (i.e. mutations
which are not strictly neutral nor are strongly selected).
The most notable yet overlooked difference between these
two theories is the effect of population sizes. Simply, population
size does not effect on neutral mutations. For nearly neutral
mutations in small populations, the effects of selection are
also extremely limited and the fate of them is also determined
by random genetic drift just like neutral mutations. However,
the effects of selection are not ignorable in a large population
for nearly neutral mutations.
The nearly neutral theory in general requires many more parameters
because of the characteristics of these mutations. For this
reason, the most frequent criticism of the nearly neutral
theory is that it is too complex. Neutralists, including Kimura,
argue that a theory needs to be simple, easy to be understood,
accepted, and applied. Yet, Ohta argues that her theory explains
the “actual natural populations” better.
The current focus of Ohta’s research is seeking a
possibility to apply the nearly neutral theory to the morphological
For more on the nearly neutral theory follow the links to
This page was written by Tomoko Y. Steen. It was last updated