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0:04
That gives us the main
useful principles
of neutral theory
and brings us to the
extension introduced
by Tomoko Ohta in 1973,
the nearly neutral theory.
Let's take a look at that one.
0:19
It's important to
remind you what
the distribution of fitness
effects of mutations
were under neutral theory.
It's often referred to as an
all-or-nothing distribution.
You can see that the
distribution has
quite a high density of
strongly deleterious mutations,
quite a high density of
neutral and some positive.
But they're not
close to each other.
You're either, if you're a
mutation in this theory,
strongly deleterious
or strictly neutral
or strongly beneficial.
Tomoko Ohta reasoned that
this was too unrealistic.
She adopts many of
the same ideas,
we can still see that,
on average, because when
you make a random
change to your protein
you're likely to damage its
function and reduce fitness.
There's still a
very high density
of negatively
selected mutations,
and there's still a
small occurrence of
positively selected
mutations because
that's the basis of adaptation.
She does believe that
there is quite a bit
of neutral evolution
or neutral mutations,
but she reasons that if
there are neutral mutations
and there are
deleterious mutations,
then some fraction should
be slightly deleterious.
They would have slightly
negative selection coefficients,
so she added those to the model.
This is basically the
first version of the model
that was introduced in 1973.
She later adds slightly
beneficial mutations,
and it's the combination
of these two that gives us
today what we call the nearly
neutral theory of evolution.
Because these selection
coefficients are small,
they're either positive but
small or negative but small,
the intensity of natural
selection acting on
these mutations is
similar in effect size
to the intensity of drift,
acting on mutations
that have s = 0.
Because the effect
sizes are similar,
these two evolutionary
processes interact.
What Tomoko Ohta
is saying is that
a large fraction
of polymorphism
and divergence
that we observe at
the genetic level in natural
settings must result
from some interaction
between mutation and drift
if this distribution of
fitness effects is correct.
