**The Neutral Equation**

According to the **Neutral Theory**, in the
absence of natural selection (**s = 0**), expected **heterozygosity**
(**H**_{e}) in a finite population of **effective size N**_{e }is a balance
between *loss *of variation by **genetic drift**
and its *replacement *by recurrent **mutation** at a
rate **µ**. Then it can be shown,

**H**_{e} = (4N_{e}**µ**) / (4N_{e}**µ** + 1)

If **N**_{e}** = 10**^{6}
and **µ****
= 10**^{-6}, then N_{e}**µ**
= 1 and **expected H**_{e}
= (4)/(4 + 1) = **0.80. **However, the *observed *range
**0.001 < ****H**_{obs}**
****< 0.25** over a **wide range of species** is much smaller than this *expected *value.
One interpretation is that the *effective***
population size (N**_{e}**)** for most
species** **is much smaller than the *observed***
population count (N**_{c}**)**. Thus, if **N**_{e} = 10^{4},
then N_{e}u = 0.001 and **H**_{e }=
(0.004)/(0.004 + 1) = (0.004)/(1) = **0.04**, a
fairly typical value.

A modified form is the **Nearly-Neutral Theory** where **0 ~
s << 1**, which would allow *larger *populations
to have
* lower ***H**_{e}.