Biology Reference
In-Depth Information
EFFECTIVE POPULATION SIZE: EPIDEMIOLOGICAL
UTILITY AND ESTIMATION
The effective population size (
N
e
) is the size of an ideal population that
has the same rate of genetic drift as the population under consideration.
The “ideal” population follows the models of Wright
47
and Fisher
48
and,
in simple terms, refers to the situation where every individual has an
equal opportunity to contribute genes to the next generation.
49,50
The
effects of genetic drift can be measured several ways such as by the
increase in inbreeding, increase in variance in allele frequency, or loss of
heterozygosity over generations. Hence, there are different definitions of
N
e
: inbreeding
N
e
, variance
N
e
, and eigenvalue
N
e
, respectively.
51
In closed
populations of constant size, the different concepts have similar or
identical values of
N
e
, but certain demographic scenarios can lead to
different estimates of
N
e
depending on which aspect of drift is being
measured.
49
e
52
My discussion will largely not make a distinction between
the different
N
e
concepts; however, the estimates I provide are more
closely related to inbreeding
N
e
. Commonly, but not always,
N
e
is smaller
than the actual census population size (
N
c
) because some parents
contribute many more offspring to the next generation than others.
Of what interest is parasite
N
e
to epidemiologists? There are both long-
term (evolutionary) and short-term (ecological) utilities of
N
e
. Evolu-
tionary importance stems from the fact that
N
e
directly determines the
rate of drift where the loss of neutral genetic variation (often quantified
via expected heterozygosity;
H
e
) each generation is expected to decline by
a rate inversely dependent on
N
e
.
51
N
e
is also needed to assess the relative
importance of the three other evolutionary mechanisms (mutation, gene
flow, and selection). For instance, equilibrium gene diversity in the
infinite alleles model is determined by
N
e
and the mutation rate (
u
)
such that
q
q þ
1
;
H
e
¼
(8.1)
4
N
e
u
.
51
Additionally,
where
selection coefficient),
change in allelic frequency is determined primarily by genetic drift rather
than selection.
47
Given these above relationships, it is clear why
N
e
is
an important parameter in conservation biology.
53
Indeed, conserva-
tionists are concerned about populations with small
N
e
because there is
lower genetic variation to respond to environmental change (i.e. lower
adaptive potential), the breeding of closely related individuals can reduce
the fitness of an outbreeding species (inbreeding depression), and
deleterious alleles can become fixed at low
N
e
.
1
e
3
All of the latter may
increase the chance for population extinction.
1
Of course, the latter is the
goal for epidemiologists. Consequently, from a disease management
if
N
e
s
<<
1(
s
q ¼
¼
