Biology Reference
In-Depth Information
assessment of the values and their confidence intervals (
Table 8.1
) does
not reveal any discernible impact of chemotherapy on the
N
e
of these
Ascaris
subpopulations (even if houses with small
n
are examined). These
genetic results parallel prior epidemiological data from Jiri where after 1
year of treatment both prevalence (year 1
24.2%) and
mean number of worms expelled per individual (2.37 and 2.67) showed
little change.
62
The latter scenario also begs the question of whether
N
e
reflects
N
c
of
Ascaris
in Jiri. Intuitively, as
N
c
increases, so should
N
e
. However, I caution
that the relationship between
N
e
and
N
c
under different demographic
scenarios is generally not well understood and may vary considerably
among species.
69
In some free-living organisms, the ratio of
N
e
/
N
c
decreases as population density increases.
69
Experimental data in flour
beetles suggest this may be caused by an increase in the variation in
reproductive success among individuals as
N
c
increases.
70
Therefore, it
might be that there is an asymptotic relationship between
N
e
and
N
c
such
that
N
e
levels off even as
N
c
gets larger (
Figure 8.2
). The latter relationship
would be important in epidemiological studies because a drop in
N
c
may
not constitute a drop in
N
e
until a critical
N
c
is reached. This would be
crucial in terms of the evolution of drug resistance because a huge
selection pressure via chemotherapy could be imposed on the population
without a drop in
N
e
. Selection is more efficient with larger
N
e
. Thus, both
worm count and genetic data are warranted in epidemiological studies if
27.2%, year 2
¼
¼
FIGURE 8.2
Hypothetical asymptotic relationship between
N
e
and
N
c
. Dotted line
denotes critical
N
c
where
N
e
no longer substantially increases as
N
c
increases. One possible
explanation for this pattern is that as population density increases, variation in reproductive
success may increase considerably, thus substantially reducing
N
e
. An important epide-
miological implication is that above this critical point,
N
c
could be reduced drastically
without a dramatic effect on
N
e
. In relation to the
Ascaris
data presented, the correlation
between household intensities and the single-sample
N
e
estimates may suggest
Ascaris
subpopulations already exist below the critical
N
c
value.
