9643

• UCREFRP Catalog Number: 9643
• Author: Seener, J. W.
• Title: Inbreeding Depression and the Survival of Zoo Populations
• USFW Year: n.d.
• USFW - Doc Type: 209-217
• Copyright Material: YES

100

NUMBER
OF OFFSPRING

C)
z
:>
~
~ 50
I-
Z
W
o
II:
W
0..

3

5

FIGURE 2. The sensitivity of the
model to changes in fecundity
(mean number of viable oft'spring
per female).

o

20
GENERATION

40

of offspring to survive to maturity as a function of the offspring inbreed-
ing coefficient. If a straight line is fitted to a plot of the inbreeding coeffi-
cient versus the negative logarithm of the proportion of the offspring
surviving to reproductive age, the slope of this line (Bt) is a measure of
the intensity of inbreeding depression. Hence, it is a measure of the ge-
netic load of the source population. I assume a linear relationship because
it is expected theoretically (Morton, Crow and Muller, 1956), although
Kosuda (1972) has found a concave upward curvature to the relationship
between the negative log of survival and inbreeding in Drosophila (a dis-
couraging sign for the future of captive populations if it is generally true).
Torroja (1964) has less extensive evidence of a concave downward curva-
ture to the inbreeding-survival relationship in Drosophila (an encourag-
ing sign consistent with Slatis' (1975) view that if a population survives
an initial episode of inbreeding, major problems with further inbreeding
depression will probably not arise). Schull and Neel (1965) found no sig-
nificant departures from linearity in a large human sample, but the low
levels of inbreeding in their sample also limited their ability to detect
departures from linearity.
Figure 3 shows the effects of variation in viability depression on sur-
vival of captive populations. The line labeled 1.0 shows i;he viability de.
pression level used as a standard for all of the calculations whose results
are shown in other figures. Extinction is almost certain within 25 genera-
tions. The remaining lines show the effect of variation in Bl over a range
of likely values. This range can be compared to a number of published
estimates of Bl (recalculated to give comparable measures). A half dozen
species of Drosophila have Bl values ranging from 0.23 to 1.8 (Mettler

212

SENNER/CHAPTER 12
INBREEDING DEPRESSION AND THE SURVIVAL OF ZOO POPULATIONS

Gregg, 1969). Viability depression in four kinds of fowl range from
in the domesticated chicken to 3.74 in the Hungarian chukar par-
dge (Abplanalp, 1974). The mean of 10 studies on Japanese humans is
7 (Schull and Neel, 1972). Slatis (1960) found the low value of 0.06, not
'ficantly different from zero, in the wisent.

~ecundity Depression

Inbred animals are more likely to be sterile than outbred animals and
. bred mothers are poorer mothers than outbred mothers. Both factors
l~uce the ultimate fecundity of a pair of animals as a function of the
{evel of parental inbreeding. Figure 4 shows the probability of population
llU1'Vival for three values of fecundity depression. The value of 0.5 for B2
(Obtained in a similar fashion to B1) was used as a standard o~ all other
figures. In comparison, Abplan~p (1974) found ~at the .coefficIent relat-
ing depression of egg hatchabIlity to maternal mbreeding ranged from
0.12 for chickens to 0.55 for the chukar partridge. Studies on Japanese
humans give values of 0.5 and 0.6 for B2 (Schull and Neal, 1972; Tanaka,

1977).

100

100

VIABILITY
DEPRESSION

(!)
z
:>
:>
II:
~ 50
I-
Z
W
U
II:
W
0..

50

1.5

o 20
GENERATION

30 60 90
GENERATION

FIGURE 3. The sensitivity of the
blodel to changes in the level of
ViabUity inbreeding depression.

FIGURE 4. The sensitivity of the
model to changes in the level of
fecundity inbreeding depression.

213