9684

• UCREFRP Catalog Number: 9684
• Author: Soule, M. E.
• Title: Chapter 9 - Thresholds For Survival
• USFW Year: n.d.
• USFW - Doc Type: Maintaining Fitness and Evolutionary Potential.
• Copyright Material: YES

percent decline in a particular reproductive trait (Frankel and Soule, in
press). Note that an f equal to 10 percent approximates the amount of
inbreeding that would theoretically occur in a population of five adults
breeding at random for a single generation, or-using the formula
f = 1 - (1 - 1I2N;l where t is the number of generations-in a popula-
tion of 25 adults breeding at random for 5 nonoverlapping generations. A
5 or 10 percent decline in fecundity might not appear to be very serious,
but if the effects of inbreeding depression on the other traits (such as
viability) are also considered, this amount of inbreeding can lower repro-
ductive performance as a whole by 25 percent (Abplanalp, 1974, for exam-
ple). Inbreeding depression can be very potent indeed.
Another way of looking at the effects of close inbreeding is in the sur-
vival of inbred lines. Table III summarizes data from a variety of orga-
nisms. A priori, it is expected that survival will depend on and be
inversely related to the load of deleterious mutations in the stock popula-
tion. Everything else being equal, this load should be greatest in stocks
lacking a history of inbreeding or artificial selection, because such lines
have not been purged of any of their deleterious genes. It is not surpris-
ing, therefore, that the Japanese quail (Cotumix cotumix), the least do-
mesticated of the birds in Table III, show the most inbreeding
depression.
Considering the above results, it is obvious that conservationists
ought to view inbreeding as anathema. Although inbreeding will purge a
stock of some of its deleterious genes, most lines do not survive a bout of
inbreeding of this intensity. Those that do survive are typically handi-
capped by lower fitness and by random changes in morphological, physio-
logical and behavioral traits. It rarely, if ever, happens that an inbred line
is equal in overall vigor or fecundity to its outbred control, particularly in
a natural setting. Therefore, to purposely inbreed one of the last stocks of
an endangered species, on the off chance that it will be "cleansed" of its
genetic load, is the height of folly.
This fiat does not apply with equal force to species which normally
inbreed. Young and Murray (1966) observed that the degree of inbreeding
depression in domesticated plants is correlated with the usual amount of
cross-pollination; in order of increasing cross-pollination and inbreeding
depression (genetic load) these plants were barley, cotton, tomato and
com.

The Basic Rule of Short-Term Survival

The data in the preceding two sections may motivate one to ask how
to best and safely manage populations in order to minimize the loss. of
fitness, but such data do not provide a basis for specific recommenda-
tions. It is as if we knew that a particular substance was poisonous but

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