Suppose the A 1 is a recessive, its effect being masked by A 2 when the two occur together
at the locus. If additionally A 1 is slightly deleterious when its effect is expressed, it
can have damaging effects on fecundity and survival of the population if its frequency
increases by the statistical luck of random pairing of alleles in each generation. The
gene pools of most populations contain many of these sublethal recessives, about enough
to kill an individual three times over if by chance they all occurred on its chromo-
somes in homozygous form and were therefore all expressed in its phenotype. Thus
a decline in heterozygosity tends to lead to a decline in fitness.
Genetic malfunction may follow as a consequence of small population size. The
following sequence may be triggered if a population becomes too small:
1 The frequency of mating between close relatives rises and random genetic drift
increases,
2 which leads to reduced heterozygosity in the offspring,
3 which exposes the effect of semi-lethal recessive alleles,
4 which reduces fecundity and increases mortality,
5 which causes the population to become smaller yet, and that trend may continue
until extinction. The population must be held at low numbers for several genera-
tions before that process is initiated. A short bout of low population size has little
effect on heterozygosity.
Loss of fitness during inbreeding can be traced largely to the process of fixation (i.e.
reduction of alleles at a locus to one type) of deleterious recessive alleles. In mam-
mals, mortality is 33% higher for the offspring of parent–offspring or full sibling
matings than for the offspring of unrelated parents (Ralls et al. 1988). Hybrid vigor
is largely the reverse, the masking of the effect of those recessives; but it might also
contain a component of heterosis where the heterozygote is fitter than either
homozygote.
Inbreeding does not automatically lead to inbreeding depression. It is seldom reported
for populations larger than a couple of dozen individuals. Nor does low hetero-
zygosity necessarily lead to inbreeding depression. Note that the average individual
of most wild populations is heterozygous at less than 10% of loci. A population that
has survived a bout of inbreeding may come out of it with enhanced fitness because
inbreeding exposes deleterious recessives and allows them to be selected out of
the population. That is precisely the method used by animal breeders to remove
deleterious alleles. Homozygosity causes an immediate problem only when the allele
is deleterious. Nonetheless inbreeding often does produce inbreeding depression. That
possibility must always be kept in mind if a population is small. Section 17.6
addresses the question: how small is too small?The cheetah (Acinonyx jubatus) has a low level of heterozygosity (O’Brien et al. 1983).
Stephen O’Brien and his colleagues (O’Brien et al. 1985b, 1986) and Cohn (1986)
studied the genetic variance of captive populations of that species originating from
southern Africa. A standard electrophoretic analysis of 52 loci (n=55 individuals)
discovered a heterozygosity value of H=0.00 as compared with H=0.063 for
people and 0.037 for lions. A more refined “two-dimensional” electrophoretic
analysis, separating the proteins first by electrical charge (as above) and then by
molecular weight, uncovered rather more variability and yielded H= 0.013 for
cheetahs as against 0.024 for people analyzed by the same method. A further
sample from East Africa returned H=0.014 (O’Brien et al. 1987).296 Chapter 17
17.3.5Inbreeding
depression
17.3.6How much
genetic variation is
needed?