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Intl. J. of Mol. Ecol. and Conserv. 2012, Vol. 2, No.1, 1-7
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4
that the important effect of inbreeding depression lies
with its tendency to aggravate the consequences of
environmental downturns. However other studies
showed evidence for direct connections of inbreeding
with population decline, for example in Glanville
fritillary butterfly large metapopulation, extinction risk
was positively related to inbreeding even after
accounting for ecological factors (Saccheri et al., 1998).
The above discussed evidence shows that inbreeding
depression is common even in wild populations and
can be a short term extinction threat especially if
populations are subjected to environmental stress or
rapid population decline. The severity of inbreeding
and genetic variability loss can be reduced by
immigration of a few individuals into a population -
what has been termed the rescue effect. In the absence
of such migrants, inbreeding depression can contribute
to the driving of populations to an extinction vortex.
3Accumulation of mildly deleterious mutations
In stable environments, mutations with phenotypic
effects are usually deleterious since populations tend
to be well adapted to their environment (Gaggiotti,
2003). Random mutations are likely to disrupt such
environmental adaptations. Selection is efficient in
eliminating detrimental mutations (with large effects
on fitness) in when
N
e
is large or moderate. Mild
deleterious mutations with selection coefficient s≤1/2
N
e
behave as neutral mutations and are therefore difficult
to remove (Wright, 1931). When
N
e
drops to a new
value, very small deleterious mutations begin to
accumulate after approximately 4/
N
e
generations and
can rapidly drive populations to extinction after
N
e
<100-1000 (Kondrashov, 1995). In small
populations, selection is hampered and this increases
the role of genetic drift thereby increasing the chance
fixation of some of the deleterious alleles from
mutations. This result in reduced fitness for the
population, which might eventually lead to extinctions
(Muller, 1964). Previously, this was thought to be a
problem only in obligately asexual species as there is
no recombination (offspring will have parent
mutations as well as newly arisen mutations) (Muller,
1964), but sexual species are also at risk of extinction
due to mutation accumulation (Lande, 1994). If this
process repeats, mutations will accumulate and there
will be further declines in fitness and population size
forming a positive feedback mechanism-a process
called mutational meltdown (Lynch and Gabriel,
1990). Recombination in sexual species can slow
down mutational meltdown to some extent, but they
are not entirely immune from it (Gaggiotti, 2003).
Empirical evidence for mutational meltdown is scarce
for wild populations, and this threat might have been
overestimated as an artifact of how the mutation
effects on mean fitness has been modeled (Poon and
Otto, 2000). In some experiment with yeast
(
Saccharomycetes cerevisiae)
, Zeyl et al (2001) used
12 replicates of 2 isogenic strains of yeast with
genomewide mutation rates that differed by 2 orders
of magnitude to demonstrate mutational meltdown.
They used an effective population size of about 250
and after more than 100 daily bottlenecks; the yeast
with higher mutation rates declined in size and had
two extinctions while the wild type remained constant.
These results support the mutational meltdown model
(Zeyl et al., 2001), but it has been criticized because
of controversies in measures of per-genome mutation
rates and mean fitness cost per mutation. These
measures are thought to be small (Garcia-Dorado et al.,
1999) which makes mutational meltdown less likely
or less important for most species.
Meltdown models ignore the effect of beneficial and
backward mutations. Consideration of these mutations
might imply that only very small populations would
face the risk of extinction due to genetic stochasticity
(Poon and Otto, 2000). Also, new mutations may be
compensatory or suppressive, which might restore
fitness losses incurred by previous mutations without
requiring true reversals (Kimura, 1990). Thus
currently it is impossible to give clear evaluation of
the importance of meltdown process.
4 Local extinction in the presents of migration
Inbreeding depression is normally reduced by
immigrants that are heterozygous for deleterious
recessive mutations (Whitlock et al., 2000), and by
heterosis mean fitness of populations may be
enhanced. However, outcrossing can reduce mean
population fitness if hybridisation disrupts coadapted
gene complexes or favourable epistatic interactions