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Intl. J. of Mol. Ecol. and Conserv. 2012, Vol. 2, No.1, 1-7
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5
(outbreeding depression). Few studies have
demonstrated outbreeding depression as it requires
tracking beyond the F
1
generation. A study of song
sparrows (Marr et al., 2002), showed signs of
outbreeding depression in the F
2
generation, and
measures of fitness was low in the F
2
generation of
crosses of the tidepool copepod (
Tigriopus californicus
)
from different populations (Burton, 1990). However,
this effect of breaking up coadaptations is only
magnified if the genetic distance between the two
populations has increased greatly (Edmands, 1999).
Thus, the threat of outbreeding might not be very
serious in most wild populations since it takes many
generations in contrasted environments for genetic
distance to be significantly very large.
The reduction or increase of fitness in a population
after receiving immigrants also depends upon
interactions among several genetic and non-genetic
factors (degree of epistasis, demography, behaviour,
environmental etc.) (Tallmon et al., 2004). It might
therefore be difficult to predict whether any given
immigration event will effect genetic rescue especially
when conservation managers lack understanding of
the interactions between the genetic and non-genetic
factors. However, Gaggiotti (2003) reviewed studies
on plants such as
Lotus scoparius
,
Ipomopsis
aggregata
and
Silene diclinis
and concluded that
outbreeding depression may be common in the wild
but the potential benefits of outbreeding usually
outweigh the threats of outbreeding depression.
5 Extinction in metapopulation context
There has been some theoretical work showing that
metapopulation can be subject to
extinctions due to
genetic factors. Genetic variation can be lost through
population turnover. This would be more pronounced
when colonizing propagules are formed by individuals
from the same deme than from all extant demes
(Maruyama and Kimura, 1980). However, if habitat
patches differ in quality (the typical case in
source-sink metapopulations), then population
turnover not have large effects (Gaggiotti, 2003).
Moreover, sink populations can maintain a large
proportion of variation in the presents of migration.
The mutational meltdown theory was extended to
cover metapopulations (Higgins and Lynch, 2001)
using individual-based models with stochastic,
demographic, environmental and genetic factors. They
concluded that mutational meltdown may be a
significant threat to large metapopulations and would
exacerbate the effects of habitat loss or fragmentation
on metapopulation viability. However there is little
empirical evidence supporting predictions made in
these theories (Gaggiotti, 2003).
Conclusions
For the above discussed genetic threats, inbreeding
seems the most likely to exacerbate decline and hasten
extinction especially where the reduction in Ne has
been very great and under stressful environmental
conditions. Most of genetic threats take many
generations to be detected; caution must therefore be
taken when making conclusions from studies because
what may be insignificant for now might be a threat in
the future. In most systems we do not know the
threshold where fitness will be an imminent threat of
extinction. Also, selection intensity on particular
measures of fitness (or life history traits) can vary over
time and space, thus the cumulative effects of selection
on multiple traits will interact to produce overall fitness
effects. This implies that short-term studies of a few
traits might result in misleading conclusions.
The division between demographic, environmental
and genetic is artificial since extinction processes
often operate together and their synergy may have a
stronger impact, especially for populations of
intermediate sizes which were previously thought not
to be under extinction risk. For very small populations,
extinction risk is more influenced by demographic and
ecological stochasticity rather than genetic threats.
Neither genetic nor demographic factors per se are
responsible for most of the populations decline; they
only become important after populations have been
driven to very low levels, particularly by human
activities. Human disturbances such as poaching,
habitat fragmentation, introduction of invasive
organisms and pollution present the greatest challenge
to populations in the wild than genetic threats. In
populations that are less affected by humans (e.g.
Checkerspot butterfly), extinction still result from