Environmental Engineering Reference
In-Depth Information
importantly, matching habitat conditions (Vander
Mijnsbrugge et al . 2010) in combination with genetic
considerations explained in section 7.2.4 .
(a)
(b)
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7.2.2
Inbreeding
b
b
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100
As a result of restricted gene fl ow between populations,
individuals within a population become related over
successive generations and mating will occur between
individuals that are related by ancestry, a process called
inbreeding. Inbreeding results in a redistribution of
genetic variation. The fraction of individuals carrying
two copies of the same allele at any particular locus
(homozygotes) increases at the expense of individuals
carrying two different alleles at a locus (heterozygotes).
In other words, inbreeding leads to a loss of heterozy-
gosity. The latter is generally accompanied by inbreeding
depression , the lower reproductive fi tness of progeny
resulting from matings between relatives than of
progeny resulting from mating between unrelated
parents, for two reasons. First, populations accumulate
recessive mutations that have slightly deleterious
effects on fi tness. In heterozygotes, their negative effects
are masked by the presence of a dominant functional
allele; loss of heterozygosity leads to increased expres-
sion of the recessive deleterious alleles in a homozygous
state. Second, many benefi cial traits show a heterozy-
gote advantage (heterozygotes have trait values higher
than either of the homozygotes, overdominance ); loss of
heterozygosity abolishes this advantage.
Inbreeding depression is common. Crnokrak and
Roff (1999) show that inbreeding depression occurs in
90% of 157 examined data sets from natural popula-
tions, including many plants. Inbreeding depression is
not only associated with reduced offspring fi tness but
also with increased population extinction rate, as for
example shown for the annual evening primrose rela-
tive, Clarkia pulchella (Newman & Pilson 1997), and for
the shore campion, Silene littorea (Vilas et al . 2006 )
(Figure 7.3). These studies corroborate results from
stochastic computer simulations showing that inclu-
sion of realistic levels of inbreeding depression signifi -
cantly increases extinction risk (O'Grady et al . 2006 ).
Inbreeding depression is especially a problem in small
populations as the extent of inbreeding is inversely
related to N e , the effective population size , defi ned as the
number of breeding individuals that would result in
the same rate of inbreeding as observed in the census
population if that population had been 'ideal' (a popu-
80
80
60
60
ab
a
a
40
40
20
20
a
IL
IH
OH
IL
IH
OH
Figure 7.3 Effects of inbreeding and genetic variation on
population extinction risk and population size of surviving
populations in a reintroduction experiment of shore
campion ( Silene littorea ) in Spain. (a) Experimental
populations initiated with inbred plants and low genetic
diversity (IL, white bars) or inbred plants with enhanced
genetic diversity (IH, grey bars) had lower survival
probability than populations initiated from outcrossed plants
(OH, black bars). (b) Surviving populations initiated from
inbred plants with low genetic diversity (white dots) also
had lower population sizes in their second year than
populations initiated from outbred plants (black dots).
Symbols indicate different populations; vertical bars
indicate population means and different letters indicate
signifi cant population type differences. (Adapted from
Vilas et al . 2006 .)
lation with random mating, constant population sizes
etc.). N e is often roughly an order of magnitude smaller
than the actual census population size, for instance
due to fl uctuations in population size. This is because
the years with small population sizes disproportionally
contribute to an increase in the extent of inbreeding in
a population.
7.2.3
Genetic drift
Transmission of alleles from parents to their offspring
is essentially a random sampling process. Allele fre-
quencies will therefore fl uctuate from one generation
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