Environmental Engineering Reference
In-Depth Information
this is called
intrinsic outbreeding depression
. For instance,
in crosses between individuals from different North
American populations of partridge pea (
Chamaecrista
fasciculata
) which have differentiated via co-adapted
gene complexes, the positive effects of heterosis are pro-
gressively counteracted by the negative effects of intrin-
sic outbreeding (Fenster & Galloway 2000). In contrast,
in many species such as the rare whorlded sunfl ower
(
Helianthus verticillatus
), the positive effects of heterosis
continue in subsequent generations without any sign
of outbreeding depression. The importance of out-
breeding depression is still much debated and is cur-
rently on the top priority list for conservation genetics
research (Frankham 2010 ).
Genetic rescue has been extremely successful in a
number of animals (Hedrick & Frederickson 2010),
including adders (
Vipera berus
), greater prairie chick-
ens (
Tympanuchus cupido pinnatus
), Mexican wolf (
Canis
lupus baileyi
) and Florida panther (
Puma concolor coryi
),
as well as in plants (Tallmon
et al
. 2004 ). Hedrick
(2005) emphasizes, however, that genetic restoration
involves more than just genetic rescue. Genetic rescue
focuses on removing detrimental variants that increase
in frequency in small isolated populations due to
genetic drift, whereas the more comprehensive process
of
genetic restoration
should not only focus on removing
detrimental variation, but also consider how increased
gene fl ow affects the other two types of variation:
neutral variation and adaptive variation. Firstly,
migrants may quickly spread in the population,
increasing their representation in the gene pool and
reducing the amount of neutral genetic variation that
might be adaptive in future environments. Secondly, a
few successful migrants may overwhelm selection that
would otherwise maintain adaptive variants, reducing
their frequency, a process called
genetic swamping
. For
instance, in experimental populations of the fi eld
mustard (
Brassica campestris
), one immigrant per pop-
ulation increased fi tness compared to no-immigrant
controls (Newman & Tallmon 2001 ). Higher levels of
immigration also increased fi tness but led to less phe-
notypic divergence than with one migrant per genera-
tion, suggesting that lower immigration is suffi cient to
increase fi tness without impeding the facilitation of
local adaptation. An example of genetic restoration in
animals is that of the Florida panther (Hedrick & Fre-
drickson 2010), the last surviving puma subspecies in
eastern North America, that had in recent times nearly
gone extinct in the state of Florida. Genetic restoration
guidelines were to introduce eight pumas from a popu-
lation in Texas that formerly had been connected to the
Florida population by gene fl ow, to achieve an initial
circa 20% ancestry from introduced Texas panthers,
followed by one migrant per subsequent generation to
eliminate detrimental variants while preventing the
elimination of adaptive variants and increase the levels
of neutral variation. Even though eventually no sub-
sequent migrants were introduced, the admixture
resulted in increased fi tness, a doubling of heterozygos-
ity, a decline in inbreeding correlates and a threefold
increase in population size (Johnson
et al
. 2010 ).
7.2.5
Corollaries and consequences
The success of genetic restoration programmes in the
past two decades has shifted our view regarding the
risk of introducing foreign genetic material for restora-
tion. Although outbreeding depression is a real phe-
nomenon, its negative effects often do not outweigh the
positive effects of heterosis, not only in fi rst-generation
hybrids, but also far beyond that. This has sparked
renewed attention for the positive effects of low levels
of gene fl ow for fi tness and adaptive evolution and has
provided faith in genetic restoration as a management
tool if properly applied. Hedrick and Fredrickson
(2010) recently devised 10 guidelines for successful
implementation of genetic restoration, four of which
specifi cally aim at minimizing the risks associated with
admixture by continued management schemes that
adjust the extent and number of repeated introduc-
tions. It is also worth taking a step back and asking
whether the risk that intentional admixture will reduce
population fi tness is always the most relevant question
to consider in an ecological restoration programme.
After all, preserving or reintroducing populations with
suboptimal fi tness may in many cases be preferred over
the fi nal absence of populations that would result if no
restoration at all is undertaken (cf. van Andel 1998;
Kiehl 2010 ).
7.3
METAPOPULATIONS
Population processes take place in landscapes consist-
ing of networks of habitat patches and, therefore,
disciplines that explicitly link local abundance to
regional occurrence are necessary to understand
species abundance patterns at larger scales and how to
restore them. Metapopulation ecology and genetics
