Environmental Engineering Reference
In-Depth Information
21.2 LOCAL PROCESSES:
ADAPTATION AND SELECTION
relations between population-level response to selec-
tion and levels of heritable variation (Fisher 1930).
Motivations for using local genotypes in restoration
vary along a spectrum from purely ideological to purely
practical (see also Chapters 7 and 8). The ideological
perspective is that restoration should maintain the
suite of genetic variation historically occupying a par-
ticular site; local genotypes and their evolutionary
history should be preserved because of their inherent
value (Hamilton 2001). The practical end of the spec-
trum holds that because natural selection can operate
to create populations of locally adapted species, resto-
ration using local genotypes should, on average, be
more successful than restoration using nonlocal geno-
types (McKay
et al
. 2005). While our view leans
towards the practical end of the spectrum, both per-
spectives may need to be refi ned if restoration is going
to address species persistence in the face of rapid envi-
ronmental change. Natural selection may well have led
to populations that are locally adapted under historic
conditions, but the persistence and/or superior per-
formance of local genotypes under future conditions
are largely unknown (Harris
et al
. 2006 ).
21.2.2
Disturbance and natural selection
In addition to the amount of genetic diversity present
in a restored system, it is important to consider the type
of natural selection the population will experience.
Certain types of anthropogenic disturbances are likely
to result in selection pressures that are consistent and
predictable, such as increases in CO
2
concentrations,
consistent size selection in harvested populations, or
the introduction of new diseases, predators, prey or
competitors. Consistent selection pressure can result in
directional selection, which occurs when fi tness is con-
sistently highest for individuals with traits values that
are either larger or smaller than current population
means (Futuyma 2005 ).
Adaptive phenotypic plasticity
,
or the ability to modify a phenotype in an adaptive way
in response to environmental conditions (Pigliucci
2001), is perhaps the simplest way species can persist
under strong directional selection. While selecting
genotypes with a high degree of plasticity for restora-
tion projects may allow greater tracking of environ-
mental change, there are limits to phenotypic plasticity,
and costs to its maintenance, that may complicate
long-term adaptive species responses (Ghalambor
et al
.
2007). For example, Phillimore
et al
. (2010) demon-
strate that even though populations of
Rana temporaria
are phenotypically plastic in their spawning time, plas-
ticity alone is likely insuffi cient to maintain viable
breeding populations in Britain under climate change
scenarios (Plate 21.1). In cases where phenotypic plas-
ticity is insuffi cient to maintain viable populations,
additional evolution
(change in gene frequencies) will be
necessary to maintain local populations under dis-
turbed conditions. In the case of
R. temporaria
, natural
or human-assisted migration of individuals from
southern to northern locations could speed the process
of evolutionary change in northern Britain, but south-
ern populations border the English Channel, and any
migratory process would almost certainly require
human intervention.
In a population with suffi cient genetic variation,
populations might be able to evolve and remain viable
without human intervention, even if conditions are
shifting rapidly. There is evidence that natural selection
can result in the maintenance, rather than extirpation,
of some local populations under contemporary (< 100
21.2.1
Maintaining evolutionary potential
Multiple restoration actions in response to changing
future conditions have been proposed, including
assisted migration
, wherein species are moved
outside their historic range (McLachlan
et al
. 2007 ),
increasing the amount of diversity in populations of
restored ecosystems by including genotypes outside the
current range (Rice & Emery 2003), the use of artifi cial
selection to create adapted populations (Jones &
Monaco 2009) and the use of natural populations from
altered sites to restore under similarly altered condi-
tions (Leger 2008). Whichever option is selected, it is
important to consider not only the contemporary
success of each method, but also the capacity of popu-
lations with different genetic composition to respond to
future challenges. Rather than reintroducing only a
historic suite of local genotypes, or only genotypes with
the greatest capacity for success under current condi-
tions, ecologists are recognizing that a new goal may
be to create populations that have the capacity to evolve
in response to uncertain future conditions. Maintain-
ing diversity in restored systems is the fi rst step towards
retaining
evolutionary potential
, as there are direct cor-
