Environmental Engineering Reference
In-Depth Information
years) directional selection (Kinnison & Hendry 2001).
Examples include evolution of native species in
response to invasion (Strauss
et al
. 2006 ). Many of
these examples are native insects adapting to newly
introduced host plants, or native species evolving in
response to introduced predators, but also include evo-
lution of competitive ability between species in the
same guild (Mealor & Hild 2007 ; Leger 2008 ). Research
into
evolutionary shifts
in harvested animal populations
provides some of the best examples of contemporary
natural selection. For example, in response to strong
and consistent harvest pressure, traits such as
decreased body size and decreased time to maturity
have evolved in consistent ways in many species in
managed systems (Kuparinen & Merila 2007; Allen-
dorf
et al
. 2008). Thus, we should expect that contem-
porary evolution will occur in restoration projects over
time, although the rates and differences among popu-
lations or species are uncertain.
Not all novel pressures are likely to be directional in
nature. In particular, climate change predictions
include increases in variance, as well as changes in
means, of major climatic factors (Pimm 2009), and
disease or insect outbreaks can be cyclical in nature.
Such shifting conditions, where optimal phenotypes
vary over time, can result in fl uctuating selection
(Futuyma 2005). With insuffi cient genetic diversity,
populations can quickly go extinct when experiencing
fl uctuating selection as a result of compounding losses
of diversity and decreasing population sizes over time.
Phenotypic plasticity is one evolutionary consequence
of fl uctuating or unpredictable environmental condi-
tions (de Jong 1995), and has been observed to evolve
under contemporary selection pressures. For example,
there is evidence of increased phenotypic plasticity in
populations of invasive species introduced to new envi-
ronments (Richards
et al
. 2006), and similar evolution
of increased plasticity may be adaptive for native species
persisting in changing environments. While native
species are not moving across continents, the biotic and
abiotic environment may shift around them in dramatic
ways, and species and populations with the greatest
phenotypic plasticity may be the ones that remain.
are all useful means to directly assess the ability of
populations and communities to persist under novel
disturbances (Jump & Pe ñ uelas 2005 ; Reusch & Wood
2007). Through these experimental methods, one can
measure the strength of selection across a range of
environments as well as responses to selection, differ-
entiating the potential for adaptive phenotypic plastic-
ity from the need for contemporary evolution (Conner
2003; Etterson 2004). Assessment of the responses of
different populations to variable selection pressures
could identify a strategy that is likely to maximize
initial establishment as well as the likelihood of popula-
tion persistence in the future. For example, a close
examination of size selection among salmonid fry at
six introduction sites indicates geographic variation in
optimal size, correlated with environmental factors at
each site (Figure 21.1; Bailey & Kinnison 2010). In this
situation, it is possible that more successful establish-
ment of these endangered populations could be
achieved by tailoring the size of released individuals to
the direction of selection at each location.
Given the diffi culty of measuring the strength of
selection across environments, coupled with the diffi -
culty of precisely predicting future climate or invasion
scenarios, a restoration strategy employing a highly
variable founder population could be used to establish
populations in a wide variety of locations, with the
assumption that natural selection can favour appropri-
ate genotypes in particular environments. In addition
to concerns about outbreeding depression (Chapter 7),
another potential problem with this approach is that if
the population mean trait values are too far from adap-
tive peaks, rapid, directional selection may result in
selective sweeps, where genetic diversity is lost as genes
become fi xed in a population due to their physical
linkage to gene regions under selection (Barrett &
Schluter 2008). In essence, potentially valuable genetic
diversity can be lost because it occurs in individuals
that possess a single maladaptive trait. These individu-
als can be quickly purged from a population, leading to
the loss of all of their associated alleles, even if some
are neutral or benefi cial. An alternative to a maximum
diversity method is to fi nd natural populations with
trait frequencies near potential optima in altered
systems, and use an immunization approach to maxi-
mize survival of restored populations (Figure 21.2;
Schlaepfer
et al
. 2005). This alternative is similar in
concept to including genotypes from outside the
current climatic zones to prepare populations for
climate change (Rice & Emery 2003). Both strategies
21.2.3 Assessing genetic diversity,
applying knowledge
Manipulative climate change experiments, reciprocal
transplant studies and artifi cial selection experiments
