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ranges into new regions in many cases involve multiple adaptive traits and
multiple genes (Reznick and Ghalambor 2001). The capacity for evolu-
tionary change in areas where competition is low and the potential for
rapid population growth high is considerable. Under these conditions,
strong directional selection by conditions of the physical or biotic envi-
ronment can cause rapid genetic change. Invasions by alien species thus
carry two potentials: a short-term threat of extinction and a longer term
promise of speciation.
Alien Species, Evolution, and Conservation
A major issue now exists in defining what is and what is not an alien.
Global climate change has now permitted many species to extend their
ranges into regions that formerly were climatically unsuitable. Should
species be regarded as aliens if they invade new areas in response to
regional climatic change that makes such new areas suitable? Considering
such species alien fails to recognize that range shifts in response to chang-
ing environmental conditions have always occurred, and thus are, in a
sense, natural. Should the goal of conservation be to confine species to
ranges defined at some arbitrary date, even in the face of changing cli-
mate? Such a strategy could lead to extinction of species if environmen-
tal change exceeds their ability for evolutionary adaptation. A modified
view of what constitutes an alien will certainly be necessary.
Rapid evolution in alien environments also has major conservation
implications. For example, populations of endangered species increasingly
are being held in captivity or translocated to areas of habitat that are con-
sidered safe, the objective being to maintain populations that can be used
to reintroduce individuals to their native areas at a future time. Rapid evo-
lutionary change in such “safe” habitats, however, may reduce the suitabil-
ity of the population for reintroduction to their original native habitats
(Conant 1988).The ubiquity of rapid evolutionary change by populations
introduced to new environments means that the genetic composition of
endangered species must be managed as a reserve of evolutionary poten-
tial, not as gene pool locked in time (Stockwell et al. 2003).
The rapid rate of introduction of species to new geographical regions
has opened a Pandora's box of evolutionary change. From the perspective
of human interest, many of the evolutionary responses of aliens them-
selves are detrimental. Responses of native species, on the other hand,
tend to mitigate some of these detrimental impacts.Thousands of uncon-
trolled experiments in evolution thus are being played out, giving theo-
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