Biology Reference
In-Depth Information
become strong agents of selection for altered modes of reproduction and
dispersal.The intimate association of weeds and other pests with crops and
domestic animals, for example, is favored not only by the care that humans
give to these domesticated species, but also by the enhanced probability
that weed and pest progeny will be transported to new world regions
(Gould 1991). Natural selection is constantly acting to fine-tune adapta-
tions for dispersal by natural and human agency (Dieckmann et al. 1999).
Adaptation of Alien Species to New Environments
Alien species show diverse patterns of evolutionary adaptability when
they arrive in new geographical regions. Each has a particular pattern of
genetic variability that influences its potential for evolutionary change. In
some cases, variability may be very limited due to the small number of
individuals in the founding population. Genetic drift in small initial pop-
ulations may further reduce variability by genetic bottlenecking. The
genetic composition of a new population may be a biased sample of vari-
ability in the source population, the so-called founder effect.Thus, evolu-
tionary responses may be constrained by lack of variability in many cases.
On the other hand, many colonizers, especially those of early succes-
sional or disturbed habitats, arrive with high phenotypic and genetic
adaptability. Founder populations of crop weeds and arthropod pests may
consist of many individuals, carrying much of the variability of their
source populations.The potential for such species to evolve races adapted
to local conditions has long been recognized (Baker 1974). Aquatic
organisms, carried in large numbers in ballast water of cargo ships, may
also possess high genetic variability. Many other aliens, especially plants,
have been introduced deliberately and in abundance, and their high
genetic variability is guaranteed by multiple introductions and sources.
Alien species may also acquire genetic variability after their arrival.
Many weedy plants, for example, are able to hybridize with closely related
crop plants. Such hybridization may provide a source of new genetic vari-
ability, including transgenes that have been introduced by genetic engi-
neering. In addition, complex patterns of hybridization may occur among
species that were once isolated geographically but have been brought
together in new regions or introduced to regions where they have close
relatives among native species. In addition, the genetic composition of
alien plants and animals is increasingly being influenced by multiple intro-
ductions that bring together genetic races from different parts of their
native range.The hybrids of such races may show greater genetic variabil-
ity than individual races in the native region.
