Biology Reference
In-Depth Information
Despite the serious impacts of alien species, their dynamic evolution-
ary potential has received little recognition. In particular, systems of reg-
ulation and management of these species have failed to recognize this
potential. Alien species are classified as beneficial, harmless, or harmful,
based on an evaluation of a limited sample of individuals at a particular
time, with the usual result that they are either permitted or prohibited in
commerce. Little attention is given to the risk of evolutionary change by
alien species once they become established in a new region.
As we shall see, all species, both native and alien, are at all times sub-
ject to evolutionary pressures that may maintain an evolutionary status
quo or may lead to gradual or rapid change in genetic characteristics.
Freed, in many cases, from the constraints of gene flow from their parent
population and from biotic pressures of former enemies, alien species
acquire exceptional evolutionary opportunities. Because populations of
alien species have become established in new physical and biotic environ-
ments, however, they are subject to altered selection pressures that are
likely to bring about rapid evolutionary change. In their new environ-
ment, alien species may encounter close relatives from which they had
been isolated geographically. Hybridization with these relatives may
enhance their evolutionary potential. Furthermore, those aliens that
become abundant and highly invasive impose strong new evolutionary
pressures on the natives with which they interact.
The example of late blight of potatoes is only one of many cases in
which alien species pose threats to human interests through their evolu-
tionary potential. Strong anthropogenic selection pressures imposed by
pesticides, antibiotics, and environmental pollutants such as heavy metals
have long been known to induce resistance in plants, animals, and
microbes. Numerous plants have shown rapid evolution of resistance to
heavy metals in spoil heaps associated with mining activity (Macnair
1987). Literally hundreds of plants, animals, and disease organisms, many
of them alien to the regions involved, have evolved resistance to pesticides
(see, e.g., National Academy of Science 1986). More than 100 plants now
show resistance to herbicides, with more being recorded annually.
The potential for alien species in general to show rapid evolutionary
responses to other pressures, however, has been appreciated only in the last
quarter century. Numerous examples of such evolutionary responses are
now available.A recent survey (Reznick and Ghalambor 2001), for exam-
ple, documented 34 studies of rapid evolution in response to agents other
than pesticides, antibiotics, or pollutant chemicals. Most of these studies
involve evolutionary changes following the introduction of a species to an
environment with novel characteristics or to a new location. Some 18 of
