Biology Reference
In-Depth Information
ductive. Thus, it appears likely that an invasive, early reproducing geno-
type evolved in the southeastern United States.
Seed dormancy is another characteristic that has evolved rapidly in
several alien plants. Cheatgrass (
Bromus tectorum
), an annual, has invaded
open habitats throughout much of North America. For such a plant, ger-
mination must be timed to correspond with the season of assured mois-
ture availability, not just irregular rainfall events. Allen and Meyer (2002)
examined the germination behavior of cheatgrass from populations in
different environments in the western United States. Plants from the
Mojave Desert had strong dormancy and required prolonged after-ripen-
ing drying. In the Mojave, seeds ripen in June, but assured moisture is not
available until the following winter rains.Thus, the dormancy mechanism
prevents germination during occasional summer thunderstorms that do
not provide enough moisture for plants to mature. Plants from mountain
areas, on the other hand, show little dormancy.These plants mature in late
July, when both temperature and moisture conditions are often favorable
for immediate growth. These dormancy patterns are genetic and have
evolved in about 100 yr.
Differences in seed dormancy and germination have been found in
several other weedy plants.Variety
vulgaris
of common groundsel, in fact,
is derived from
Senecio denticulatus
ssp.
vulgaris
,from which it differs in
lacking seed dormancy (Moritz and Kadereit 2001). Canada thistle (
Cir-
sium arvense
) also exhibits ecotypes differing in seed dormancy and ger-
mination patterns (Moore 1975).
Dispersal Adaptation and Genetic Variability
Thus, we can see that a diverse set of characteristics can preadapt organ-
isms to be successful invaders of new geographical regions, especially with
the assistance of the rapidly expanding systems of international travel and
commerce. These transport systems, coupled with the massive human
modification of landscapes, place strong selection pressures on plants and
animals. Patterns of rapid evolution in response to these pressures are only
recently being recognized.
The evolutionary responses that dispersing and newly established
aliens are able to make depend on the genetic variability that exists among
this select group of individuals.The number of colonists, their geograph-
ical origins, the frequency of dispersal events, and the mode of reproduc-
tion all influence the evolutionary potential of an alien colonist.We shall
examine these important variables in the next chapter.
