Biology Reference
In-Depth Information
season, and some clones experienced 100% mortality. The Willapa Bay
plants also exhibited levels of leaf nitrogen about 70% higher than
those of the other two populations and were highly preferred by
leafhoppers.
It is likely that the higher leaf nitrogen levels of Willapa Bay plants are
at least partly due to a founder effect—the initial plant that became estab-
lished in 1894 was by chance a high-nitrogen genotype. But the lower
resistance of many Willapa Bay plants to leafhopper herbivory is more
likely the result of genetic drift or natural selection (Daehler and Strong
1997b). Willapa Bay plants also showed slower growth and shorter, thin-
ner stems and leaves than plants from the other populations, suggesting
that selection has favored these characteristics at the expense of resistance
to
Prokelesia
herbivory.Thus, in a little more than a century, smooth cord-
grass had adapted to new habitat conditions characterized by reduced
herbivore pressure.
Rapid Evolution to New Habitat Conditions
Invasion of a new environment by a species is an experiment in evolu-
tion.A subset of genotypes of a species is introduced into an environment
that is similar in some physical and biotic features, but different in many
others. Just as natural selection tends to promote genetic adaptation of
populations to local conditions within the native range, the same process
tends to occur in the new alien environment. In most cases, the new alien
population has a high degree of spatial isolation from populations in the
native region, precluding strong gene flow that might offset the action of
natural selection under the new conditions.
Given adequate genetic variability, invading species thus are often able
to make rapid evolutionary adjustments. In a laboratory selection exper-
iment with plants, for example, velvet-leaf (
Abutilon theophrasti
) and green
amaranth (
Amaranthus retroflexus
) showed significant genetic shifts in just
four generations of selection under treatments involving different soil
moisture levels, competitor diversity regimes, and intraspecific densities
(Zangerl and Bazzaz 1984). Shifts involved many biological features: veg-
etative structure, photosynthetic rate, flowering phenology, and reproduc-
tive allocation. In field experiments, tall fescue (
Festuca arundinacea
) culti-
vars exhibited significant shifts in patterns of genetic variation in only
3 yr of selection under grazing and nongrazing regimes (Vaylay and van
Santen 2002).
