Biology Reference
In-Depth Information
2001). One, the long-winged cone-head (
Conocephalus discolor
), possesses
two forms, short-winged and long-winged. Localities colonized within
the past 20 yr show higher frequencies of the long-winged form than do
localities occupied for more than 20 yr. The evidence is strong that this
difference is at least partly genetic. The second species, Roesel's bush
cricket (
Metrioptera roeselii
), also has two forms, a flightless short-winged
form and a long-winged form capable of flight. The long-winged form
shows higher frequencies in more recently colonized localities. Similar
observations exist for ground beetles expanding their ranges northward in
southern Canada (Niemalä and Spence 1991). In these cases, it appears
likely that under climatic conditions favoring range expansion, the more
dispersive forms of these insects are favored. Under stable climates, the
more sedentary forms may be favored.We shall consider this relationship
in more detail in chapter 20.
For organisms dispersed over long distances by vehicles of transport
and commerce, strong selection exists for tolerance of the special condi-
tions of these agents. For example, broad salinity tolerance should favor
the survival of aquatic organisms during transoceanic voyages, during
which ballast water exchanges may occur (Ricciardi and Rasmussen
1998). Broad salinity tolerance should also favor establishment in areas
into which ballast water is ultimately discharged. Whether or not rapid
evolutionary responses have occurred as a consequence of such selection
is not yet known.
Rapid Evolution of Enhanced Establishment
Capability
Strong selection also exists for reproductive success by individuals that
have reached new, favorable environments. Genetic variants that have a
reduced reliance on outcrossing or on biotic pollination mechanisms are
likely to be favored in small, isolated populations. Gradual evolutionary
shifts from outcrossing to self-fertilization may be one of the evolution-
ary processes underlying the time lag phenomenon associated with the
rise of many introduced species to the status of seriously invasive species.
In several cases, introduced plant species have rapidly evolved higher
rates of self-fertilization (Brown and Marshall 1981). Smooth cordgrass
(
Spartina alterniflora
), introduced to San Francisco Bay in the mid-1970s,
has spread over open mudflats to form discrete, circular, clonal patches
with varying degrees of isolation from each other.This species is normally
strongly outcrossing in its reproductive mode. The San Francisco Bay
