Biology Reference
In-Depth Information
considered variety
hibernicus
of common groundsel, with a diploid num-
ber of 40.The hybrid has also experienced chromosome doubling, giving
rise to a new groundsel,
S. cambrensis
, with a diploid chromosome num-
ber of 60 (Ashton and Abbott 1992).
Speciation can also occur without hybridization. New species of plants
may arise by evolutionary change in a single parent species, without alter-
ation of chromosome number. Divergence of allopatric populations due
to selection for different characteristics is perhaps the most frequent pat-
tern of speciation. Such change can also occur in sympatry. Smooth cord-
grass (
Spartina alterniflora
), native to the eastern coast of North America,
was introduced to San Francisco Bay in the 1970s (Daehler et al. 1999).
This species has spread aggressively in the upper intertidal zone of the bay
area. In 1991, a dwarf form of cordgrass, reaching only about one-fifth the
height of typical smooth cordgrass, was found in several locations. The
dwarf form appears to be identical to smooth cordgrass in chromosome
number and genomic DNA markers. Thus, it may be interpreted most
conservatively as a dwarf ecotype of smooth cordgrass. Its distinctive
growth form, lack of plants of intermediate stature, and restriction to a
higher tidal zone than typical smooth cordgrass suggest that it might rep-
resent a distinct species.
Recent speciation may also be occurring in green algae of the genus
Caulerpa
.Two forms of
Caulerpa
have long been present in the eastern
Mediterranean Sea. One,
C. racemosa
var.
turbinata
, has been known since
the mid-1920s, and the second,
C. racemosa
var.
lamourouxii
, appeared in
the 1950s. Both, restricted to relatively warm waters, are noninvasive and
were probably introduced in the ballast water of cargo ships (Verlaque et
al. 2000). Beginning in 1984, however, an invasive form,
Caulerpa taxifo-
lia
, appeared and has spread rapidly throughout the Mediterranean Sea,
including the colder waters of the western basin. This form has subse-
quently colonized coastal waters in southern California and along the
coast of Australia near Sydney (Meusnier et al. 2002). This invasive form
reproduces vegetatively to produce dense mats over submerged substrates,
crowding out other organisms.
The origin of this invasive form in the Mediterranean has been the
object of speculation. It was once believed to be a hybrid between
Caulerpa racemosa
var.
turbinata
and an unknown tropical form of
Caulerpa
(Durand et al. 2002), but it now seems certain that it originated in aquar-
ium culture in Monaco and was carelessly released into ocean waters
(Meusnier et al. 2002). Recent analyses of nuclear and chloroplast DNA
of the various forms of
Caulerpa
,however, indicated that the invasive form
