Biology Reference
In-Depth Information
in feeding morphology (Schluter and McPhail 1992, 1993). Many sets of
species differ in the number of gill rakers; those with the larger number
are plankton feeders and those with the smaller number are benthic feed-
ers. Recent invasions of freshwater rivers, lakes, and impoundments by
many marine organisms have set the stage for evolutionary adaptation and
speciation (Lee and Bell 1999).
In Tasmania, Australia, populations of
Galaxias truttaceus
have become
isolated in lakes that have been separated from streams in the last
3,000-7,000 years (Ovenden and White 1990). The stream
Galaxias
are
anadromous, spawning in autumn and spending 3 months at sea before
returning to streams.The lake
Galaxias
spawn in spring and are thus iso-
lated in space and time from their stream relatives.Although not the result
of recent invasion of new environments in the strict sense, silverside fish
(
Odontesthes
spp.) in coastal waters of southern Brazil show similar pat-
terns of rapid speciation (Beheregaray et al. 2002).
The threespine stickleback (
Gasterosteus aculeatus
), a widespread marine
and anadromous fish of the northern hemisphere, has invaded postglacial
freshwater environments in many locations. Many of these invasions have
given rise to incipient species that have differentiated in sympatry or para-
patry (McKinnon and Rundle 2002). For example, Schluter and McPhail
(1992) examined the pattern of distribution and divergence of marine
and freshwater sticklebacks in coastal British Columbia, Canada. The
marine form (
Gasterosteus aculeatus
) occurs in estuarine habitats. Since the
end of Pleistocene glaciation, about 13,000 yr ago, this marine form has
colonized many rivers and lakes. Many of the lakes became separated from
the ocean about 12,500 yr ago, but some may have been reconnected for
a period about 11,000 yr ago. In five lakes on islands in the Strait of Geor-
gia, two sympatric forms of sticklebacks exist, apparently reflecting inva-
sions dating from these two periods.These forms are best regarded as sep-
arate species, as they are strongly reproductively isolated, the incidence of
hybrid individuals being only about 1%. In addition, they are divergent in
morphology and feeding ecology, with one form being specialized for
planktonic and one for benthic feeding.
Anadromous and freshwater forms of the threespine stickleback occur
in many, perhaps hundreds, of streams and rivers that have become avail-
able only in postglacial times (McKinnon and Rundle 2002). In some
river systems, differentiated lake and stream forms exist.The differentiated
forms of these fish typically differ in multiple traits and appear to be the
result of divergent selection, rather than of genetic drift or founder effects.
