Biology Reference
In-Depth Information
later, in 1950, a third whitefish was introduced from Lake Neuchâtel.This
form initially differed only slightly in gill raker number.Within only a few
generations, however, the number of gill rakers of this last form had
diverged significantly from that of the fish already present. Several other
transplanted whitefish in Europe and North America have also exhibited
changes in gill raker characteristics over short intervals (Lindsey 1981).
Unfortunately, detailed documentation of the degree of genetic basis for
such shifts is lacking, although other studies have shown that gill raker
structure has a strong genetic basis.
The pumpkinseed sunfish (
Lepomis gibbosus
) offers one of the best
examples of evolutionary change in habitat and resource use (Robinson
and Wilson 1996; Robinson et al. 2000). Pumpkinseeds and bluegill sun-
fish (
Lepomis macrochirus
) coexist in lakes in midwestern North America,
but bluegills were originally absent from lakes east of the Appalachians
and north of the St. Lawrence River. In lakes where bluegills have been
absent, pumpkinseeds exhibit pelagic and littoral ecotypes. The pelagic
forms have more slender bodies, smaller pectoral fins, and more closely
spaced gill rakers than littoral forms. These characteristics are adaptive in
relation to feeding on zooplankton. The littoral forms of the pumpkin-
seed feed largely on benthic arthropods and mollusks. Bluegills now have
been introduced to many lakes where they have recently come into con-
tact with pumpkinseeds. In these lakes, pumpkinseeds have become
restricted to the littoral zone and prevented from feeding on zooplankton
(Robinson et al. 2000).
In many lakes, bluegill sunfish themselves show pelagic and littoral
forms that differ in morphology and behavior (Ehlinger and Wilson
1988). The degree to which these differences have a genetic basis is
uncertain.
In other instances, fish have evolved distinct races adapted to freshwa-
ter lake and stream habitats in coastal areas released from Pleistocene
glaciation (Moodie 1972; Hendry et al. 2002).The threespine stickleback
(
Gasterosteus aculeatus
), many populations of which occur in coastal
marine waters, has established purely freshwater populations in many
locations deglaciated within the past 15,000 yr. These populations have
adapted to freshwater habitats in morphology and behavior and in many
cases have developed divergent ecotypes (Foster 1995). On coastal islands
and the mainland of British Columbia, Canada, pairs of incipient species
adapted to lake versus stream or benthic versus open-water habitats have
evolved in several locations (McPhail 1994). On Vancouver Island, British
Columbia, Canada, for example, sticklebacks from Misty Lake differed in
