Biology Reference
In-Depth Information
Most also show some degree of hybridization as well as a strong degree
of assortative mating because of differences in habitat preferences or habi-
tat-related courtship traits.
Incipient speciation is likely in several other taxa of northern fish
(Schluter 1996). In the case of rainbow smelt (
Osmerus mordax
) in Lake
Saint-Jean, Ontario, Canada, dwarf and normal forms coexist and use the
same spawning areas in two inflowing rivers (Saint-Laurent et al. 2003).
Smelt populations in Lake Saint-Jean probably date from between 8,700
and 10,300 yr ago. Genetic analyses revealed that fish spawning in the two
inflowing rivers were distinct and that within each river population dwarf
and normal forms existed. The dwarf forms grow more slowly, reach
maturity more quickly, and reproduce once at maturity.The normal form
grows faster, matures more slowly, and reproduces repeatedly. Dwarf fish
possess more closely spaced gill rakers, suggesting a greater tendency to
feed on plankton in open water. No evidence was found that these forms
resulted from separate invasions of Lake Saint-Jean by smelt.Thus, diver-
gent selection, strong enough to outweigh gene flow between dwarf and
normal forms, was apparently responsible for differentiation of the two
forms.
The development of reproductive isolation between different popula-
tions of colonizing species can begin very soon after colonization.
Hendry et al. (2000) documented incipient speciation between popula-
tions of sockeye salmon (
Oncorhynchus nerka
) breeding in different loca-
tions within the Lake Washington watershed, Washington State. Sockeye
salmon were extirpated in this watershed by anthropogenic impacts in the
early 1900s. Successful reintroductions were made between 1937 and
1945 using hatchery-propagated juveniles originally obtained from Baker
Lake,Washington.These introductions resulted in a large population that
breeds in the Cedar River, which flows into the southern end of Lake
Washington. In 1957, a small population breeding at Pleasure Point, on
the eastern shore of Lake Washington, was discovered. The Cedar River
sockeyes spawn in gravel beds in flowing water that varies substantially in
temperature from fall through spring, whereas the Pleasure Point fish
spawn in quiet-water gravel beds in which little change in temperature
occurs seasonally.
Comparisons of the fish were made at these two breeding sites in
1992, a maximum of 56 yr or 13 fish generations after the original intro-
ductions to the Lake Washington watershed (Hendry et al. 2000; Hendry
2001). By examining otoliths, calcareous structures of the inner ear, the
site at which individual fish were hatched could be determined because
