Biology Reference
In-Depth Information
In 1980, Stearns (1983b) collected fish from reservoirs with stable and
fluctuating water levels and propagated them in the laboratory under
identical conditions. The life history parameters of laboratory-hatched
offspring were then analyzed to determine if genetic differences existed
between populations. Fish from reservoirs with stable water levels tended
to have smaller offspring than those from fluctuating reservoirs and to be
longer lived.The relation of fecundity to body length tended to be neg-
ative for fish from stable reservoirs and positive for those from fluctuating
reservoirs. Other differences were related to reservoir size, depth, and
intensity of fluctuation.Thus, although it is difficult to pinpoint the selec-
tive forces operating in these reservoirs, substantial evolutionary change
was evident.
Some predatory fish have also made rapid evolutionary adjustments to
changes in the predatory regime affecting them. Threespine sticklebacks
(
Gasterosteus aculeatus
), native to coastal marine waters, have invaded
stream and lagoon environments in many locations and have often
become isolated in such environments by natural coastline changes or
human modification of the coastline. In these environments, the predation
regime on small fish is usually less intense than in the ocean. The three-
spine stickleback has a series of lateral armor plates varying in number
from none to about ten, with most populations showing some variation
in number. This characteristic is genetically controlled (Hagen and
Gilbertson 1973).Where freshwater habitats have been invaded, the long-
term result is usually reduction in the number of armor plates and a short-
ening of the dorsal spines (Kristjánsson et al. 2002). In many locations
these changes have obviously occurred in postglacial time, but the length
of isolation involved usually is not precisely known.This pattern of rapid
evolution has apparently occurred many times, but these differentiated
populations are also prone to extinction because of the transitory nature
of the habitats occupied (Bell 2001).
In a fjord in western Iceland, however, a dam was constructed in 1987
to create a lagoon for trout and salmon ranching.The water in this lagoon
soon became fresh, so a population of threespine sticklebacks was isolated
in a new environment with an altered predation regime. Kristjánsson et
al. (2002) examined the sticklebacks isolated in the lagoon in 1999, 12 yr
and a maximum of 12 generations after isolation.They found that the fish
had shown significant reduction in their armor plates and spine length.
Some morphological adaptation to the two major habitats of the lagoon,
areas of mud bottom versus lava rock bottom, had also occurred.Although
this example does not strictly involve an alien species invasion, the evolu-
