Biology Reference
In-Depth Information
body shape and feeding morphology from sticklebacks in the inflowing
stream (Hendry et al. 2002). Lake fish had more slender bodies, inter-
preted as an adaptation for sustained swimming in large water bodies, and
more gill rakers, correlated with planktonic feeding. Fish from the inlet
stream, with deeper bodies, were adapted for short swimming bursts, and
with fewer gill rakers, for benthic feeding. Reciprocal transplant experi-
ments, in which fish from lake and stream habitats were placed together
in cages in the two habitats, showed that each race grew best in its own
habitat type (Hendry et al. 2002). Estimates of gene flow among lake
and stream populations indicated that genetic interchange was higher
between lake and outlet stream fish than between lake and inlet stream
fish and that it probably constrained the degree of divergence of the for-
mer populations.
In some cases, very rapid evolution of threespine stickleback popula-
tions isolated in freshwater areas has been observed. In Bergen, western
Norway, a pond was created in 1960 by closing the connection of an inlet
to the sea, resulting in its conversion to freshwater. Marine sticklebacks
were isolated by the formation of this pond (Klepaker 1993).Three phe-
notypes of these fish could be recognized in the 1960s, based on the num-
ber and arrangement of lateral armor plates: low, partial, and complete.
These phenotypes appear to be defined by dominant and recessive alleles
of two genes. Sticklebacks in nearby marine waters showed only the com-
plete and partial plate phenotypes, with 98.4% of all fish having the for-
mer. In addition, less than 1% showed four, rather than three, dorsal spines.
By 1991, 31 yr after isolation, sticklebacks in the pond showed only
85.9% complete plate phenotype, and 12.7% exhibited four dorsal spines.
These changes are in the direction of the morphology shown by stickle-
backs in other isolated freshwater areas.
Fewer cases are known of rapid evolution in response to biotic condi-
tions by predatory fish introduced to new waters in recent time. Stearns
(1983a, 1983b), however, analyzed life history characteristics of mosqui-
tofish (
Gambusia affinis
) that had been introduced to irrigation reservoirs
in Hawaii in 1905 for mosquito control. Some of these reservoirs contin-
ued to be used for irrigation and experienced fluctuations of water level
related to irrigation withdrawals. Others were abandoned, after which
water levels remained stable. Studies in 1973 and 1974 showed that sev-
eral life history parameters varied between reservoirs with fluctuating and
nonfluctuating water levels, possibly related to food availability differences
(Stearns 1983a). Much of the difference in these parameters was evidently
related to short-term changes correlated with water fluctuations.
