Biology Reference
In-Depth Information
invaded the entire length of the Mississippi River. Allozyme analysis of
mussels from seven locations from Minnesota south to Louisiana revealed
large differences in allele frequencies for two of the six loci examined.
Because gene flow from upstream to downstream localities in the Missis-
sippi River must be substantial, these differences indicate that selection for
different alleles at different latitudes must be strong.
Other recent North American invaders have shown rapid evolution.
The fruit fly
Drosophila subobscura
has evolved a geographical cline in wing
length comparable to that in its Old World range, since its introduction in
about 1978 (Huey et al. 2000; Gilchrist et al. 2001).This cline apparently
evolved in a period of between 10 and 20 yr. The Asian tiger mosquito
(
Aedes albopictus
) shows allozyme variation within its rapidly expanding
new range (Black et al. 1988a, 1988b). Much of this variation, however,
appears likely to be due to genetic drift in the small local populations that
are typical of this species both in North America and in its native Asian
range. In Florida, however, rapid selection for loss of photoperiodically
induced egg diapause has apparently occurred (Lounibos et al. 2003).
Within about 10 yr, the percentage of eggs hatching under short pho-
toperiods (10 h) increased from less than 2% in Georgia to more than
30%, in the Miami, Florida, area.
Among vertebrates, anadromous fish introduced to new regions show
some of the most extensive evolutionary responses. Chinook salmon
(
Oncorhynchus tshawytscha
), native to the Pacific coast of North America,
were introduced to New Zealand in 1901-1907. From the initial intro-
ductions, salmon have colonized several river systems along the eastern
coast of South Island. These populations now differ genetically among
themselves and from their source population in California in several mor-
phological and reproductive features, changes that have occurred in about
30 generations (Kinnison et al. 1998a, 1998b; Quinn et al. 2001).The tim-
ing of spawning migrations into freshwater differ as much as 7 wk among
the several major river systems occupied. Fish also differ genetically from
each other in ways related to the length of the migrations from the ocean
to spawning areas in different river systems in New Zealand (Kinnison et
al. 2001). In general, fish with longer upriver spawning runs tend to be
larger bodied and to have a smaller ovary mass and smaller eggs. Experi-
mental releases of salmon fry in 1994-1995 and recovery of adults in
1997-1998 showed that fish migrating to spawning areas 86 km farther
inland and 413 m higher in elevation experienced greater losses of body
and ovary mass than fish migrating to spawning areas near the coast.Thus,
