Biology Reference
In-Depth Information
the incubation temperature creates distinctive otolith patterns. By this
method, fish resident at the two breeding sites could be distinguished
from new immigrants from the opposite site. Resident fish at the two
breeding sites differed significantly in morphology in ways that correlated
with characteristics of the two habitats. Males had shallower bodies in the
river population, as expected for fish adapted for maximum swimming
efficiency. Females in the river population were larger bodied, which
enables them to spawn at greater depths in the river gravel, reducing the
chance that the eggs will be lost by scouring by the flowing water. Dif-
ferences were also found in features adapting the fry to the different tem-
perature regimes of the two sites. All of these differences are known to
have a strong genetic basis in salmon. Furthermore, the differentiation of
the Pleasure Point population occurred in spite of an immigration rate of
about 39%, indicating that gene flow into the Pleasure Point population
was much less.Thus, although full reproductive isolation had not yet been
achieved, selection was favoring reduced interbreeding.
Animals that have colonized oceanic islands or other islands of recent
origin also show many instances of incipient speciation. On the island of
Madeira, located in the Atlantic 600 km west of North Africa, the house
mouse (
Mus musculus
) was introduced by humans, either via early Por-
tuguese ships that carried them to the islands or byViking visitors as early
as the ninth century (Gündüz et al. 2001). In either case, within a few
hundred years, six distinct chromosomal races have evolved in coastal val-
leys separated by mountain barriers that reach from the interior highlands
to the coast (Britton-Davidian et al. 2000). Owing to chromosomal
fusions, these races possess reduced chromosome numbers relative to
mainland European mice. Formation of fertile hybrids among these races
appears to be impossible, so they are reproductively isolated and consti-
tute sibling species. Mitochondrial DNA analyses suggest that the
Madeiran mice are most similar to present-day populations in northern
Europe, an observation consistent with an introduction by Viking ships
(Gündüz et al. 2001).
Speciation and Extinction: Long-term Prospects
The earth is experiencing an anthropogenic mass extinction of species
comparable in magnitude to the catastrophic mass extinctions of past geo-
logical times.As many authors have shown, these extinctions reflect com-
bined impacts of several types of human activity, especially habitat
destruction, exploitation of plant and animal populations, and introduc-
