Geography Reference
In-Depth Information
the next step in blackcap evolution is to become sedentary, year-round resi-
dents of Britain. Maybe then they will be oi cially crowned a new life form.
I hope their connection to “people of the city” earns the city blackcap a suit-
able Latin binomial.
Sylvia urbanus,
anyone?
Birds and other organisms respond to novel environments such as cities in one
of three ways. First, many adapt their behaviors, customs, or bodies to the new
urban world, and survive. Second, some that exploit cities even change their
character so fundamentally that they evolve into entirely new species. Finally,
avoiders neither adapt nor transform. Their phenotypes are rigid, unable to
of er any possible solution to the challenges of urban life. They become lo-
cally or globally extinct. In Seattle, Pacii c wrens, Wilson's warblers, and hairy
woodpeckers are eliminated from our most urbanized lands because native
groundcover and snags are removed. In Edmonton, Canada, the din of the city
is claiming western tanagers, least l ycatchers, and red-breasted nuthatches.
Elsewhere the story repeats. Some combination of changing habitat, novel
predators and competitors, disease, human persecution, noise, movement
barriers, altered climate and disturbance regimes, pollutants, and toxins stim-
ulate the evolutionary responses of some birds but prove too much for others.
Why do some evolve while others do not?
Confronting change as a large population, rather than a small one, is fun-
damental to adaptive evolution. Extremely small populations, such as the few
Pacii c wrens that remain in subirdia, are vulnerable to the vagaries of chance
and often express little phenotypic variability. Bad luck may af ect small pop-
ulations in some years as extreme weather events kill many or cause wide-
spread reproductive failure. Chance also may deal small populations a skewed
sex ratio that ef ectively limits the pool of breeders to a few paired animals.
These sorts of accidental events make small populations even smaller.
