Geography Reference
In-Depth Information
long ago. But juncos beyond subirdia may also naturally have bright tails. In
San Diego, Dr. Pamela Yeh observed that urban juncos evolved substantially
dif erent tails in only twenty years. She suggested that the longer breeding
season enjoyed by city juncos favored males that cared for young rather than
those that squabbled among themselves for territory. If caring males had fewer
white tail feathers than i ghting males, juncos with darker tails should become
commonplace in the city. I checked the credentials of the Burke Museum jun-
cos and scored their tails to test Yeh's thesis.
Dead juncos talk, and the story they told corroborated Yeh's hypothesis.
Junco tails from the wilds—the forests of Oregon, California, the Olympic
Peninsula, the peaks of the highest Cascade Mountains, and the north slope
of Alaska—averaged 42 percent white, but those from urban western Wash-
ington averaged only 35 percent white. Through time, urban junco tails changed
little, but it was change in the direction expected; before 1940, when Wash-
ington cities were small, tails were 37 percent white, but from 1988 to 2012,
when cities were large, tails of male junco specimens were 35 percent white.
I put the juncos back in the drawer, excited at the prospect that evolution had
occurred in my backyard, and was likely continuing.
Dark-eyed juncos of ered an insight into the keys of rapid evolution that
we now know is frequent in novel ecosystems such as the city. The ability of
colonizing animals to adjust their behavior or appearance to a new situation
boosts natural selection in two complementary ways. First, expression of a
wide variety of behaviors, such as when city juncos start and end breeding,
gives natural selection the raw material it needs to act upon. This is called
“phenotypic plasticity.” If, in this example, breeding for a long time each year
results in more young, then juncos expressing this phenotype will be favored
by natural selection. In a novel environment, such as a city, it is likely that
phenotypes, rare among colonizers, will suddenly be at a premium. A junco
that breeds early in the forest, for example, may be doomed by snow, while one
that breeds late into autumn may not be able to fatten sui ciently for migration.
