Geography Reference
In-Depth Information
often it simply slows down adaptation by introducing less i t individuals into
the city.
The strength of selection is a fundamental driver of the degree and speed
of evolution, so species in particularly extreme, fundamentally dif erent envi-
ronments, and perhaps under the inl uence of sexual as well as natural selec-
tion, should rapidly evolve or become extinct. Species in newly urbanizing
areas or in the center versus the periphery of a city may therefore evolve or
become extinct most rapidly. Sexual selection in novel environments may in-
crease the rate of evolution in secondary sexual characteristics, such as was
apparently the case for junco tail feathers in San Diego.
From an airplane, cities and towns look like islands. Their shores, more
abrupt than fringing reefs, are awash in a sea of agriculture and wildland
rather than water. Some are large. Others are small. Many form archipelagos
connected by highways. A few are isolated. Birds on these isles of subirdia are
evolving, but unlike the radiation of species that is typical of evolution on oce-
anic islands, we know of only a few new forms that have been created. The
ease with which country birds move into cities, while a thrill to urban bird
watchers and an essential seed to the evolutionary process within cities, also
limits the radiation of diversity.
The evolution of urban birds to their extreme environments has been
ongoing for a few thousand years at best. During this time, intentionally or
simply as a by-product of our actions, urbanization has given us the rock
pigeon, house sparrow, and Italian sparrow. As urban areas become even
more extreme, producing entirely new feeding and nesting opportunities
and challenging birds with novel soundscapes, landscapes, and birdscapes,
the rate of evolution is quickening. Chimney swifts, house i nches, barn swal-
lows, black redstarts, and white storks join blackcaps, tree sparrows, and
