Geography Reference
In-Depth Information
These constraints are relaxed in the warm, feeder-rich city. A new type of
junco—one with the behavior of breeding early and often—are favored in the
city. If the behavior of breeding all season is inherited by the many young
produced, then this trait will quickly come to characterize urban juncos; that
is, it will evolve rapidly.
Pamela Yeh suggested that a dozen or so juncos that initially settled in
San Diego in the 1980s had capitalized on the extended length of the breeding
season. It was likely that these colonizers originated from birds that typically
spent the winter in San Diego but bred in nearby mountains where short
breeding seasons are the rule. For reasons unknown, some stayed, and of
those that did, the ones who bred longest left the most descendants. The jun-
cos that stayed evolved bolder and less easily stressed personalities as well,
which allowed them to breed throughout the long nesting season with less
interruption. In fact, those juncos that took full advantage of the long summer
in San Diego l edged twice as many young as did those that retained their
mountain temperaments and breeding habits. Plasticity paid big dividends,
allowing the rapid evolution of personality and productivity. This superpro-
ductivity was critical in enabling the founders to persist, and this persistence
is how the long breeding season gave natural selection a second boost.
Once city juncos expressed their phenotypic plasticity and natural selec-
tion favored those that bred repeatedly, then this behavior worked on other
aspects of the junco's physique. During the long breeding season, females
preferred males that attended to their multiple broods rather than those that
quickly left to i ght other males. Males also had fewer rivals in the city, be-
cause buildings and roads separated junco territories, and bird density was
low. Darwin recognized the dif erences in female preferences and male com-
petition, or “sexual selection,” as important and often complementary forces
to natural selection. Phenotypes that are advantageous in competition with
others of the same sex or in attracting a mate of the opposite sex can evolve by
