Geography Reference
In-Depth Information
chance of rapid adaptive genetic evolution. Eurasian collared doves have been
able to colonize and spread throughout European and U.S. cities, even into
extreme places such as Alaska, in part because of their ability to produce mul-
tiple broods each year.
Short generation times—the average time between the birth of parents
and the birth of their young—also reduce the risk of extinction by increasing
genetic variability, boosting population growth rates, and increasing a popu-
lation's ability to respond to natural selection. Generation time is shortest in
small animals, so small birds should evolve especially rapidly in urban envi-
ronments. Evolution might also be generally more rapid in warmer cities
where breeding occurs year-round and in species, such as rock pigeons, that
have generation times that are shorter than one year.
The social behavior and cognitive abilities of birds also may reduce their
risk of extinction. Species that l ock and live many years, especially those
with large brains relative to their body sizes, often have increased behavioral
variability, which can enhance survival during colonization and evolve within
a generation by the mechanism of social learning. Advantageous innovations
are often visible to l ock mates and available for rapid incorporation by a pop-
ulation able to observe and learn or imitate. Corvids, parrots, gulls, and tits
may survive and rapidly evolve in urban areas because social learning allows
them to develop traditions and avoid random extinction. Birds that learn
songs may also rapidly evolve cultural dialects in urban areas that can be im-
portant reproductive isolating mechanisms that limit gene l ow and enable
populations to evolve local adaptations and distinctions.
Isolated populations in novel environments are at risk of extinction but
also are primed for rapid evolution. Urban populations that are strongly con-
nected to rural populations may import genes and cultures that may not be
adapted to city life. While this variation can be critical to evolution, as we saw
when house and Spanish sparrows hybridized to form the Italian sparrow,
