Geography Reference
In-Depth Information
The mobility of birds means that gene l ow is typically high among bird
populations. As we learned from the Italian sparrow, gene l ow and resulting
hybridization can be creative, but often gene l ow reduces distinctions among
populations and prevents speciation, as was true in the case of the European
blackbird and Australian silvereye. Even in the extensively urbanized Seattle
area, which reduces the mobility of song sparrows and has imparted some
slight ge ne tic dif erences between nearby birds, substantial distinctions are
lacking. Here, mobility is an equalizer, but sometimes it is the instigator of
change, curtailing gene l ow and sending urban and rural populations on the
path to speciation.
Blackcaps are small, insectivorous warblers common throughout Europe.
For millennia these drab gray birds with sleek black top hats left their Ger-
man breeding grounds to winter in the pleasant Mediterranean climate of
southern Spain, Portugal, and North Africa. It is hard to imagine foregoing a
winter in Casablanca for one in the British Isles, but for blackcaps there are
several advantages, namely, a shorter migratory route, access to bird feeders,
a less variable climate, and a jump-start on the next year's breeding. As a re-
sult, what was i fty years ago a rare but normal deviation in migratory direc-
tion and distance is now a well-established migratory route. In the 1960s few
Brits saw blackcaps at their feeders, but by 2004 nearly one in three feeders
hosted a blackcap. This new migratory population orients to the northwest
after breeding and is evolving a host of distinctions from the ancestral south-
west migrants. And they are doing so despite breeding side by side, a feat rare
in animals that is called “sympatric speciation.”
Natural selection was able to create a second blackcap that migrates from
south-central Europe to the British Isles because it had phenotypic variability
to work upon, heritability that connected this variability across generations,
advantage for extreme traits shown by the ancestors, and, most important, a
way to isolate breeders in one gene pool from those in the other. Heritable
phenotypic variation in migration direction and distance provided the foun-
 
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