Geography Reference
In-Depth Information
dation for selection because, as with juncos in San Diego, some blackcaps
ended up in the novel environment that is British subirdia. At this northern
latitude, day length shortens rapidly in the autumn and reaches a minimum
less than that experienced by blackcaps on the ancestral southern wintering
grounds. This change heightened the birds' responses to rapid increase in
day length and exceptionally long days that lay ahead. As spring arrived the
British warblers' gonads and migratory restlessness grew more rapidly than
did those of birds wintering to the south. As a result of this physiology and a
l ight plan shorter by three hundred miles, the northern warblers arrived back
on the breeding grounds more than two weeks before those using the tradi-
tional southern route. Fattened on suet and seed, the northern birds quickly
set up territories in the prime locations and selected mates. Because of their
early arrival, northern birds paired with other northern birds; few if any
southern birds were even on the breeding grounds. This “assortative mating”
was the barrier that separated northern and southern gene pools and pro-
vided the critical reproductive isolation needed to keep the heritable migra-
tory behaviors of birds wintering in the north and south from mixing each
summer.
Natural selection reinforced genetic isolation among blackcaps. If north-
ern and southern birds managed to pair, their hybrid of spring inherited mi-
gratory instructions that sent them on an intermediate and deadly l ight path.
Scientists demonstrated this in the lab with forced pairings; in the autumn,
hybrid young oriented west and exhibited just enough l ight to end up in the
Bay of Biscayne, of the western shore of France. Parents that bred true to
their migratory route would produce the most survivors, thus keeping the
gene pools pure.
The advantage of arriving i rst, and perhaps exceptionally i t, on the breed-
ing grounds spurred growth in the proportion of blackcaps migrating north.
As more birds rely on nearby subirdia for their winter fare, their features are
also changing, just as are those of juncos and house sparrows. Long-distance
