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dif er in the shape and color of their fl owers. As a result they attract dif er-
ent pollinators—one species attracts native bees and the other attracts hum-
mingbirds.
Almost certainly, these two species arise from their recent common
ancestor because dif erent populations of the ancestral species lived in places
where only one of the two pollinators was prevalent. The two populations
were then selected to attract the pollinator that was commonest in their
region. Now, because the two species are visited by dif erent pollinators, they
can no longer exchange genes even when they live in the same meadow.
Schemske found that although the two species are ef ectively isolated
from each other, gene fl ow is still possible between them. Hybrids can be
readily produced when the experimenter takes over the role of pollinator,
and deliberately paints the stigmas of one species with pollen from the other.
The resulting hybrids are fully fertile, though they are a little less fi t than the
original species.
When Schemske mated these hybrid fl owers with each other, he produced
a wide variety of new fl ower types with mixed-up characteristics. Some of
them do not attract either pollinator. This means that in nature, any accidental
hybrids would be at a disadvantage. This disadvantage has helped to drive the
two species towards their dif erent pollinators. When he tracked the genes of
hybrid fl owers that attracted one, both or neither pollinator, Schemske was able
to identify some of them. These genes infl uence fl ower color and the amount
of nectar produced. Bees prefer more orange-colored fl owers, and humming-
birds prefer fl owers that make copious amounts of nectar. These results pro-
vide a beautiful example of how speciation actually happens in the real world.
So, returning to our two forest plant species, let us suppose that they
have speciated by some similar process. Like Schemske's monkeyfl owers
they can no longer exchange genes in their native environment, even when
they happen to grow next to each other. Now what? The disadvantage of
hybridization has helped to drive the forest trees to become two dif erent
species, but that disadvantage has now disappeared because hybridization
no longer happens.
Here is where the entire ecosystem starts to play a role in driving the spe-
cies further apart. If such closely related species are going to thrive in their
 
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