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species next door and waste their genes by copulating with them. There will
be selection for behavioral and chemical reproductive isolating mechanisms
that make those alien creatures less desirable.
But once two species are well and truly driven apart by selection for such
isolating mechanisms, any further selection for these mechanisms dimin-
ishes and eventually disappears. Now the two species are so dif erent that
they no longer attract each other. Does their subsequent evolution now slow
to a crawl, governed only by the gradual accidental accumulation of further
mutational dif erences over long periods of time? Or do selective pressures
exist that can continue to drive the species even further apart, making them
more dif erent from each other and increasing the overall diversity of the
ecosystem in which they live?
If the two species come to occupy clearly dif erent ecological niches, the
answer is yes. Natural selection will continue to drive their divergence as
they adapt to the dif erent niches. But in a rainforest the ecological niches are
crowded and overlapping. The further divergence of two species is likely to
be a slow process, especially if they live near each other so that their habitats
remain similar. Once speciation has happened, the further accumulation of
mutations that make the species more and more dif erent should be as slow
as watching paint dry.
But here is where the evolution of entire ecosystems comes into play.
Interactions with the entire ecosystem can speed up species divergence. Tree
species can become extremely dif erent from each other even if they have
similar light and water and nutrient requirements.
Consider two species of plant in a forest that had a recent common
ancestor. Because they are so closely related they are still rather similar in
their appearance, with similar biochemistries, stem structures, leaf shapes,
and so on.
Why have they become dif erent species? There are many possible rea-
sons. For example, pollinators may play a role, as has been shown in beau-
tiful experiments by Doug Schemske of Michigan State University and his
colleagues.
Schemske studied two species of monkey-fl owers that live in overlap-
