- Predators are causing guppy populations to become more drab
by preying on the most brightly colored individuals and eliminating them from
the gene pool.
- Guppy populations are evolving to more closely match
-- or stand out from -- their environment.
- Female guppies are choosing to mate with the most brightly
colored males, giving those males a higher probability of passing their genes on
to the next generation.
Maybe you have a different hypothesis.