In the natural stream communities of Trinidad, guppy populations live close together, but evolve differently. Upstream, fewer predators mean more guppies but less food for each; they grow slowly and larger, reproduce later and less, and die older. Downstream, where predators thrive, guppies eat more, grow rapidly, stay small, reproduce quickly and die younger.
While it is clear to ecologists that an ecosystem shapes the evolution of animals living in it, population biology experts such as Joseph Travis of The Florida State University believe the reverse can also be true, making the relationship between evolution and ecology a model of reciprocity. Now, he can prove it.