Guppies are a classic example of rapid evolution. “Downstream, your life is nasty, brutish and short,” says FSU biologist Joseph Travis. “You mature as fast as you can and reproduce. Upstream, you can expect to live longer and perhaps take your time.” Scientists had believed that differences between upstream and downstream guppies are a response to their different risks of predation.

However, Travis and his colleagues found that something else is driving the rapid evolution of upstream guppies—living in an overcrowded world. They evolved to have fewer offspring since there is less food available due to a higher population. The team tracked every aspect of the life of more than 40,000 guppies over a decade, noting how fast they matured and how large they grew.

• Joseph Travis