The plate on the left contains about equal numbers of colonies of two different bacteria. After the bacteria compete and evolve, the lighter ones have taken the lead in the plate on the right.
Courtesy of Michael Wiser
This is an artist's illustration of Kepler-62f, a planet in the "habitable zone" of a star that is slightly smaller and cooler than ours. Kepler-62f is roughly 40 percent larger than Earth.
A helicopter is unloaded from an LC-130 in Antarctica last December. Researchers on this mission were studying the Pine Island Glacier, one of the fastest-receding glaciers on the continent.
August Allen/National Science Foundation
Health officials around the world are on constant lookout for the deadly bird flu. Here a worker collects chickens on a farm in Kathamndu, Nepal, where the virus was suspected of infecting poultry last October.
Prakas Mathema/AFP/Getty Images