Evolution can perform spectacular makeovers: today's airborne songbirds descended from the wingless, earthbound dinosaurs that roamed millions of years ago, for example. But some organisms seem to change very little, even over eons. Scientists have long wondered how these species withstand the pressures of natural selection. The prevailing hypothesis for this “stasis paradox” has been that natural selection keeps some species unchanged by selecting for moderate or average traits (so-called stabilizing selection) rather than selecting for more extreme traits that would cause a species to change (directional selection). But a study published in the Proceedings of the National Academy of Sciences USA contradicts this idea, showing that evolution constantly favors different traits in seemingly unchanging animals that improve short-term survival. In the long term, though, “all that evolution cancels out and leads to no change,” says the study's lead author, James Stroud, assistant professor and Elizabeth Smithgall-Watts Endowed Faculty in the School of Biological Sciences.