The word evolution is nearly synonymous with change. One species diversifies into many. A disaster triggers a mass extinction among marine life. A microbe becomes resistant to our drugs. Each of these changes, large or small, is a classic example of evolution in action. But what about lack of change? Most of the species we observe around us today look about the same as they did in our grandparents’ time. And the fossil record includes many species that seem hardly to have changed at all for millions of years. How does this conspicuous stability square with evolutionary theory? New research supports one explanation.