Improbable destinies

A Harvard museum curator draws on the latest breakthroughs in evolutionary biology to examine how tiny, random convergences, from mutations to butterfly sneezes, have triggered remarkable evolutionary changes. "Earth's natural history is full of fascinating instances of convergence: phenomena like eyes and wings and tree-climbing lizards that have evolved independently, multiple times. But evolutionary biologists also point out many examples of contingency, cases where the tiniest change--a random mutation or an ancient butterfly sneeze--caused evolution to take a completely different course.…