When you think of evolution, perhaps you imagine a very long tree of life, extending across millions of years, starting with one-celled creatures and ending with us: human beings (Homo sapiens). But evolution also happens more quickly. The development of antibiotic-resistant species of bacteria is also evolution in action: some of the tiny creatures that cause illnesses in human beings no longer respond to medicine that once cure those illnesses. This challenges doctors and scientists to come up with new ways of protecting people. Evolutionary geneticists like Elizabeth Alter examine DNA and other genetic evidence to help us understand how different species change over time. Alter focuses her work on oceanic and fresh-water creatures including the mummichog, a fish that lives in polluted water near New York City.