Still, as we have all heard by now, this season’s flu vaccine is only 62 percent effective (an average of the protection against each of the major strains prevalent this year). While that’s far better than crossing your fingers and hoping for the best, the risk of getting sick even when vaccinated is much greater than with vaccines for diseases such as polio. Why is it so hard to develop and maintain a highly protective vaccine for flu? The reason is, in a word, evolution.

It might be hard to imagine evolution working quickly enough to touch our day-to-day lives, and when the theory was still young, the speed of evolution was an open question in science. A key experiment that helped answer the question was carried out right in New England, during another blustery winter over a century ago.

In 1898, the region was experiencing record-setting bad weather. Rather than bemoan the cold temperatures, an extravagantly mustached scientist named Hermon Bumpus, an assistant professor of comparative zoology at Brown University, decided to profit from misfortune in the name of science.

After a brutal early February storm, Bumpus was brought 136 dead or stunned house sparrows. (His paper is curiously mute about the source—was he well known to friends and neighbors for an interest in dying birds?) Once Bumpus saw that about half the birds recovered after they were warmed, while the others died, he knew he had a potential gold mine of information on his hands. Darwin’s theory of evolution by natural selection, then still just a few decades old, predicted that certain individuals would survive and reproduce while others failed. If this process had just happened with the sparrows, then the survivors might show characteristics that the others did not.

So Bumpus methodically measured the size of bills, wings, legs, and other body parts in the survivors and those not so fortunate, and then compared the two groups. They differed substantially. Unusually large or unusually small birds fared worse than those clustered around the average size of the group, and Bumpus postulated that stabilizing selection, a kind of natural selection that winnows out the extremes and favors those in the middle, had been at work. Through the pattern of death and survival, the genes in the population had changed in relative frequency. In other words, evolution had occurred.

Bumpus had confirmed that evolution happened in the wild; his study is still used by biology classes today in calculating the action of evolution. But he had also shown that it could, literally, occur overnight. What is required is a strong selective agent, like the storm, and the variation to ensure that at least some individuals survive to reproduce. Since Bumpus’s time, and ever more readily in the last two decades, scientists have confirmed so-called contemporary evolution—genetic change in a population within 100 generations or fewer—in scores of species.

Today, the most common modern-day equivalent of the Providence blizzard is human activity. Trophy hunters can change a population of sheep or deer by shooting the rams with the largest horns or antlers; similarly, fishing seems to have to have altered the life schedules of salmon and other commercially important fish by removing specimens that grow to a large size before they have a chance to reproduce. Keep picking off those slow-maturing larger individuals, and soon the average salmon is smaller and precocious, a process called fishery-induced evolution. Where viruses like influenza are concerned, meanwhile, the strong selective agent is the human immune response—which can be bolstered by vaccines.